Lunar Perigee and Apogee Home Sun & Moon Moon Lunar Perigee and Apogee Lunar Perigee and Apogee The Moon's orbit around Earth is elliptical. The point of the orbit closest to Earth is called perigee, while the point farthest from Earth is known as apogee. The difference between apogee and perigee. The difference in Moon's Distance–from its center–to the center of Earth, at perigee and apogee. timeanddate.com Elliptical Orbit The Moon's orbit around Earth is elliptical, with one side closer to Earth than the other. As a result, the distance between the Moon and Earth varies throughout the month and the year. On average, the distance is about 382,900 kilometers (238,000 miles) from the Moon's center to the center of Earth. The point on the Moon's orbit closest to Earth is called the perigee and the point farthest away is the apogee. Supermoons & Micromoons A Supermoon versus a Micromoon.

A Full Moon looks bigger and brighter when it occurs around perigee and smaller and dimmer at apogee. The Moon's phase and the date of its approach to its perigee or apogee are not synced. When a Full Moon or New Moon occurs close to the Moon's perigee, it is known as a Supermoon . On the other hand, when a Full Moon or New Moon occurs close to the Moon's apogee, it is known as a Micromoon . The Moon passes through the 2 extreme points–or apsides–perigee and apogee about once a month. The time it takes for the Moon to travel from perigee to perigee, is called the anomalistic month, and takes around 27.55455 days. This is not to be confused with the synodic month , which lasts a little longer, and is the time it takes the Moon to orbit once around Earth, from New Moon through all the Moon phases to the next New Moon.

Moon phases and events Close to Earth The Supermoon on November 14, 2016 , was the closest a Full Moon has been to Earth since January 26, 1948 . The next time a Full Moon is even closer to Earth will be on November 25, 2034 (dates based on UTC time). Moonrise is the best time to view the Moon, weather permitting, of course. At this time, illusion mixes with reality to make a low-hanging Moon that looks unnaturally large when compared to foreground objects. Taking pictures of the Moon Lunar Libration In addition to its counterclockwise orbit around Earth, the Moon rotates around its axis at a constant speed. Like all celestial objects with elliptical orbits, the Moon's speed varies on its path around the Earth. It speeds up when it is at its perigee and slows down when it is at the apogee. This means that at its perigee, the Moon's orbital speed is faster than its rotational speed. When the Moon rocks slightly from north to south and wobbles a little from east to west, it is called lunar libration.

This motion makes it possible, over time, to see up to 58% of the Moon’s surface from Earth, but only 50% at a time. Tidal Effects Moon's gravitational pull causes tides. Moon's gravitational pull causes tides. ©bigstockphoto.com/duallogic The tides on Earth are mostly generated by the Moon’s gravitational pull, in combination with the Sun. The tidal range, which is the difference between high and low tide, is greatest at Full and New Moon because the Sun and Moon are aligned on the same or opposite sides of Earth, and the oceans are affected by the combined gravitational pull of both the Sun and the Moon. During Supermoons, the gravitational pull leads to even larger variation between high and low tides. It causes higher spring tides, known as perigean spring tides. This has nothing to do with the season spring, but rather it is a synonym for jump or leap. The tidal range is smallest during the 2 Quarter Moons (Half Moons). These tides are known as neaps or neap tides, from Anglo-Saxon, meaning without the power. Natural Disaster Trigger?

Although the Sun and the Moon’s alignment cause a small increase in tectonic activity, the effects of the Supermoon on Earth are minor. Many scientists have conducted studies and haven’t found anything significant that can link the Super Moon to natural disasters. According to NASA, the combination of the Moon being at its closest and at Full Moon, should not affect the internal energy balance of the Earth since there are lunar tides every day. Moon Origins, Facts and Phenomena The Physical Moon and its History (The Moon, The Whole Moon & Nothing But The Moon) The Moon's Phases The light we see emanating from the Moon on any moonlit night is the result of reflected sunlight being reflected back to Earth. The familiar phases the Moon passes through in a month are caused by the different angles from which we see the sunlit side. The dark far side of the Moon only reflects 7% of the incoming light. Because of the Moon's reflective properties and since it illuminates no light of its own, it is said by mystics to be "receptive" (borrowing its light from the sun).

The Moon moves in an elliptical path tilted at a 5 degree angle to the ecliptic, and moves westward in its orbit across the sky. It takes about 12 hours for the Moon to travel from horizon to horizon. As it crosses the sunrise terminator, where the Sun is rising, the Moon is in its waxing phase, where it is increasing in light. As it crosses the sunset terminator, where the Sun is setting, the Moon is in its waning phase and is decreasing in light. If you look up in the sky on any given night and the Moon's right side is brightly lit, it is in its waxing phase. If the Moon's left side is brightly lit, then it is in its waning phase. The Period of Rotation It takes 27.3 days to complete one orbit. It also rotates on its axis in 27.3 days, so it always keeps the same face pointing toward earth. A complete cycle from one new Moon to the next takes 29.5 days or one synodic month to complete. Because the Moon's period of rotation is locked in phase with its orbit around the Earth, we usually only see the one side.

However, the Moon has a slightly non-circular orbit, moving quicker when closest the Earth and more slowly when far away. This causes the Moon to appear to wobble a bit, and the usual face we see drifts sideways, to one side or the other, so that a few degrees of the far side can occasionally be seen. While its speed in orbit changes rhythmically, its axis rotation remains uniform. When the Moon is closest to Earth, the axial rotation falls behind a bit, turning its face to the East. While at a greater distance, the Moon moves ahead, turning its face to the West. The tides , eclipses , and phases of the Moon are in direct correlation to the movement of the sun, moon, and earth. When the Moon is full, earthquakes, storms and floods increase in frequency. The Moon begins its rhythmic cycle as the Moon's nearside crosses the sunrise terminator, from east to west, to produce a crescent Moon. The Moon waxes to a 1st quarter Moon, where the Moon is half illuminated, to a gibbous Moon. Then when the Earth, Sun and Moon are nearly in a straight line, with the Earth in the middle, a Full Moon occurs.

This occurs when the Moon is in opposition to the position of the Sun. From the Full Moon, the Moon begins its waning cycle, crossing the sunset terminator to produce a waning gibbous Moon, to the last quarter. Finally we see only a waning crescent. The complicated motion of the Moon results in a difference in the way we measure the monthly cycles. Time is measured by a reference point in the sky. We measure the time between one New Moon and the time it takes to return to the same position with respect to a fixed position in the sky. This is called a Sidereal month and is equal to 27.32166 days. The time between the transits of the Moon through one of its nodes is called the Draconic month; this equals 27.212220 days. The time between one New Moon to the next New Moon is called the Synodic month; this equals 29.530589 days and the time the Moon moves from equinox to equinox is called a Tropical month; this equals 27.321.58 days.

