Moon Glossary: Lunar Terms and Definitions Moon Posters Moon Glossary: Lunar Terms and Definitions A | B | C | D | E | F | G | H | I | L | M | N | O | P | Q | R | S | T | U | W | X, Y, Z 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 Saber's Beads  Detached points of light seen along the limb of very young and old lunar crescents. The striking resemblance to 2nd and 3rd contacts during a total solar eclipse was first noted by American astronomer Stephen Saber.

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. Earth Is Farthest from the Sun for 2014 Today Earth Is Farthest from the Sun for 2014 Today By Joe Rao, Space.com Skywatching Columnist | July 3, 2014 07:26am ET MORE A view of the Earth's arctic as seen by NASA's NPP Suomi observation satellite. On July 3, 2014, the Earth will be at aphelion, its farthest point from the sun for 2014. Credit: NASA/NPP Suomi With the Fourth of July holiday weekend nearly upon us, the national weather forecast for today, July 3, calls for high temperatures to hit at least 90 degrees Fahrenheit (32 degrees Celsius) from the Middle Atlantic states and points south through the Southeastern states and across the Deep South.

So it may surprise you to find out the Earth is actually farther from the sun today than it is on any other day in the year. Scorching temperatures at or above 100 degrees F (38 degrees C) are expected across parts of California and southern Nevada as well as southern Arizona. In some desert regions, temperatures could soar to above 110 degrees (43 degrees C). Temperatures of 90 degrees F are also expected over the Southwestern states and points north through inland sections of Washington state and Oregon, as well as Idaho. Despite those hot temperatures, the Earth will reach aphelion, the point in its orbit when the planet is farthest from the sun, at 8 p.m. EDT (5 p.m. PDT/0000 GMT) tonight. At that moment, the distance between the Earth and sun will be 94,506,462 miles (152,093,407 kilometers) as measured from center to center. [ Earth Quiz: Do You Know Your Home Planet? ] How far, the sun?

During this annual milestone, the Earth is 3,104,641 miles (4,996,435 km) farther from the sun than it was on Jan. 3, when Earth was at perihelion, its closest point to the sun for 2014. The difference in distance between aphelion today and perihelion at the beginning of the year is 3,099,789 miles, or 3.28 percent. In terms of radiant heat received by the Earth, that makes a difference of nearly 7 percent. If you ask people in which month of the year they believe Earth is closest to the sun, most probably would guess June, July or August. But the warm weather doesn't relate to the Earth's distance from the sun . It's because of the 23.5-degree tilt of the Earth's axis that the sun is above the horizon for different lengths of time at different seasons. The tilt determines whether the sun's rays strike the planet at a low angle or more directly.

At New York's latitude, the more nearly direct rays at the summer solstice of June 21 bring about three times as much heat as the more slanting rays at the winter solstice on Dec. 21. Heat received by any region is dependent on the length of daylight and the angle of the sun above the horizon. Hence the noticeable differences in temperatures that are registered over different parts of the world. You live here, so we figure you ought to be well grounded in Earth facts. But you might find these questions a little tough and tricky. Good luck! The Sun - Universe Today Universe Today by Matt Williams The Sun is the center of the Solar System and the source of all life and energy here on Earth. It accounts for more than 99.86% of the mass of the Solar System and it’s gravity dominates all the planets and objects that orbit it. Since the beginning of history, human beings have understood the Sun’s importance to our world, it’s seasons, the diurnal cycle, and the life-cycle of plants. Because of this, the Sun has been at the center of many ancient culture’s mythologies and systems of worship.

From the Aztecs, Mayans and Incas to the ancient Sumerians, Egyptians, Greeks, Romans and Druids, the Sun was a central deity because it was seen as the bringer of all light and life. In time, our understanding of the Sun has changed and become increasingly empirical. But that has done nothing to diminish it’s significance. Name: The name “The Sun” is a proper English noun which evolved from the Old English sunne, which may be related to the word south. Other Germanic forms of the name – ranging from sunne and sonne in Old Frisian to sunna in Old High German and Old Norse to sunno in Gothic. All Germanic terms for the Sun stem from the Proto-Germanic “sunnon“, which is in turn derived from the sauel or sauol of Proto-Indo-European. The English name for Sunday stems from the Old English Sunnandæg (literally “Sun’s day”) which was in use before 700 CE. This name resulted from the Germanic interpretation of Latin dies solis, which is itself a translation of the Greek heméra helíou.

The Latin name for the Sun, Sol, is widely known but is not in common use. However, the adjectival form solar is used widely to refer to phenomena or attributes pertaining to the Sun. Characteristics: The Sun is a G-type main-sequence star that comprises about 99.86% of the mass of the Solar System. The Sun has an absolute magnitude of +4.83, which is estimated to be brighter than about 85% of the stars in the Milky Way – most of which are red dwarfs . With a diameter of 696,342 ± 65 km and a mass of approximately 1.988 × 1030 kg (1.9 trillion quadrillion metric tons), the Sun is 109 times as large as Earth and 333,000 times as massive. Being a star, the Sun’s density varies considerably between its outer layers and its core. On average, it has a density of 1.408 g/cm3, which is roughly one-quarter that of Earth.