The Moon's perigee and nodes also move east and west with periods of 8.85 years and 18.61 years respectably. This results in different lengths of the Sidereal and Draconic months. Watch a Full Year of the Moon in Five Minutes What is a supermoon? | Human World | EarthSky What is a supermoon? By Bruce McClure and Deborah Byrd in Human World | Space | November 8, 2016 Are supermoons hype? In our opinion … no, just modern folklore. And they cause real physical effects, such as larger-than-usual tides. Full moon, via Kristal Alaimo-Moritz at Sepulveda Basin Wildlife Reserve, Van Nuys, California. A supermoon is a new or full moon closely coinciding with perigee – the moon’s closest point to Earth in its monthly orbit. An astrologer, Richard Nolle, coined the term supermoon over 30 years ago, but now many in astronomy use it as well. Are supermoons hype? In our opinion … gosh, no, just modern folklore.

They’ve entered the popular culture (check out Sophie Hunger’s music video in this post , for example). And they can cause real physical effects, such as larger-than-usual tides. According to the definition of supermoon coined by Nolle, the year 2017 has a total of four supermoons. The new moons of April, May and June and the full moon of December all qualify as supermoons. Follow the links below to learn more about supermoons. Enjoying EarthSky so far? Sign up for our free daily newsletter today! Photographs or other instruments can tell the difference between a supermoon and ordinary full moon. The supermoon of March 19, 2011 (right), compared to an average moon of December 20, 2010 (left). Image by Marco Langbroek of the Netherlands via Wikimedia Commons. What is a supermoon? We confess: before a few years ago, we in astronomy had never heard that term. To the best of our knowledge, astrologer Richard Nolle coined the term supermoon over 30 years ago. The term has only recently come into popular usage.

Nolle has defined a supermoon as: … a new or full moon which occurs with the moon at or near (within 90% of) its closest approach to Earth in a given orbit. That’s a pretty generous definition, which is why there are so many supermoons. By this definition, according to Nolle: There are 4-6 supermoons a year on average. Some astronomers have complained about the name … but we like it! And it’s entered the popular culture. for example, Supermoon is the title track of Sophie Hunger’s 2015 album. It’s a nice song! Check it out in the video below. What did astronomers call these moons before we called them supermoons? We called them a perigee full moon , or a perigee new moon . Perigee just means “near Earth.” The moon is full, or opposite Earth from the sun, once each month. It’s new, or more or less between the Earth and sun, once each month. And, every month, as the moon orbits Earth, it comes closest to Earth. That point is called perigee. The moon always swings farthest away once each month; that point is called apogee.

No doubt about it. Supermoon is a catchier term than perigee new moon or perigee full moon. We first became familiar with the supermoon label in the year 2011 when the media used it to describe the full moon of March 19, 2011. On that date, the full moon aligned with proxigee – the closest perigee of the year – to stage the closest, largest full moon of 2011. About three or four times a year, the new or full moon coincides closely in time with the perigee of the moon—the point when the moon is closest to the Earth. These occurrences are often called ‘perigean spring tides.’ The difference between ‘perigean spring tide’ and normal tidal ranges for all areas of the coast is small. In most cases, the difference is only a couple of inches above normal spring tides. Image and caption via NOAA. When are the supermoons of 2017?

By Nolle’s definition, the new moon or full moon has to come within 362,146 kilometers (225,027 miles) of our planet, as measured from the centers of the moon and Earth, in order to be considered a supermoon. By that definition, the year 2017 has a total of four supermoons. The first supermoon, for 2017, comes with the April 26 new moon. The new moons on May 25 and June 24 are also considered supermoons, according to Nolle’s definition, and that same definition dictates that the full moon of December 2017 – plus the full moons on January 2 and 31, 2018 – will be supermoons, too.

Thus, three successive full moon supermoons (aka near-perigee full moons) come in December 2017 and January 2018: Full moon of 2017 December 3 at 15:47 UTC Full moon of 2018 January 2 at 2:24 UTC Full moon of 2018 January 31 at 13:27 UTC The new moon on May 25, 2017 will present the closest supermoon of the year (357,207 kilometers or 221,958 miles). What’s more, this will be the first time since the year 2009 that the new moon (instead of the full moon) ushers in the closest supermoon of the year. Want more detail? Okay. In 2017, the moon comes closest to Earth on May 26 (357,207 kilometers), and swings farthest away on December 19 (406,603 kilometers).

That’s a difference of 49,396 kilometers (406,603 – 357,207 = 49,396). Ninety percent of this 49,396-figure equals 44,456.4 kilometers (0.9 x 49,396 = 44,456.4). Presumably, any new or full moon coming closer than 362,146.6 kilometers (406,603 – 44,456.4 = 362,146.6) would be “at or near (within 90% of) its closest approach to Earth.” Around each new moon (left) and full moon (right) – when the sun, Earth, and moon are located more or less on a line in space – the range between high and low tides is greatest. These are called spring tides. A supermoon – new or full moon at its closest to Earth – accentuates these tides. Image via physicalgeography.net Spring tides will accompany the supermoons.

Will the tides be larger than usual at the April, May and June 2017 new moons and the December 2017 full moon? Yes, all full moons (and new moons) combine with the sun to create larger-than-usual tides, but closer-than-average full moons (or closer-than-average new moons) elevate the tides even more. Each month, on the day of the new moon, the Earth, moon and sun are aligned, with the moon in between. This line-up creates wide-ranging tides, known as spring tides. High spring tides climb up especially high, and on the same day low tides plunge especially low. The closest new moon of the year on May 25 and the year’s closest full moon on December 3 are bound to accentuate the spring tide all the more, giving rise to what’s called a perigean spring tide . If you live along an ocean coastline, watch for high tides caused by these perigean full moons. Will these high tides cause flooding? Probably not, unless a strong weather system accompanies the perigean spring tide.

Still, keep an eye on the weather, because storms do have a large potential to accentuate perigean spring tides. Dates of closest full supermoons in past and future years. More often than not, the one day of the year that the full moon and perigee align also brings about the year’s closest perigee (also called proxigee). Because the moon has recurring cycles, we can count on the full moon and perigee to come in concert in periods of about one year, one month and 18 days. A lunar month refers to the time period between successive full moons, a mean period of 29.53059 days. An anomalistic month refers to successive returns to perigee, a period of 27.55455 days. Hence: 14 lunar months x 29.53059 days = 413.428 days 15 anomalistic months x 27.55455 days = 413.318 days Therefore, the full moon and perigee realign in periods of about 413 days (one year and 48 days).

So we can figure the dates of the closest full moons (<357,000 km) in past and future years: Extra-close perigee full moons from 2010 to 2020 2010 Jan 30 2019 Feb 19 2020 Apr 08 There won’t be an extra-close perigee full moon in 2017 (<357,000 km) because the extra-close coincidence of full moon and perigee won’t happen again (after November 14, 2016) until January 2, 2018. Looking further into the future, the perigee full moon will come closer than 356,500 kilometers for the first time in the 21st century (2001-2100) on November 25, 2034 (356,446 km). The closest full moon of the 21st century will fall on December 6, 2052 (356,425 km).

By the way, some astronomers will call all the full moons listed above proxigee full moons. But, like many of you, we’ll have fun just calling ’em supermoons. What does a full supermoon look like? Most astronomers say you can’t really detect any difference with your eye between a supermoon and any ordinary full moon, although some careful observers say you can. This great moon photo is from EarthSky Facebook friend Rebecca Lacey in Cambridge, Idaho. What is a Black Moon? We had never heard the term Black Moon until early 2014. It doesn’t come from astronomy, or skylore, either. Instead, according to David Harper , the term comes from Wiccan culture. It’s the name for the second of two new moons in one calendar month. January 2014, for example, had two new moon supermoons, the second of which was not only a supermoon, but a Black Moon. Does a Black Moon have to be a supermoon in order to be called Black? No. You can read more about Black Moons here . The next Black moon by the above definition will occur on October 30, 2016.

Sten Odenwald at astronomycafe.net lists some other names for the second new moon in a month: Spinner Moon, Finder’s Moon, Secret Moon. However, we’ve also come across another definition for Black Moon: the third of four new moons in one season. This last happened with the new moon supermoon of February 18, 2015, because this particular new moon was the third of four new moons to take place between the December 2014 solstice and the March 2015 equinox. The next Black Moon by this definition will occur on August 21, 2017, to feature a Black Moon total solar eclipse in the United States. Bottom line: The term supermoon doesn’t come from astronomy. It comes from astrology, and the definition is pretty generous so that there are about 6 supermoons each year. This post explains what a supermoon is, how many will occur in 2017, which moon is the most “super” of all the 2017 supermoons, and gives a list of upcoming full supermoons for the years ahead.

Moon Glossary: Lunar Terms and Definitions Moon Glossary: Lunar Terms and Definitions Moon Glossary: Lunar Terms and Definitions A Albedo — This term is a unit-less measure that refers to the how much an object diffusely reflects light from the sun. Angular Diameter — The measurement of the diameter of a distant object from the perspective of the angle resulting between the observer and the object's outer edges. Also known as the 'visual diameter'. Annular Eclipse — Refers to an solar eclipse where the Moon is between the Sun and the Earth, but with the Moon appearing smaller than the Sun to the observer, leaving a bright ring around the circumference of the Moon. Anomalistic Month — The length of time that the Moon takes to pass between perigee (or apogee) passages — that is, the 27.55455 day period during which the Moon moves from its closest (or farthest) point to the Earth, all the way around and back again. Anorthositic Rock — A calcium-rich rock type found in abundance on the Moon. Aphelion — The farthest point from the Sun in a planetary body's orbit. Apogee — The farthest point from the Earth in the Moon's orbit.

Apogean Tide — The name for low tide when the Moon is at it's farthest point from the Earth. Apolune — The farthest point from the Moon in an object's orbit around it. B Basalt — Volcanic rock formed by rapidly cooling lava. Found on the moon and the earth. Breccia — A type of rock that is composed of a matrix of different materials, minerals and fragments of other rocks. Found on the moon and the earth. C Celestial equator — The projection of the Earth's equator into space. It could be considered an invisible belt that surrounds the Earth in the minds of astronomers. Celestial Mechanics — The specialized areas of astrology that describes the gravitational effects of heavenly bodies as well as their motion. Colongitude — Also called selenographic colongitude, it is the longitude of the Moon's morning terminator. Conjunction — The term applied when two planets are in close proximity to each other in the sky, from the perspective of an Earth-bound observer. Crater Wall — The cliff-like wall formed by the impact of a meteor with a celestial body such as a planet or moon.

Crescent Moon — The famous image of the Moon frequently used in the media, consisting of only a thin crescent slice of the Moon being visible from Earth. This phase of the moon occurs just after the New Moon phase, which is also known as Dark of the Moon. There is also a Crescent Moon phase just prior to the next New Moon as well. D Dark of the Moon — So named because during this phase, the Moon is not visible in the sky, Dark of the Moon is also known as New Moon. Declination — Declination is the position of a celestial body, such as the Moon, in the equatorial coordinate system. Declination is measured by degrees in relation to the celestial equator. Diurnal — In astronomy, diurnal generally refers to the motion of an object in a 24 hour period. An example would be Moon-rise. These activities repeat every 24 hours. The diurnal arc describes the amount of time a celestial object takes to transition from fully risen to fully set. DST — Daylight Savings Time. E Earthshine — The light of the sun that is reflected back into space by the Earth, and which can illuminate other objects such as the Moon.

Eclipse — Any interference between the light from the Sun and the object being illuminated. The Moon frequently moves between the Earth and the Sun, blocking out the Sun's rays. Ecliptic — A term applied to the invisible path in the sky that the Sun moves through during the year, in relation to Earth and the other planets. Elongation — The angle found between a planet and the Sun, from the perspective of the Earth. Ephemeris — An astronomical text which contains the position of celestial bodies in the sky as seen from Earth at specific times. These positions are given in coordinates that astronomers can then use to locate and view these bodies. Equatorial Tide — A tide with a period of 328 hours (approximately every two weeks) that occurs when the Moon is positioned above the equator. F Far Side — The side of the Moon which faces away from the Earth and which is not visible via planetary observation. First Quarter Moon — During this phase of the Moon fifty percent of it is illuminated and visible from the Earth. It occurs after the Crescent phase and before the Waxing Gibbous phase. Full Moon — During this phase of the Moon it is entirely illuminated and visible from the Earth.

The Moon is opposite the Sun in this phase, with the Earth in between. G Gibbous Moon — There are two Gibbous phases of the Moon, with the first representing the growing moon between the First Quarter and the Full Moon and the second when it describes the Moon growing smaller as it shrinks down from the Full Moon to the Last Quarter. These phases are referred to as the Waxing and Waning Gibbous, respectively. Gravity — Gravity is the attractive force which governs the motion of the celestial bodies. Gravity controls the orbits of all planets in our solar system as well as our solar system's relative motion to the Universe. It also plays a significant role in the distribution of mass throughout the Universe. Grazing Occultation — A phenomenon in which the varied topography of the Moon's surface causes stellar objects such as stars to disappear and reappear at the northern or sudden limit of the Moon intermittently. This occurs due to the motion of the Moon itself across the path between a certain patch of the heavens and the Earth. Greenwich Mean Time — Time derived from the annual average movement of the sun over the zero degrees longitude position of the Greenwich Observatory in England.

The mean is used in order to compensate for the Sun's non-uniform motion. It has since been replaced in astronomical circles by Universal Time. H Half Moon — Term that is used to describe the First Quarter Moon and the Last Quarter Moon. I Intercalation — The source of leap years, or the addition of an extra day or other period of time in order to reconcile the solar year with that of the calendar we use. This is necessary because the solar year contains approximately 365.25 days, making it necessary to add a full extra day to the calendar every four years. In the past, much longer periods of time were used during intercalation. Intercalary days or months can also be added to the lunar calendar. L Lacus — Areas on the Moons surface that have the topography of lakes are prefaced with this Latin term for lake. Last Quarter — The phase of the Moon between the Waning Gibbous and the Waning Crescent, where fifty percent of the Moon is still visible before the Waning Crescent phase wipes it from the sky. Latitude — Coordinate system used on a planetary body to give the location of a point in relation to its equator.