However, models of the Sun estimate that it has a density of 162.2 g/cm3 closer to the core, which is 12.4 times that of Earth. Although our Sun appears to be yellow, it is actually white. It merely appears to be yellow because of the effect of the atmosphere. Our Sun is brighter than most of the other stars in the galaxy (which are also red dwarfs) and only about 5% of stars in the Milky Way are larger than the Sun. The Sun is a member of the Population I group of stars, which describes luminous, hot, and young stars that are typically found in the spiral arms of galaxies. Estimates of the Sun’s temperature also vary the closer one gets to its core.

Within the center, temperature are estimates to be as high as 15.7 million K (15,699,726.85 million °C/28,259,540.33 million °F), while the Corona experiences temperatures of about 5 million K (4,999,726.85 °C/ 8,999,540.33 °F), and the visible surface (photosphere) reaches an effective temperature of 5778 K (5504.85 °C/9940.73 °F). Because the Sun is made of plasma, it’s also highly magnetic. It has north and south magnetic poles like the Earth, and the magnetic field lines create the activity we see on the surface. The darker sunspots – cooler areas which last for a few months and vary greatly in size – are created when magnetic field lines pierce through the Sun’s photosphere. Sunspots appear in cycles and sometimes there are none visible at all. Coronal mass ejections and solar flares occur when these magnetic field lines snap and reconfigure.

The amount of activity on the Sun rises and falls over an 11-year cycle. At the low point, called solar minimum , there are few, if any sunspots. And then at the high point of the cycle, solar maximum , there are the most sunspots and the greatest amount of solar activity. The Sun is by far the brightest object in the sky, with an apparent magnitude of -26.74, which is about 13 billion times brighter than the next brightest star (Sirius, which has an apparent magnitude of -1.46). The mean distance of the Sun to Earth is approximately 1 astronomical unit or AU (150,000,000 km/93,000,000 mi), though this changes due to variations in Earth’s orbit. At this average distance, light travels from the Sun to Earth in about 8 minutes and 19 seconds. The energy of this sunlight supports almost all life on Earth by photosynthesis, and drives Earth’s climate and weather.

Composition and Structure: The Sun is composed primarily of the chemical elements hydrogen and helium, which account for 74.9% and 23.8% of the mass of the Sun in the photosphere, respectively. All heavier elements account for less than 2% of the Sun’s mass, with oxygen (roughly 1% of the Sun’s mass), carbon (0.3%), neon (0.2%), and iron (0.2%) being the most abundant. The interior of the Sun is differentiated between multiple layers, which includes a core, a radiative zone, a convective zone, a photosphere , and an atmosphere. The core is the most dense and hottest region of the Sun (150 g/cm³/15.7 million K) and accounts for about 20–25% of the Sun’s overall radius. The Sun takes about 1 month to rotate once on its axis; however, this is a rough estimate because the Sun is a ball of plasma. Recent analysis has indicated that the core has a rotation rate that is faster than the outer layers of the Sun.

At the outer layers, near the equator, it rotates about once every 25.4 days; whereas nearer to the poles, it takes up to 36 days to complete a single rotation. It is also in the core where the majority of the Sun’s energy is produced through nuclear fusion, which converts hydrogen into helium. Nearly 99% of the thermal energy created by the Sun occurs within this region – which accounts for 24% of the Sun’s interior. By 30% of the radius, fusion processes have nearly ceased. The rest of the Sun is heated by this energy, which is transferred outwards to the solar photosphere before escaping into space as sunlight or high-energy particles. The interior structure of the Sun. Credit: Wikipedia Commons/kelvinsong At the radiative zone, which extends from 0.25 out to about 0.7 solar radii, thermal radiation is the primary means of energy transfer. In this layer, the temperature drops with increasing distance from the core, from approximately 7 million K in the interior to 2 million K at the outer edge. The density also drops a hundredfold – from 20 g/cm³ to only 0.2 g/cm³.

Between the radiative zone and the convective zone, there is a transition layer known as the tachocline. This region is defined by a sharp change in the uniform rotation of the radiative zone and the differential rotation of the convection zone, which results in a large shear. It is currently theorized that a magnetic dynamo in this layer is what is responsible for generating the Sun’s magnetic field. In the convective zone, which extends from the surface to approximately 200,000 km below the surface (0.7 solar radii), the temperature and density of plasma is lower. This allows thermal convection to develop as material heated below expands and rises, which then cools and contracts once it reaches the photosphere, causing it to sink again and for the convective cycle to continue. The visible surface of the Sun, otherwise known as the photosphere, is the layer below which the Sun becomes opaque to visible light. Above the photosphere, visible sunlight is free to propagate into space, and its energy escapes the Sun entirely. The photosphere is tens to hundreds of kilometers thick, being slightly less opaque than the air on Earth.

Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the center than on the edge or limb of the solar disk. At the photosphere, temperature and density reaches its lowest point – approximately 5,700 K and a density of 0.2 g/m3 (about 1/6,000th the density of air at sea level). Last, there is the Sun’s atmosphere, which is made up of three distinct layers – the chromosphere , the transitional region , and the corona . The chromosphere (literally “sphere of color”) is roughly 2,000 kilometers deep and has a very low density (10-4 times that of the photosphere and 10-8 times that of Earth’s atmosphere). This, combined with the brightness of the photosphere, makes the chromosphere normally invisible. However, during a total eclipse, its reddish color can be seen. Above the chromosphere is the thin transition region (200 km thick), where temperatures rise rapidly from 20,000 K in the upper layer to close to 1,000,000 K at the corona.

This is facilitated by the full ionization of helium in the transition region, which significantly reduces radiative cooling of the plasma. This layer is not well-defined, instead forming a kind of nimbus around features in the chromosphere, and is in constant, chaotic motion. The transition region is not easily visible from Earth’s surface, but is visible in the ultraviolet spectrum. Lastly, there is the corona. In the lower region, the particle density is extremely low and the average temperature is about 1 – 2 million K – with the hottest regions ranging between 8 and 20 million K. This is believed to be due to the Sun’s magnetic field causing particle acceleration, which in turn creates kinetic (and thermal) energy. Artist’s impression of the Sun’s heliosphere, showing the extent of the Voyager 1 and 2 spacecraft’s exploration. Credit: NASA/Feimer The corona is the extended atmosphere of the Sun, and the flow of plasma outward from the Sun into interplanetary space (aka. “ solar wind “) forms the solar magnetic field into a spiral shape.

This is known as the heliosphere , a magnetic sphere that extends beyond the heliopause (more than 50 AU from the Sun) and protects the Solar System from charged particles coming from the interstellar medium (aka. “interstellar wind”). Evolution and Future: The current scientific consensus is that the Sun formed around 4.57 billion years ago from the collapse of part of a giant molecular cloud that consisted mostly of hydrogen and helium, and probably gave birth to many other stars. As one fragment of the cloud collapsed, it also began to rotate (because of conservation of angular momentum) and heated up with the increasing pressure. Much of the mass became concentrated in the center, whereas the rest flattened out into a disk that would eventually accrete to form the planets and other Solar System bodies. Gravity and pressure within the core of the cloud generated a lot of heat as it accreted more matter from the surrounding disk, eventually triggering nuclear fusion. From this grand explosion, the Sun was formed. The Sun is presently in it’s main sequence phase, which is characterized by the ongoing production of thermal energy through nuclear fusion. Currently, more than four million tonnes of matter is converted into energy within the core, producing neutrinos and solar radiation.

At this rate, the Sun has converted 200 times the mass of our Earth into energy (about 0.03% of its total mass). The Sun is becoming hotter because the helium atoms in its core are gradually occupying less volume than all the hydrogen that’s been fused. The core is therefore shrinking, allowing the outer layers of the Sun to move closer to the center and experience a stronger gravitational force. This stronger force increases the pressure on the core, which in turn is making the core denser. It is estimated that the Sun has become 30% brighter in the last 4.5 billion years, and is increasing in brightness at a rate of about 1% every 100 million years. At the end of its main sequence phase, the Sun will not go supernova since it does not have sufficient mass. Instead, once the hydrogen in the core is exhausted in 5.4 billion years, the Sun will begin to expand and become a red giant. It is hypothesized that it will grow large enough to encompass the orbit’s of Mercury, Venus, and maybe even Earth .

Once it reaches the Red-Giant-Branch (RGB) phase, the Sun will haves approximately 120 million years of active life left. But much will happen in this amount of time. First, the core (full of degenerate helium), will ignite violently in a helium flash – where approximately 6% of the core and 40% of the Sun’s mass will be converted into carbon within a matter of minutes. The Sun will then shrink to around 10 times its current size and 50 times its luminosity, with a temperature a little lower than today. For the next 100 million years, it will continue to burn helium in its core until it is exhausted. By this point, it will be in its Asymptotic-Giant-Branch (AGB) phase, where it will expand again (much faster this time) and become more luminous. Over the course of the next 20 million years, the Sun will then become unstable and begin losing mass through a series of thermal pulses.

These will occur every 100,000 years or so, becoming larger each time and increasing the Sun’s luminosity to 5,000 times its current brightness and its radius to over 1 AU. At this point, the Sun’s expansion will either encompass the Earth, or leave it entirely inhospitable to life. Planets in the Outer Solar System are likely to change dramatically, as more energy is absorbed from the Sun, causing their water ices to sublimate – perhaps forming dense atmosphere and surface oceans. After 500,000 years or so, only half of the Sun’s current mass will remain and its outer envelope will begin to form a planetary nebula. The post AGB evolution is even faster, as the ejected mass becomes ionized to form a planetary nebula and the exposed core reaches 30,000 K. The final, naked core temperature will be over 100,000 K, after which the remnant will cool towards a white dwarf . The planetary nebula will disperse in about 10,000 years, but the white dwarf will survive for trillions of years before fading to black.