There are ninety degrees of latitude north and south of the equatorial line. Librations — The gentle rocking motion of the Moon as it orbits the Earth that allows observation of the side that normally faces away from our planet. In total, through this irregular motion fifty-nine percent of the Moon can be seen. Limb — The outermost edge of a planetary body or celestial object. Longitude — Coordinate system used on a planetary body to give the location of a point in relation to a reference meridian. Meridians are lines drawn by cartographers and astronomers that pass through the northern point on the horizon, meeting at the celestial pole. As there is no natural starting meridian, the base or center meridian is chosen arbitrarily on each planet or moon. On Earth's Moon, Sinus Medii represents the zero degree point for longitude, with ninety degrees of longitude available to the east and west. Lunar Day — There are two definitions of this term. The first refers to the period of time it takes for the Moon to spin completely on its axis in terms of its position to the sun. The second is the amount of time it takes for the Moon to complete a single orbit around the Earth. Due to the eccentric orbit, a Lunar Day varies in length.

Lunar Eclipse — This event occurs when the Full Moon moves through the shadow cast by the Earth as it passes between the Sun and the Moon. Lunar Interval — The difference in time between a phase of the Moon or tide occurring at the Greenwich meridian and a local meridian. This interval represents the passage of the Moon during this time. Lunar Rays — Lines scored into the crust of a celestial body caused by mass ejected on impact from a meter. Found on both the Moon and Mars. Lunitidal Interval — The period of time that occurs between the Moon passing over a point on Earth and the next high tide for that point. M Magnitude — The brightness of a celestial body. A lower magnitude number indicates a brighter object. Mare — Name given to plains composed of basalt on the Moon. From the Latin word for 'sea', so-called due to their large landmass. These basalt deposits were left by the eruptions of now-extinct volcanoes. Mascon — A region in the crust of a celestial body which is denser than average and as such acts to create a local gravitational anomaly. On the moon, there are several basins which demonstrate these properties.

Meridian — Meridians are lines drawn by cartographers and astronomers that pass through the northern point on the horizon of a planetary body or moon, meeting at the celestial pole. Meteoroid — A meteoroid is any body drifting in space that has a large enough mass to be detected but which is significantly smaller than an asteroid. The exact limit at which a meteoroid is reclassified as an asteroid is up for debate, but generally, any object smaller than 50 meters in diameter is considered a meteoroid. Moon — The natural satellite in orbit around the planet Earth. Also the classification for any natural satellite found in orbit around any other planet. Moon Rise — Similar to sunrise, it is the first appearance that the Moon makes over the Earth's horizon, and as such it is relative to the geographical position of the observer. Moon Set — The opposite of Moon Rise, it is the when the Moon disappears behind the Earth's horizon, relative to the observer. N Nadir — The point with a negative ninety degree inclination in relation to the observer, or the point directly beneath their feet. Neap Tide — When the Moon is at its First Quarter or Last Quarter, its forces are partially cancelled out by the Sun. This leads to a lower high tide than normal.

Near Side — The side of the Moon that is visible from the Earth. New Moon — If the Moon is on the same side of the Earth as the Sun, then the face of the Moon that can be seen from the Earth is no longer illuminated by the Sun's rays — as only the opposing side is facing the Sun. As a result, it is invisible in the Earth's sky. Nodes — Also called Lunar Nodes, these are the points where the orbit of the moon intersects the path of the Sun in terms of how it appears to move against the stars. This solar path is called the ecliptic. Nodical Month — A period of 27.212220 days representing the movement of the Moon starting and finishing at a specific Lunar Node. O Occultation — The act of one celestial body obscuring another as a result of moving between the observer and the object being observed. The most well known occultations are the lunar and solar eclipses. Old Crescent Moon — Phase of the Moon that occurs between the Last Quarter and the New Moon. In this phase, the Moon is visible only as a very thin crescent.

Opposition — The term used to describe two celestial bodies as being opposite each other in the sky, relative to the observer. An example is when the Moon is opposite the Sun, with the observer on the Earth in the middle. Orbital Eccentricity — Objects orbit in an elliptical fashion; eccentricity can be thought of as the amount by which the orbit of a celestial body deviates from a circular shape. P Palus — A Latin term meaning 'swamp' that is used to describe topographical features on the moon which resemble dark plains or swamps. Parallax — The apparent motion of an observed object against the background caused by the movement of the observer. An example of this is the motion of the stars as seen from observatories on Earth. Partial Eclipse — When a celestial body gets between another object and a light source, it casts 2 shadows. The umbra blocks all of the light from the light source (usually a sun), and the penumbra blocks only a portion of that light. A partial eclipse occurs when an object finds itself in the penumbra. Penumbra — The name given to the shadow cast by a celestial object that only blocks a portion of the light.

Perigee — The closest point to the Earth in the Moon's orbit. Perigean Tide — The tide that occurs when the moon is at its closest point to the earth during its orbit. These tides are higher than normal. Perihelion — The closest point to the Sun in a planetary body's orbit. Perilune — The closest point to the Moon in an object's orbit around it. Phases — When applied to the Moon, the phases refer to the different illumination that it undergoes during it's orbit around the Earth and the Sun. The most commonly known phases of the Moon include Full Moon and the Quarter Moon. Q Quadrature — Term that describes two celestial bodies appearing to be ninety degrees apart, from the perspective of the observer. An example is when the Moon appears to be at a right angle to the Sun, as viewed from Earth. R Radius — Half the diameter of any sphere or circle. Regression of nodes — The westward movement of the nodal points of the Moon's orbit where it intercepts the orbit of the sun, also known as the elliptic. Revolution — A way to describe the movement of one celestial body as it orbits another. A complete revolution is a complete orbit.

Rille — Grooves in the Moon's surface which resemble canals or canyons. Rotation — The motion of a sphere which is spinning around its own axis. An example would be a basketball spinning on the tip of a finger. S Saros Cycle — This cycle predicts the occurrence of eclipses, as after a period of 18 years and 11.3 days the Earth, Moon and Sun all line up in the formation necessary to generate this celestial event. Satellite — Any object that orbits another celestial body. Selenography — The study of the topography and features of the Moon's surface. Sidereal Month — Period of 27.32166 days which represents the Moon's movement through space relative to a start point amongst the stars. Sinus — Latin term that describes topographical features of the moon that resemble bays. Solar Eclipse — Term that describes when the Sun is obscured by the Moon from the perspective of Earth. Spring Tide — When the conjunction or opposition of the Sun, the Earth, and the Moon occurs, approximately at the times of the Full Moon and the New Moon, the Sun acts to augment the tidal forces of the Moon, creating a higher than normal tide.

T Tektites — Objects made from natural glass that are created from the impact of meteorites. They can be found on the Earth and the Moon. Terminator — The line which delimits night (shadowed portion) and day (sunlit portion) on a celestial body. The Moon's phases illustrate this. Tides — The rising and falling levels of the ocean in comparison with the bodies of land on Earth. Tides are the result of the gravitational forces of the Sun and the Moon acting upon the planet. Transit — The movement of a celestial body across another, from the viewpoint of an observer. Tropical Month — The period of time, (27.321582 days ), that it takes for the Moon to return to a specific celestial longitude (its position in reference to the stars). U Umbra — The name given to the shadow cast by a celestial object that entirely blocks out illumination. W Waning Moon — The term used to describe the period of the Moon as it moves from a Full Moon to a New Moon, decreasing in visibility with respect to an Earth-bound observer.

Waxing Moon — The term used to describe the period of the Moon as it moves from a New Moon to a Full Moon, increasing in visibility with respect to an Earth-bound observer. X, Y, Z Young Crescent Moon — Name for the initially visible crescent of the Moon immediately after the New Moon. Zenith — The point with a ninety degree inclination in relation to the observer, or the point directly above them. Standard NASA - Eclipses and the Moon's Orbit Eclipses and the Moon's Orbit Fred Espenak, NASA's GSFC 4.5 Eclipse Cycles 4.1 Introduction The Moon revolves around Earth in an elliptical orbit with a mean eccentricity of 0.0549. Thus, the Moon's center-to-center distance from Earth varies with mean values of 363,396 km at perigee to 405,504 km at apogee. The lunar orbital period with respect to the stars (sidereal month) is 27.32166 days (27d 07h 43m 12s).

However, there are three other orbital periods or months that are crucial to the understanding and prediction of eclipses. These three cycles and the harmonics between them determine when, where, and how solar and lunar eclipses occur. The mutual gravitational force between the Sun and Moon is over twice as large as between the Moon and Earth. For this reason, the Sun plays a dominant role in perturbing the Moon's motion. The ever changing distances and relative positions between the Sun, Moon, and Earth, the inclination of the Moon's orbit, the oblateness of Earth, and (to a lesser extent) the gravitational attraction of the other planets all act to throw the Moon's orbital parameters into a constant state of change. Although the Moon's position and velocity can be described by the classic Keplerian orbital elements, such osculating elements are only valid for a single instant in time (Chapront-Touze' and Chapront, 1991). Nevertheless, these instantaneous parameters are of value in understanding the Moon's complex motions particularly with respect to the three major orbital cycles that govern eclipses. 4.2 Synodic Month The most familiar lunar cycle is the synodic month because it governs the well-known cycle of the Moon's phases.

The Moon has no light of its own but shines by reflected sunlight. As a consequence, the geometry of its orbital position relative to the Sun and Earth determines the Moon's apparent phase. The mean length of the synodic month is 29.53059 days (29d 12h 44m 03s). This is nearly 2.21 days longer than the sidereal month. As the Moon revolves around Earth, both objects also progress in orbit around the Sun. After completing one revolution with respect to the stars, the Moon must continue a little farther along its orbit to catch up to the same position it started from relative to the Sun and Earth. This explains why the mean synodic month is longer than the sidereal month. According to astronomical convention, New Moon is defined as the instant when the geocentric ecliptic longitudes of the Sun and Moon are equal. When the synodic month is measured from New Moon to New Moon, it is sometimes referred to as a lunation, and we will follow that usage here. Historically, the phases of the Moon have been used as the basis of lunar calendars by many cultures around the world.

The major problem with such calendars is that the year, based on the solar calendar, is not evenly divisible by a whole number of lunations. Consequently, most lunar calendars are actually lunisolar calendars (e.g., Chinese, Hebrew, and Hindu) that include intercalary months to keep the seasons in step with the year. The duration of the lunation actually varies from its mean value by up to seven hours. For instance, Table 4-1 contains details for all lunations in 2008. The first column lists the decimal date of every New Moon throughout the year (Terrestrial Dynamical Time), while the second column gives the duration of each lunation. The third column is the difference between the actual and mean lunation. The first lunation of the year (Jan 08) was 03h 23m longer than the mean. Continuing through 2008, the length of each lunation drops and reaches a minimum of 05h 48m shorter than the mean value (Jun 03).

The duration now increases with each succeeding lunation until the maximum value of the year is reached of 06h 49m longer than the mean (Dec 27). Table 4-1 New Moon and Lunation Length in 2008 Date of New Moon (Dynamical Time) Length of Lunation +06h 49m 186.6° What is the cause of this odd behavior? The last column in Table 4-1 gives a clue; it contains the Moon's true anomaly at the instant of New Moon. The true anomaly is the angle between the Moon's position and the point of perigee along its orbit. In other words, it is the orbital longitude of the Moon with respect to perigee. Table 4-1 shows that when New Moon occurs near perigee (true anomaly = 0°), the length of the lunation is at a minimum (e.g., Jun 03). Similarly, when New Moon occurs near apogee (true anomaly = 180°), the length of the lunation reaches a maximum (e.g., Dec 27). Click to enlarge. This relationship is quite apparent when viewed graphically.

Figure 4-1 plots the difference from mean lunation (histogram) and the Moon's true anomaly (diagonal curves) for every New Moon from 2008 through 2010. The left-hand scale is for the difference from mean lunation, while the right-hand scale is for the true anomaly. The shortest lunations are clearly correlated with New Moon at perigee, while the longest lunations occur at apogee. From the figure, the length of this cycle appears to be about 412 days. The reason why must wait until the next section. The Moon's orbital period with respect to perigee is the anomalistic month and has a duration of approximately 27.55 days. The lock-step rhythm between the lunation length and true anomaly can be explained with the help of the anomalistic month and Figure 4-2. It illustrates the Moon's orbit around Earth and Earth's orbit around the Sun. The relative sizes and distances of the Sun, Moon, and Earth as well as the eccentricity of the Moon's orbit are all exaggerated for clarity. The major axis of the Moon's orbit marks the positions of perigee and apogee. Click to enlarge.

Two distinct cases-each consisting of two revolutions of the Moon around Earth-are depicted in Figure 4-2. The first case covers the New Moon geometry around perigee. The orbit marked A shows New Moon taking place near perigee at position a1. One anomalistic month later (orbit B), the Moon has returned to the same position relative to perigee (marked b1). However, Earth has traveled about 30° around its orbit so the Sun's direction relative to the Moon's major axis has shifted. The Moon must travel an additional distance of Δb in its orbit before reaching the New Moon phase at b2. This graphically demonstrates why the synodic month is longer (~1.98 days) than the anomalistic month. The second case takes place about half a year later. New Moon then occurs near apogee (orbit C, position c1). After one anomalistic month, the Moon has returned to the same location with respect to apogee (orbit D, position d1). Once again, Earth has traveled about 30° around its orbit so the Moon must revolve an additional distance of Δd before reaching the New Moon phase at position d2.