Artist’s impression of a red giant star. Credit: ESO Position in the Milky Way: The Sun lies close to the inner rim of the Milky Way’s Orion Arm , in the Local Interstellar Cloud (or Gould Belt). This places it at a distance of 7.5 – 8.5 thousand Parsecs (25,000 – 28,000 light years) from the Galactic Center . The Sun is contained within the Local Bubble , a cavity in the interstellar medium that contains rarefied hot gas. The Sun, and thus the Solar System, is found in what scientists call the galactic habitable zone , a zone which contains several characteristics supportive to life. These include the right mix of elements, an orbit that keeps it away from the dangerous spiral arms, and being at a sufficient distance from the galactic center so that it is not disrupted by its gravitational forces or too much radiation. The general direction of the Sun’s galactic motion is towards the star Vega in the constellation of Lyra , at an angle of roughly 60 sky degrees to the direction of the Galactic Center.

Of the 50 nearest stellar systems within 17 light-years from Earth (the closest being the red dwarf Proxima Centauri at approximately 4.2 light-years), the Sun ranks fourth in mass. The Sun’s orbit around the Milky Way is believed to be elliptical, with the addition of perturbations due to the galactic spiral arms and non-uniform mass distributions. In addition, the Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit. It takes the Solar System about 225–250 million years to complete one orbit through the Milky Way (a galactic year), so it is thought to have completed 20–25 orbits during the lifetime of the Sun. History of Observation: The Sun has been an object of veneration throughout prehistory and ancient human history. Most cultures believed it to be supernatural in nature or a deity, one who’s presence was intimately tied to time, the seasons, and the cycle of life. Worship of the Sun was central to civilizations such the ancient Egyptians, Sumerians, Inca, Aztecs and Mayans, as well as numerous cultures in Europe, West Asia and Africa.

The earliest known examples of Sun worship are found in Proto-Indo-European mythology, where the sun is depicted as traversing the sky in a chariot (aka. a “solar chariot). In Germanic mythology, the solar chariot is depicted as Sol; in Vedic (and subsequently Hindu) cultures as Surya; and in Norse mythology as Solvognen. The gilded side of the Trundholm, the Norse sun chariot. Credit: Public Domain In Mesapotamia, Utu was the deity of the Sun – the god of justice and the descendent of Nannar (god of the Moon). To the Babylonians and Assyrians, Shamas (or Samas) was the equivalent, and similar gods were worshiped in the Akkadian and Hebrew pantheons – and well as throughout the Arabian peninsula – under different names. To the ancient Egyptians, the sun was associated with Ra, the god who ruled the sky, the Earth and the underworld. The Sun itself was named Aten, which was either the body or the eye of Ra.

From the 25th century BCE onward, worship of Ra became widespread across Egypt, with many depictions of him being carried across the sky in a solar vessel accompanied by the lesser gods. In the case of New World civilizations, the Incas, Mayans and Aztecs all believed that human sacrifices were necessary to appease the Sun god and maintain the cycle of life. To the Aztecs, Huitzilopochtli – the god of war, the sun, human sacrifice and the patron of Tenochtitlan – was responsible for all their victories and defeats in battle, and could only be appeased through the offering of blood. To the Greeks, the Sun god was known as Helios, the son of the Titan Hyperion and the Titaness Theia. Similar to the Egyptian’s representations of Ra, Helios was commonly depicted as being carried by a chariot drawn by fiery horses. However, unlike their ancient forebears, the Greeks viewed the Sun as one of the seven planets, since it revolved once a year along the ecliptic through the zodiac. Coin of Roman Emperor Constantine I depicting Sol Invictus/Apollo with the legend SOLI INVICTO COMITI (ca.

315 CE ). Credit: cngcoins.com The Roman Empire adopted Helios into their own mythology as Sol. The title Sol Invictus (“the undefeated Sun”) was applied to several solar deities, and was depicted on several types of Roman coins during the 3rd and 4th centuries CE. The birth of “the undefeated Sun” was celebrated around this same time, on the 25th of December, soon after the winter solstice to mark the end of the days growing shorter. In Chinese mythology, the Deity of the Sun was known as Ri Gong Tai Yang Xing Jun (or Tai Yang Gong, “Grandfather Sun”) – aka. Star Lord of the Solar Palace, Lord of the Sun. Tai Yang Xing Jun is usually depicted with the Star Lord of the Lunar Palace and Lord of the Moon, Yue Gong Tai Yin Xing Jun (Tai Yin Niang Niang / Lady Tai Yin). Several famous temples and monuments were constructed in ancient times with worship of the Sun or solar phenomena in mind.

For example, stone megaliths that marked the summer or winter solstice have been observed in Egypt, Malta, England (Stonehenge), Ireland, and at the ancient city of Chichen Itza in southern Mexico. Over time, ancient astronomers began to develop a scientific understanding of the Sun, based on ongoing observations of its movements. In the early 1st millennium BCE, Babylonian astronomers noted that the Sun’s motion along the ecliptic was not uniform. This would later be learned to be the result of the Earth’s elliptical orbit around the Sun. An illustration of the Ptolemaic geocentric system by Portuguese cosmographer and cartographer Bartolomeu Velho, 1568. Credit: Bibliothèque Nationale, Paris In the fifth century BCE, Greek philosopher Anaxagoras reasoned that the Sun was not the “chariot of Helios”, but a flaming ball whose light was reflected by the Moon.

By the 3rd century BCE, Eratosthenes estimated the distance between the Earth and the Sun at either 4,080,000 stadia (755,000 km) or 804,000,000 stadia (148 – 153 million km, or 0.99 – 1.02 AU), the latter of which is correct to within a few percent. It was also during the 3rd century BCE that Greek astronomer Aristarchus of Samos would propose the idea that the Sun was at the center of the universe and the planets revolved it. This view would be adopted later by Seleucus of Seleucia (ca. 190 BC – ca. 150 BC), and would go on to be articulated by Islamic and Indian astronomers during the Middle Ages, particularly through the Samarkand Observatory. The contribution of Arab and Islamic scholars include Al-Battani (858 – 929 CE), who discovered that the direction of the Sun’s apogee (the point when the Sun appears to be moving slowest against the fixed stars) is subject to change.

Egyptian astronomer Ibn Yunus (950 – 1009) observed more than 10,000 entries for the Sun’s position for many years using a large astrolabe. From an observation of a transit of Venus in 1032 CE, the Persian astronomer and polymath Ibn Sina (aka. Avicenna, ca. 980 – 1037) concluded that Venus is closer to Earth than the Sun. Ibn Rushd, the 12th century Andalusian astronomer, also provided a description of sunspots in the 12th century. Observations of sunspots were recorded earlier during the Han Dynasty (206 BCE – 220 CE) by Chinese astronomers, who maintained records of these observations for centuries. Sunspot plate from Scheiner’s Tres Epistolae (1612). Credit: galileo.rice.edu Nicolaus Copernicus ‘ mathematical model of a heliocentric system revolutionized astronomy, and helped to usher in our modern understanding of the Sun’s importance in our universe.

In addition to explaining observational discrepancies in the motion’s of the planets, the heliocentric model effectively placed the Sun at the center of the known universe. The development of the telescope in the early 17th century also allowed for detailed observations of the Sun and planets. Thomas Harriot, Galileo Galilei , Christoph Scheiner, and other astronomers were able to make accurate illustrations of the positions of sunspots on the Sun’s surface. In 1672, Giovanni Cassini and Jean Richer were able to determine the distance to Mars, and were thereby able to calculate the distance to the Sun. In 1666, Isaac Newton became the first scientist to observe the Sun’s light using a prism, and showed that it is made up of light of many colors. In 1800, William Herschel built on this by discovering infrared radiation using a series of thermometers and a prism. By noting temperature changes beyond the red part of the solar spectrum, he helped usher in the study of electromagnetism by determining that certain forms energy are invisible. Studies of the Sun’s light spectrum also led to advancements of spectroscopic studies in the 19th century.

This culminated with Joseph von Fraunhofer’s discovery and recording of over 600 absorption lines in the spectrum, the strongest of which were grouped together and named “Fraunhofer lines”, after their founder. The Sun viewed in the Extreme Ultra-Violet (EUV), with a comparison between solar minimum (left) and maximum (right). Credit: SOHO/NASA Another major area of study during the 19th century that would have an impact on our understanding of the Sun was the development of thermodynamics. A major contributor to this field was William Thomson (aka. Lord Kelvin, 1824 – 1907) who suggested that the Sun is a gradually cooling liquid body that is radiating an internal store of heat. Kelvin and Hermann von Helmholtz also proposed that a gravitational contraction mechanism was responsible for the Sun’s energy output. They also estimated the Sun’s age at 20 million years – which was in stark contrast to geological estimates that that placed the Earth’s age at at least 300 million years. By the 20th century, a documented solution for the Sun’s energy output was finally offered.

The first came from Ernest Rutherford (1871 – 1937), who suggested the Sun’s output was maintained by an internal source of heat, and suggested radioactive decay as a possibility. But it would be Albert Einstein who would provide the essential clue to the Sun’s energy output with his mass-energy equivalence (E = mc²). In 1920, British astronomer and physicist Sir Arthur Eddington proposed that the pressures and temperatures at the core of the Sun could produce nuclear fusion whereby hydrogen atoms merge into helium nuclei, resulting in the production of energy from the net change in mass. This would later be confirmed by numerous studies conducted by physicists, which would also lead to the conclusion that the fusion of hydrogen is responsible for the formation of all known elements in the universe. Exploration: With the beginning of the space age in the mid-20th century, the opportunity to observe the Sun with robotic space probes became possible for the first time. The first missions to the Sun were NASA’s Pioneer 5 , 6 , 7 , 8 and 9 satellites, which were launched between 1959 and 1968.