An inspection of orbits B and D reveals that the orbital arc Δd is longer that Δb. This means that the Moon must cover a greater orbital distance to reach New Moon near apogee as compared to perigee. Furthermore, the Moon's orbital velocity is slower at apogee so it takes longer to travel a given distance. Thus, the length of the lunation is shorter than average when New Moon occurs near perigee and longer than average when New Moon occurs near apogee. Earth's elliptical orbit around the Sun also factors into the length of the lunation. With an eccentricity of 0.0167, Earth's orbit is about one third as elliptical as the Moon's orbit. Nevertheless, it affects the length of the lunation by producing shorter lunations near aphelion and longer lunations near perihelion. During the 5000-year period covered in this catalog, there are 61841 complete lunations.

The shortest lunation began on -1602 Jun 03 and lasted 29.26574 days (29d 06h 22m 40s; 6h 21m 23s shorter than the mean). The longest lunation began on -1868 Nov 27 and lasted 29.84089 days (29d 20h 10m 53s; 7h 26m 50s longer than the mean). Thus, the duration of the lunation varies over a range of 13h 48m 13s during this time interval. The histogram presented in Figure 4-3 shows the distribution in the length of the lunation over 5000 years. To create the histogram, the durations of individual lunations were binned into 30-minute groups. It might seem reasonable to expect a simple bell-shaped Gaussian curve. However, the results are surprising because the distribution in lunation length has two distinct peaks.

This bifurcation can be understood if the lunation length, which depends primarily on the Moon's distance, is considered as a series of sine functions. The extremes of a sine function always occur more frequently than the mean, which is just what is seen in Figure 4-3. For a more detailed discussion, see Meeus (1997). Click to enlarge. 4.3 Anomalistic Month The anomalistic month is defined as the revolution of the Moon around its elliptical orbit as measured from perigee to perigee. The length of this period can vary by several days from its mean value of 27.55455 days (27d 13h 18m 33s). Figure 4-4 plots the difference of the anomalistic month from the mean value for the 3-year interval 2008 through 2010. Also plotted is the difference between the mean longitudes of the Sun and perigee. This is just the angle between the Sun and the Moon's major axis in the direction of perigee.

The left-hand scale is the length of the anomalistic month minus the mean value, while the right-hand scale is for the difference in longitude (Sun-perigee). For comparison, the lunation length minus its mean value is also plotted (light gray). Click to enlarge. The variation in the length of the anomalistic month is much larger than that of the lunation. Figure 4-4 shows the anomalistic month is typically within 1 day of its mean value. But once or twice every 7 to 8 months, the anomalistic month is significantly shorter than the mean by 2 to nearly 3 days. The difference in longitude of the Sun and perigee show that the shortest anomalistic months are correlated with values of 90° and 270°, when the line of apsides is perpendicular to the Sun's direction. In comparison, the longest anomalistic months take place when the difference in longitude passes through 0° or 180°. The line of apsides is then directed towards or away from the Sun. The maximum duration of the anomalistic month is then about 28.5 days (1.0 day longer than the mean).

The Earth-Sun distance also influences the anomalistic month by causing greater extremes near perihelion. This currently occurs in early January each year. In an earlier discussion on the synodic month, it was assumed that the lunar orbit's line of apsides has a fixed and permanent direction in space. In fact, the length of the mean anomalistic month (27.55 days) exceeds the mean sidereal month (27.32 days) by 0.23 days. Thus, the Moon's major axis slowly shifts with a mean rate of 0.11140° per day in the direct sense, that is, in the same direction as the Moon's orbital motion. This corresponds to an average of 40.7° per year, so it takes 8.85 years (3231.6 days) for the line of apsides to make one complete revolution with respect to the stars. What impact do the varying length of the anomalistic month and the direct (eastward) rotation of the Moon's elliptical orbit have on the length of the lunation?

To answer this, one must first consider Earth's elliptical orbit around the Sun, which has a mean eccentricity of 0.0167. The center-to-center distance between Earth and the Sun varies with mean values of 147,098,074 km at perihelion to 152,097,701 km at aphelion. The direction of Earth's orbital line of apsides also changes but at a rate far slower than the Moon's. Having a direct (eastward) shift with a mean value of 0.0172° per year, it takes about 20,500 years for Earth's major axis to make one complete revolution. This is only 0.0004 of the lunar rate, so it can be treated as fixed for the purpose of the following discussion. At certain times, the perigee of the lunar orbit and the perihelion of Earth's orbit can have the same ecliptic longitude. Ignoring the 5.1° tilt of the Moon's obit, the major axes are then essentially parallel to each other and point in the same direction.

As time passes, the major axis of the lunar orbit slowly rotates east with respect to Earth's major axis until it becomes perpendicular to it 2.21 years later. In another 2.21 years (4.42 years from the start), the major axes of the orbits are again parallel to each other, but the perigee and the perihelion are 180° apart as they point in opposite directions. After an additional period of 2.21 years, the axes are once more perpendicular. Finally, the Moon's perigee and Earth's perihelion again share the same ecliptic longitude after a total interval of 8.85 years. The length of each lunation minus the mean lunation is plotted in Figure 4-5 for the 20-year period from 2008 through 2027. The periodic rhythm between the lunation length and the true anomaly, as described earlier (via Figure 4-1), can now be seen over the course of two decades. The 412-day mean period of this cycle corresponds to the time between two consecutive alignments of the major axis in the direction of the Sun.

It is slightly longer than a year because of the slow eastward shift of the Moon's major axis. Click to enlarge. An interesting feature revealed in Figure 4-5 is how the extremes in the lunation length slowly vary over a period of nearly 9 years. The envelope defined by the minima and maxima appears to oscillate over a range of values from �2 h to �6 h. This behavior is evidence revealing the influence of the 8.85-year cycle in the alignment of the major axes of the orbits of the Moon and Earth. The amplitude of the envelope is due to the eccentricity of Earth's orbit. When Earth is at perihelion, its orbital velocity is at its maximum value so Earth travels a larger distance around its orbit in a given time as compared to aphelion. Thus, the Moon must travel a greater distance to align with the Sun, which results in a longer lunation. Near aphelion, the opposite conditions produce a shorter lunation. Using the axis scale on the right, the diagonal lines in Figure 4-5 plot the angle between the Moon's perigee and Earth's perihelion.

This is the difference between the Moon's mean longitude of perigee and Earth's true longitude of perihelion. When the angle between the perigee and perihelion is 0°, the length of the lunation varies from a minimum of 29.273 days (-6.17 hours from mean) to a maximum of 29.820 days (+6.93 hours from mean). Similarly, when the angle between the perigee and perihelion is 180°, the length of the lunation varies from a minimum of 29.452 days (-1.88 hours from mean) to a maximum of 29.628 days (+2.33 hours from mean). To summarize, the greatest extremes in the length of the lunation occur when the longitudes of the Moon's perigee and Earth's perihelion are equal. The smallest extremes in the lunation length occur when their longitudes differ by 180°. Although the Moon's major axis rotates eastward at a mean rate of 0.1114° per day, the true rate varies considerably.