These probes orbited the Sun at a distance similar to that of Earth, and made the first detailed measurements of the solar wind and the solar magnetic field. In the 1970s, the Helios 1 and 2 probes – a U.S.-West-German collaboration that studied solar winds from inside Mercury’s orbit at perihelion – provided scientists with significant new data on solar wind and the solar corona. The Skylab Space Station, which was launched by NASA in 1973, also made numerous discoveries using its solar observatory – known as the Apollo Telescope Mount . These included the first observations of coronal mass ejections and of coronal holes , now known to be intimately associated with solar wind. In 1980, NASA launched the Solar Maximum Mission , a spacecraft designed to observe gamma rays, x-rays and UV radiation released by solar flares. Unfortunately, an electrical failure caused the probe to go into standby until it could be retrieved and repaired by the Space Shuttle Challenger in 1984. The mission subsequently acquired thousands of images of the solar corona before re-entering Earth’s atmosphere in June 1989.

In 1991, the Japan Aerospace and Exploration Agency (JAXA) deployed the Yohkoh (Sunbeam) satellite, which observed solar flares at X-ray wavelengths. Until 2001, when an annular eclipse caused it to lose its lock on the Sun, it observed an entire solar cycle and determined that the corona was much more active in regions away from peak activity than previously thought. Launched in 1995, the joint ESA-NASA Solar and Heliospheric Observatory (SOHO) has gone on to become one of the most important solar missions in history. Situated at the Lagrangian point between Earth and the Sun, SOHO has provided a constant view of the Sun at many wavelengths since its launch. Originally intended to serve a two-year mission, a mission extension through 2012 was approved in 2009, and a follow-on mission was launched in 2010 – the Solar Dynamics Observatory (SDO). All these satellites observed the Sun from the plane of the ecliptic, and so have only observed its equatorial regions in detail.

The first attempt to study the Sun from the polar regions was the Ulysses probe, a joint ESA-NASA mission which was launched in 1990. Once it assumed its scheduled orbit, the probe began observing the solar wind and magnetic field strength at high solar latitudes, finding that high-latitude solar wind moved slower than expected (750 km/s), and that there were large magnetic waves emerging from high latitudes that scattered galactic cosmic rays. In 2006, the Solar Terrestrial Relations Observatory (STEREO) mission was launched, which consisted of two identical spacecraft being launched into orbits that caused them to alternately pull further ahead of and fall gradually behind Earth. This enables stereoscopic imaging of the Sun and solar phenomena, such as coronal mass ejections. Many more solar missions are planned for the coming years and decades. These include the Indian Space Research Organisation’s (ISRO) scheduled mission of Aditya – a 100 kg satellite that is slated for launch in 2017–18. Its main instrument will be a coronagraph for studying the dynamics of the Solar corona.

In 2017, the ESA plans to launch the Solar Orbiter , which will study how the Sun creates and controls its heliosphere. The mission will fly as close as 0.28 AU to capture its measurements. In 2018, NASA plans to launch its Solar Probe Plus , which will approach the Sun from a distance of 8.5 solar radii to take direct measurements of the particles and energy emanating from the Sun’s corona. Lastly, there is NASA’s Solar Sentinels mission, an as-of-yet unscheduled mission that will involve a group of six spacecraft – four stationed inside the orbits of Venus and Mercury, one behind the Sun and one orbiting Earth. Together, they will study the sun during solar maximum, research energetic particles, coronal mass ejections and interplanetary shocks in the inner heliosphere. This data will be used to forecast space weather for future human spaceflight missions. The Sun does much more for us than just provide light and heat. It also provides all the energy that allows for chemical reactions and metabolic reactions, which is what began the life cycle here on Earth in the first place.

The continuous energy that it gives us, combined with our atmosphere’s protective presence, ensures that this life cycle keeps going. The Sun also lets off potentially harmful rays, solar winds and material that would kill us if it were not for the Earth’s magnetic field. However, the solar winds carry this charged material out to the edge of the Solar System where it forms a magnetic field that in turn prevents other interplanetary material from getting in. Without this barrier (the heliopause), the Solar System would be battered by cosmic rays. In this respect, the Sun is a great provider, and the Earth is ideally situated to benefit from its presence. We are not too close, nor too far so as to be too hot (like Venus) or too cold (like Mars). We are also secure in the knowledge that by the time the Sun expands to the point where life can no longer exist on Earth, we will either be long gone, or have evolved beyond the point where we live on only one planet. Kepler's Laws and Newton's Laws Kepler's Laws and Newton's Laws Kepler's Laws Johannes Kepler (1571-1630) developed a quantitative description of the motions of the planets in the solar system.

The description that he produced is expressed in three ``laws''. Kepler's First Law: The orbit of a planet about the Sun is an ellipse with the Sun at one focus. Figure 1 shows a picture of an ellipse. It is constructed by specifying two focus points, F1 and F2, of the ellipse. All points on the ellipse, such as P in Figure 1, have the property that the sum of the distance between P and F1 and the distance between P and F2 is a constant. The dimension of an ellipse is often described by giving its major axis and minor axis. In descriptions of orbits in the solar system, however, it is more common to use the semi-major axis to describe the size of the orbit, and the eccentricity of the ellipse to describe its shape. The eccentricity is given by the ratio of the distance between the two focus points to the length of the major axis of the ellipse.