Figure 4-5 illustrates the variation by plotting the difference between the true longitudes of the Moon's perigee and Earth's perihelion. This quasi-sinusoidal oscillation about the difference in the mean longitudes shows peak departures of �30° from average. Indeed, the Moon's major axis can swing both east and west of its mean value, taking on an actual retrograde shift west during some anomalistic months. This dynamic behavior is due to the gravitational pull of the Sun on the Moon as it orbits Earth. Consequently, a continuous torque is applied to the lunar orbit in an unsuccessful effort to permanently align the major axis towards the Sun. The annual orbit of the Earth-Moon system around the Sun coupled with the Moon's synodic orbit around Earth mean that the conditions for such a permanent alignment are always changing. The overall effect is to twist and distort the shape and orientation of the Moon's elliptical orbit. It was stated earlier that the Moon's mean orbital eccentricity is 0.0549, but this too is subject to large changes because of solar perturbations. Figure 4-6 plots the variation in the Moon's orbital eccentricity from 2008 through 2010.

The instantaneous eccentricity (light gray curve) oscillates with a period tied to the synodic month and ranges from 0.0266 to 0.0762 over this 3-year interval. Superimposed on the instantaneous eccentricity is the eccentricity at the instant of perigee, which occurs at the beginning of each anomalistic month (heavy black curve). The straight diagonal lines represent the difference between the mean longitudes of the Sun and perigee. In other words, it is the angle between the Moon's perigee-directed major axis and the Sun. Oscillating about this line is the difference between the true longitudes of Sun and perigee. The scale for these angles appears along the right side of Figure 4-6. The extreme range of the Moon's orbital eccentricity at perigee during the 5000 years covered by the catalog is 0.0255 to 0.0775. Click to enlarge. Figure 4-6 shows that the eccentricity reaches a maximum when the major axis of the lunar orbit is pointed directly towards or directly away from the Sun (angles of 0° and 180°, respectively).

This occurs at a mean interval of 205.9 days, which is somewhat longer than half a year because of the eastward shift of the major axis. The eccentricity reaches a minimum when the major axis of the lunar orbit is perpendicular to the Sun (angles of 90° and 270°). Such changes in orbital eccentricity produce significant variations in the Moon's distance at perigee and apogee. Figure 4-7 plots the Moon's distance for all perigees and apogees from 2008 through 2010. Also shown is the orbital eccentricity at perigee as well as the angle between the perigee directed major axis and the Sun. The closest perigee (minimum perigee distance) and farthest apogee (maximum apogee distance) occur when the eccentricity is at maximum. This corresponds to times when the Moon's major axis points directly towards or directly away from the Sun (angles of 0° and 180°, respectively). The farthest perigee (maximum perigee distance) and closest apogee (minimum apogee distance) occur when the eccentricity is at minimum.

At such times, the major axis is oriented perpendicular to the Sun. During the 3-year interval covered in Figure 4-7, the Moon's perigee distance ranges from 356,568 to 370,216 km while the apogee distance ranges from 404,168 to 406,602 km. Click to enlarge. Over the 5000-year period of the catalog, there are 66,276 perigees and apogees. During this epoch, the distance of the Moon's perigee varies from 356,355 to 370,399 km while the apogee varies from 404,042 to 406,725 km. The minimum and maximum extremes in orbital eccentricity are 0.0255 to 0.0775 and the extremes in the length of the anomalistic month are 24.629 days (2.925 days shorter than the mean) to 28.565 days (1.011 days longer than the mean).

A histogram showing the distribution in the length of the anomalistic month is presented in Figure 4-8 where the durations of individual anomalistic months have been binned into 2-hour groups. The sharply asymmetric distribution shows that anomalistic months longer than the mean cluster over a much shorter range of values compared to anomalistic months shorter than the mean. Click to enlarge. 4.4 Draconic Month The plane of the Moon's orbit is inclined at a mean angle of 5.145° to the plane of Earth's orbit around the Sun. The intersection of these planes defines two points or nodes on the celestial sphere. The node where the Moon's path crosses the ecliptic from south to north is the ascending node, while the node where the Moon's path crosses the ecliptic from north to south is the descending node. The draconic month is defined as one revolution of the Moon about its orbit with respect to the ascending node. The mean length of this nodical period is 27.21222 days (27d 05h 05m 36s). However, the actual duration can vary by over 6 h from the mean.

Figure 4-9 plots the duration of the draconic month minus its mean value for 2008 through 2010. The shortest month over this 3-year period is 27.05115 days (27d 01h 14m), while the longest month is 27.38409 days (27d 09h 13m). Click to enlarge. The most significant characteristic of this variation is that it is synchronized with the ascending node relative to the Sun's position along the ecliptic. The mean angle between the Sun and the ascending node (i.e., difference in mean longitude) is also plotted in Figure 4-9 (diagonal lines) to illustrate this relationship. The longitude difference at the start of each draconic month is plotted as a black dot. Longitude values can be read using the scale along the right side of the figure. The longest draconic months occur when the difference in the mean longitudes of the Sun and the ascending node is either 0° or 180°.

In contrast, the shortest months occur when the angle between the Sun and the ascending node is either 90° or 270°. The mean draconic month is 0.10944 day (2h 36m 36s) shorter than the sidereal month. Consequently, the lunar nodes slowly rotate west or retrograde (opposite the Moon's orbital motion) along the ecliptic at a rate of 0.05295° per day. One complete rotation of the ascending node about the ecliptic requires 18.6 years (6793.48 days) with respect to the fixed stars. Figure 4-10 plots the instantaneous inclination of the lunar orbit over the 3-year period 2008-2010. The mean angle between the Sun and the ascending node (i.e., difference in mean longitude) is also plotted. The largest inclination of 5.30° occurs when the difference in longitude is either 0° or 180°. In other words, the inclination is always near its maximum value for both solar and lunar eclipses.

The smallest inclination of 5.00° occurs when the difference in longitude is either 90° or 270°. Note the small monthly oscillations in the inclination when near its minimum. The figure also plots the longitude of the instantaneous ascending node. Its westward motion draws to a near standstill whenever the Sun aligns with either of the nodes. This corresponds to a difference in longitude of either 0° or 180°. The mean interval in the periodic variation of both the draconic month and the orbital inclination is 173.3 days. This is the average time it takes for the Sun to travel from one node to the other. It is also equivalent to the interval between the midpoints of two eclipse seasons. The period is slightly less than half a year because of the retrograde motion of the nodes. Click to enlarge. The length of the draconic month is strongly modulated by the position of the nodes with respect to the major axis of the Moon's orbit. The histogram in Figure 4-11 shows how the draconic month changes from 2008 through 2017.