The periapsis, or the shortest distance between the orbiting body and the central mass, is determined by the product of the semi-major axis and the complement of the eccentriciy (1 - e): if the body is orbiting the sun, this is the perihelion, symbolized by q): q = a (1 - e). A circle is a special case of an ellipse, with an eccentricity of 0, or so that q = a. Kepler's Second Law: A line joining a planet and the Sun sweeps out equal areas in equal intervals of time. Figure 2 illustrates Kepler's Second Law. Consider the line between the Sun and point A on the elliptical orbit. After a certain amount of time, the planet will have moved along the orbit to point B, and the line between the Sun and the planet will have swept over the cross hatched area in the figure. Kepler's Second Law states that for any two positions of the planet along the orbit that are separated by the same amount of time, the area swept out in this manner will be the same.

Thus, suppose that it takes the planet the same amount of time to go between positions C and D as it did for the planet to go between positions A and B. Kepler's Second Law then tells us that the second cross hatched area between C, D, and the Sun will be the same as the cross hatched area between A, B, and the Sun. Kepler's Second Law is valuable because it gives a quantitative statement about how fast the object will be moving at any point in its orbit. Note that when the planet is closest to the Sun, at perihelion, Kepler's Second Law says that it will be moving the fastest. When the planet is most distant from the Sun, at aphelion, it will be moving the slowest. Kepler's Third Law: The squares of the sidereal periods of the planets are proportional to the cubes of their semimajor axes. We have defined the semimajor axis of the orbit above, in our discussion of Kepler's First Law. The sidereal period of a planet's orbit is the time that it takes a planet to complete one orbit around the Sun. Kepler discovered a quantitative relationship between these two properties of the orbit.

If P is the period of the orbit, measured in years, and a is the semimajor axis of the orbit, measured in Astronomical Units, then P2 = a3 Newton's Laws Kepler's Laws are wonderful as a description of the motions of the planets. However, they provide no explanation of why the planets move in this way. Moreover, Kepler's Third Law only works for planets around the Sun and does not apply to the Moon's orbit around the Earth or the moons of Jupiter. Isaac Newton (1642-1727) provided a more general explanation of the motions of the planets through the development of Newton's Laws of Motion and Newton's Universal Law of Gravitation. Newton's Laws of Motion One way to describe the motion of an object it to specify its position at different times. Consider the car in Figure 3. We can tell where it is at different times as it travels down a road. It starts at milepost 0. One minute later it is between mileposts 1 and 2 at a distance of about 1.3 miles from the start. Two minutes later, the car has gotten to a distance of about 3.3 miles from the start.

In general, we could specify a unique position for the car at any time. For example, we might have written down where the car was at a time 1.5 minutes after the start, and even if we hadn't, we're pretty sure that the car was, in fact, somewhere. Mathematicians call this kind of a relationship a function. When we say that the position of the car is a function of time, it just means that there is a unique location for the car for any time. For a planetary orbit, we can describe the orbit in the same way, by providing the position of the planet along the orbit for all times. Another useful property for describing motion is the velocity of the object. Velocity is defined to be the change of position with change in time. Thus, for our car moving along the road, we can find the velocity by dividing the distance travelled by the time it takes to travel that distance. In our example, during the first minute, the car travels 1.3 miles along the road. Thus, the car's velocity would be 1.3 miles per minute (or about 78 miles per hour!) on the average during that first minute.

It is important to note that physicists are very particular about the definition of velocity, and when we state a velocity we always make a statement about the direction of the motion. In our one dimensional case, this corresponds to my statement that the the car moved along the road. In general, if we were looking at a road map, we might say that the velocity was 1.3 miles per minute towards the East -- if the street ran towards the East. Velocity always is specified by both a value and a direction. A final useful property for describing motion is the acceleration of the object. Just as the velocity describes the rate of change in the position of the object, the acceleration describes the rate of change of the velocity. In our example, the car moved farther during its second minute of travel than it did during its first minute. The average velocity during the second minute would be 2 miles per minute (120 miles per hour), since the car covered two miles from 1.3 to 3.3 during the one-minute time interval from 1 minute after the start to 2 minutes after the start.

The velocity increased a lot (0.7 miles per minute) between the first minute of travel and the second minute of travel, and we describe this change by the acceleration. In this case, the car's velocity increased by 0.7 miles per minute in a time interval of one minute. Thus, we'd say that the average acceleration of the car during this time was 0.7 miles per minute PER MINUTE --- acceleration is the rate of change of the velocity. Like velocity, acceleration has both a value and a direction implied. In our example, the direction was ``along the road'', but in a more general case, the acceleration is not necessarily in the same direction as the velocity. An especially good example for understanding the solar system is the case of uniform circular motion. Lets consider the case below of a car moving around a circle. The speed is constant in this motion, but the direction is changing continuously -- note the arrows showing the direction of motion in the figure -- so there must be an acceleration here. The acceleration in this special case of circular motion is called the centripetal acceleration.