The 173-day alignment of the Sun with a node appears as the rapid oscillation in the month length. The quasi-sinusoidal envelopes surrounding the minima and maxima form two longer period oscillations. Over the 10-year period covered in this figure, the minimum month duration varies from 27.089 to 27.011 days (3.0 to 4.8 hours shorter than the mean). The maximum month duration ranges from 27.261 to 27.472 days (1.2 to 6.2 hours longer than the mean). Click to enlarge. The difference in the mean longitudes of perigee and the ascending node appear as diagonal lines in Figure 4-11. This is the angle between these orbital parameters measured along the ecliptic. The greatest extremes in the draconic month occur when the angle between perigee and the ascending node is 0°. Likewise, the smallest extremes of the month take place when the difference in longitude is 180°.

The mean rates of the major axis and the ascending node are 0.11140° east and 0.05295° west per day, respectively. Therefore, the mean period between alignments of the axis and node is 2190.4 days or 6.0 years. This period is clearly seen in Figure 4-11. There are 67,111 draconic months during the 5000 years covered in this catalog. The shortest and longest months are 27.004 days (0.208 days or 5.0 hours shorter than the mean) and 27.487 days (0.275 days or 6.6 hours longer than the mean), respectively. A histogram of the distribution in the length of the draconic month over the five millennia appears in Figure 4-12 where the duration of individual draconic months have been binned into 30-min groups. The width and bifurcated symmetry of the distribution resemble the distribution for the lunation (synodic month) in Figure 4-4. Click to enlarge.

4.5 Eclipse Cycles The interaction and harmonics of the synodic, anomalistic, and draconic months not only determine how frequently eclipses occur, but they also control the geometric characteristics and classification of each eclipse. The commensurability of these periods over long time scales results in several important eclipse cycles, which are discussed in Periodicity of Solar Eclipses and Periodicity of Lunar Eclipses . Acknowledgments Permission is freely granted to reproduce this data when accompanied by an acknowledgment: "Eclipse Predictions by Fred Espenak (NASA's GSFC) and Jean Meeus" References Espenak, F., and Meeus, J., Five Millennium Canon of Solar Eclipses: -1999 to +3000 (2000 BCE to 3000 CE), NASA Tech. Pub. 2006-214141, NASA Goddard Space Flight Center, Greenbelt, Maryland (2006).

Espenak, F., and Meeus, J., Five Millennium Catalog of Solar Eclipses: -1999 to +3000 (2000 BCE to 3000 CE), NASA Tech. Pub. 2009-214174, NASA Goddard Space Flight Center, Greenbelt, Maryland (2009). Espenak, F., and Meeus, J., Five Millennium Canon of Lunar Eclipses: -1999 to +3000 (2000 BCE to 3000 CE), NASA Tech. Pub. 2009-214172, NASA Goddard Space Flight Center, Greenbelt, Maryland (2009). Espenak, F., and Meeus, J., Five Millennium Catalog of Lunar Eclipses: -1999 to +3000 (2000 BCE to 3000 CE), NASA Tech. Pub. 2008-214173, NASA Goddard Space Flight Center, Greenbelt, Maryland (2009).

Gingerich, O., (Translator), Canon of Eclipses, Dover Publications, New York (1962) (from the original T.R. von Oppolzer, book, Canon der Finsternisse, Wien, [1887]). Meeus, J., Mathematical Astronomy Morsels III, Willmann-Bell, pp. 109-111, (2004). Meeus, J., Grosjean, C.C., and Vanderleen, W., Canon of Solar Eclipses, Pergamon Press, Oxford, United Kingdom (1966). van den Bergh, G., Periodicity and Variation of Solar (and Lunar) Eclipses, Tjeenk Willink, and Haarlem, Netherlands (1955). von Oppolzer, T.R., Canon der Finsternisse, Wien, (1887). Return to: Moon makes year's closest pass to Earth in full phase | TribLIVE Moon makes year's closest pass to Earth in full phase The Christian Science Monitor | Sunday, June 23, 2013, 8:39 p.m.

Getty Images LAKE BUENA VISTA, FL - JUNE 23: In this handout photo provided by Disney Parks, the 'supermoon' is seen with the Epcot center geodesic sphere in the foreground on June 23, 2013 at Walt Disney World Resort in Lake Buena Vista, Florida. This 'supermoon' is the closest and largest full moon for all of 2013. A 'supermoon' will not occur again until August 10, 2014. (Photo by David Roark/Disney Parks via Getty Images) Email Newsletters Sign up for one of our email newsletters. “Supermoon” officially arrived about 7 a.m. Sunday morning, with the moon making its closest swing by Earth this year. About a half hour later, the moon reached full status, making it appear 12 percent larger and 30 percent brighter than a regular full moon. Don't worry if you missed it. The effect should linger for a few nights, meaning Monday and even Tuesday should give a good sense of this year's supermoon. The show is the product of a cosmic quirk.

Because planets and moons orbit in ellipses, not circles, there are times when they are closer to what they're orbiting and times when they're farther away. For moons, the point when they're closest to their planet is called perigee; the point farthest away is called apogee. For Earth and the moon, perigee and apogee happen once each month, because the moon orbits the Earth once every 27 days. But because of small inconsistencies in the orbits, the moon's closest approach and its farthest distance are always slightly different. The moon hit its monthly perigee Sunday morning, but what makes it worthy of the name “supermoon” is that it was the closest perigee of 2013. Moreover, because it coincided almost perfectly with a full moon, the effect was enhanced. The moon's farthest apogee this year has already happened and is set to repeat itself next year during the Jan. 15, 2014, full moon. To see the visual difference in size between a perigee and apogee moon, click here. But what makes the moon so special?

If everything in the solar system orbits in an ellipsis, shouldn't we have a “supersun,” too. In fact, we do. Since we orbit the sun once a year, supersuns only happen once a year. Our closest swing to the sun, called perihelion, also already happened this year, and will happen again on Jan. 4, 2014. (An “un-supersun,” when the sun seems smallest at aphelion, is just around the corner: July 5.) But will there be this much buzz next January with sungazers filling Twitter with pictures of a gigantic sun? Don't count on it. The reason? the sun is obviously much farther away from Earth than is the moon, so the effect is not so noticeable. For Earth, the distance between aphelion and perihelion is about 3 million miles. But even at perihelion, the Earth is still 91 million miles from the sun. In addition, Earth's orbit around the sun is more nearly circular than is the moon's orbit around us, with only Venus and Neptune having more circular orbits than Earth.

Mercury has the most eccentric orbit – ranging from a perihelion of 29 million miles to an aphelion of 43 million miles – meaning it has a truly dramatic supersun (if you can stand the 800 degree Fahrenheit temperatures on the sun-side surface). By contrast, the moon's orbit around Earth is the most elliptical orbit of any major moon in the solar system. Combine that with the fact that the moon is comparatively close to Earth, so it looks large in our skies, and supermoon earns its name.

Question: What name is given to the position of the Moon in it's orbit, when it is closest to the Earth?
Answer: PERIGEE