It is always in the direction of the center of the circle, as indicated in the figure, and it has a value, A, of A = v2 / R where v is the speed of the object along its circular path, and R is the radius of the circle. Newton's First Law of Motion: A body remains at rest or moves in a straight line at a constant speed unless it is acted upon by an outside force. If you look back at the definition of acceleration, you will see that: (1) a body at rest is not accelerating; and (2) a body moving in a straight line at a constant speed is not accelerating either. Thus, the first law of Newton says that objects do not accelerate unless they are acted upon by an outside force. Newton's Second Law of Motion: If a force, F, works on a body of mass M, then the acceleration, A, is given by F = M A The first law said that if there is acceleration, then there is a force. Newton's second law gives a quantitative relationship between the force and the acceleration that is observed. The relationship depends on a new property of the object, its mass.

The mass is simply a measure of the amount of material in the object; mass is conventionally measured in grams or kilograms. Note that the second law implies that, for a given force, a less massive body will accelerate more than a more massive body. This is consistent with the world you are familiar with. Shove your kid brother, he might move a long way; shove Shaquille O'Neal with the same force and he won't move that far... Newton's Third Law of Motion: If one body exerts a force on a second body, the second body exerts an equal and opposite force on the first. This law is sometimes called the ``Action-Reaction'' law. Consider what happens if you are in one row boat and you pull on a line attached to a second row boat. When you pull the line, you exert a force on the second boat. But, by the third law, the other boat exerts an equal and opposite force back on you. Thus, if the second row boat has a large shipment of bricks in it so it is very heavy, your lighter boat may do all the moving even though you are doing all the pulling.

Implications for the Planets The elliptical orbits of the planets have such small eccentricities that, to a very good approximation, we can think of them as circles. (Only very precise measurements, like those available to Kepler, are able to detect the difference.) This means that we can use the idea of uniform circular motion to analyze planetary motion. In that section, we revealed that a body in uniform circular motion was constantly accelerating towards the center of its circular track. Thus, according to Newton's first law of motion, there must be a force acting on the planet that is always directed toward the center of the orbit -- that is toward the Sun! Newton's second law of motion allows us to state what the magnitude of that force must be. The required force is just the mass of the Earth times its acceleration. We know that the acceleration of an object moving in uniform circular motion is A = V2/R. Thus, we can calculate the force that is required to keep the Earth on its circular path and compare it to physical theories about what that force might be. This is what Newton later did, although he did it first for the Moon rather than the Earth, to learn about the force of Gravity.

Finally, let us consider an implication of the ``action-reaction'' law. If there is a force that attracts the Earth toward the Sun, then there must be an equal and opposite force attracting the Sun towards the Earth. Why, then, doesn't the Sun move? The answer is that it does move, but by a very small amount since the mass of the Sun is about half a million times that of the Earth. Thus, when subjected to the equal and opposite force required by the third law, it accelerates about half a million times less than the Earth as well. For this reason, to a very good approximation, we can treat the Sun as stationary in our studies of planetary motion. Newton's Universal Law of Gravitation By now you must be wondering: ``What is the Force that keeps the Earth going around the Sun?'' Newton's great discovery was the force of {\sl gravity}, which is an attractive force that occurs between two masses. The Universal Law of Gravitation is usually stated as an equation: Fgravity = G M1 M2 / r2 where Fgravity is the attractive gravitational force between two objects of mass M1 and M2 separated by a distance r.

The constant G in the equation is called the Universal Constant of Gravitation. The value of G is: G = 6.67 X 10-11 meters3 kilograms-1 seconds-2 Newton's great step was developing this law and using it, with his laws of motion, to explain the motion of lots of different things --- from falling objects to planets. Amazingly, out of these simple and general rules, Newton was able to show that all of Kepler's descriptive laws for orbits followed as a direct consequence. When you combine Newton's gravitation and circular acceleration, which must balance in order for the object to remain in orbit, you get a nice relation between the period, distance, and mass of the central body. It beings by equating the centripetal force (Fcent) due to the circular motion to the gravitational force (Fgrav): Fgrav = Fcent Fgrav = G m1 m2 / r2 Fcent = m2 V2 /r Let the Earth be m1 and the Moon be m2. For circular motion the distance r is the semi-major axis a.

The orbital velocity of the Moon can be described as distance/time, or circumference of the circular orbit divided by the orbital period: V = 2 pi r /P so setting the forces equal yields G m1 m2 / a2 = m2 V2 /a note that the m2 will cancel, so that circular orbital motion is independent of the mass of the orbiting body! G m1 / a2 = ((2 pi a)2/P2)/a which we rearrange to place all the a-terms on the right and all the P-terms on the left: G m1/(4 pi2) P2 = a3 which should look startlingly like Kepler's third law, but this time for the Earth's mass (or any other) instead of the sun's mass. To use a and P to solve for mass, manipulate once more so that m1 = a3 (4 pi2/G) / P2

Question: What word is used to descibe the position of a planet when it is closest to the sun?
Answer: PERIHELION