How far is a light-year? | Astronomy Essentials | EarthSky Astronomy Essentials <a href="">Fill out my Wufoo form!</a> 238,048 subscribers and counting ... How far is a light-year? By Bruce McClure in Astronomy Essentials | Space | July 11, 2016 How can we comprehend the distances to the stars? This post brings the scale of light-years down to the scale of miles and kilometers. The large yellow shell depicts a light-year; the smaller yellow shell depicts a light-month. Read more about this image at Wikimedia Commons . Stars other than our sun are so far distant that astronomers speak of their distances not in terms of kilometers or miles – but in light-years. Light is the fastest-moving stuff in the universe. If we simply express light-years as miles and kilometers, we end up with impossibly huge numbers. But the 20th century astronomer Robert Burnham Jr. – author of Burnham’s Celestial Handbook – devised an ingenious way to portray the distance of one light-year and ultimately of expressing the distance scale of the universe, in understandable terms. He did this by relating the light-year to the Astronomical Unit – the Earth-sun distance.

One Astronomical Unit, or AU, equals about 93 million miles (150 million km). Another way of looking at it: the Astronomical Unit is a bit more than 8 light-minutes in distance. A light beam takes 8 minutes to travel the 93 million miles (150 million km) from the sun to the Earth. Image via Brews OHare on Wikimedia Commons . Robert Burnham noticed that, quite by coincidence, the number of astronomical units in one light-year and the number of inches in one mile are virtually the same. For general reference, there are 63,000 astronomical units in one light-year, and 63,000 inches (160,000 cm) in one mile (1.6 km). This wonderful coincidence enables us to bring the light-year down to Earth. If we scale the astronomical unit – the Earth-sun distance – at one inch, then the light-year on this scale represents one mile (1.6 km). The closest star to Earth, other than the sun, is Alpha Centauri at some 4.4 light-years away.

Scaling the Earth-sun distance at one inch places this star at 4.4 miles (7 km) distant. The red star in the center of this picture is Proxima Centauri, our sun's nearest neighbor among the stars. A beam of light from this star takes about 4 years to travel to Earth.

Image via hyperphysics.phy-astr.gsu.edu Scaling the Astronomical Unit at one inch (2.5 cm), here are distances to various bright stars, star clusters and galaxies: Alpha Centauri: 4 miles (6.4 km) Sirius: 9 miles (14.5 km) Vega: 25 miles (40 km) Fomalhaut: 25 miles (40 km) Arcturus: 37 miles (60 km) Antares: 600 miles (966 km) Pleiades open star cluster: 440 miles (708 km) Hercules globular star cluster (M13): 24,000 miles (38,600 km) Center of Milky Way galaxy: 27,000 miles (43,500 km) Great Andromeda galaxy (M31): 2,300,000 miles (3,700,000 km) Whirlpool galaxy (M51): 37,000,000 miles (60,000,000 km) Sombrero galaxy (

M104): 65,000,000 miles (105,000,000 km) There are 33 stars within 12.5 light years of our sun.

Image via Atlas of the Universe . Light is the fastest-moving stuff in the universe. It travels at an incredible 186,000 miles (300,000 km) per second. That’s very fast. If you could travel at the speed of light, you would be able to circle the Earth’s equator about 7.5 times in just one second! A light-second is the distance light travels in one second, or 7.5 times the distance around Earth’s equator. A light-year is the distance light travels in one year. How far is that? Multiply the number of seconds in one year by the number of miles or kilometers that light travels in one second, and there you have it: one light-year. It’s about 5.88 trillion miles (9.5 trillion km). This scale starts close to home but takes us all the way out to the Andromeda Galaxy, the most distant object most people can see with the unaided eye. Image via Bob King / Skyandtelescope.com . Bottom line: The scale of light-years expressed as miles and kilometers. What is a light year?

| Space | EarthSky By Christopher Crockett in Space | February 20, 2013 Hint: A light-year is a unit of distance, not time. A light-year is how astronomers measure distance in space. It’s defined by how far a beam of light travels in one year – a distance of six trillion miles. Think of it as the bigger, badder cousin of the inch, the mile, the kilometer, and the furlong. If you like to keep up with what’s going on in astronomy, it’s worth spending a little bit of time understanding what the deal is with this funny unit of measurement. Astronomers use a beam of light as their reference to finding distances to space objects. Light beam image via Shutterstock Have you ever been on your way to meet some friends and, while en route, called to tell them you’re running late? If you’re like me, you might tell them “I’m about ten minutes away”. You’re using how long it will take you to get there as a substitute for how far away you are.

Astronomers do the same thing when figuring out the distance to a star or galaxy, but instead of relying on how fast they could drive or walk, they use a beam of light as their reference. Why do things this way? First, light is convenient. Throughout the universe, all light travels at exactly the same speed: about 670 million miles per hour. We don’t usually think about light traveling anywhere because when we flick on a light switch – there it is! We don’t have to wait for the room to light up. It happens instantaneously. Except that it’s not instantaneous, just insanely fast. In fact, let’s pause for a second and reflect on how ludicrously quick the speed of light is. If you could travel at the speed of light, you could circle the globe of Earth almost eight times in one second. Credit: NASA/NOAA/GSFC/Suomi NPP/VIIRS/Norman Kuring Traveling at that speed, you would encircle the globe of Earth almost eight times in one second. Impressive! Certainly faster than traveling by airplane.

So if you were to travel off the Earth in a straight line at light speed, you’d get pretty far in the same amount of time, right? Actually, since space is so vast, you wouldn’t have even made it to the moon. Travel to the moon takes about a second-and-a-half, at light speed. Travel to the sun at light speed takes about eight minutes. Can you even imagine? Traveling at a speed where you cross nearly 200,000 miles every second for eight full minutes would only get you to the center of our solar system. A light beam needs only 8 minutes to travel the 93 million miles (150 million kilometers) from the sun to the Earth. Image via Brews OHare on Wikimedia Commons. The fact that light takes time to get anywhere has an interesting side effect. If light from the sun takes eight minutes to get to us, then we’re actually seeing the sun as it was eight minutes ago. We’re looking into the past! While this sounds a little crazy, it’s actually something with which you’re already familiar. If you’ve ever seen fireworks, for example, you know that you see the explosion and then a few seconds later you hear it.

If you close your eyes during the fireworks show, you’d only have your ears to know when things were happening. Since it takes some time for the sound to get to you, you’d always be hearing things a few seconds after they happened. The same happens with light: we only see something once the light from that event actually gets to our eyes. When we’re looking across a room, the time delay is only a few billionths of a second. But when we start looking across large enough distances, the light becomes noticeably delayed like the sounds are from exploding fireworks. The red star in the center of this picture is Proxima Centauri, our sun's nearest neighbor among the stars. A beam of light from this star takes about 4 years to travel to Earth. Image via hyperphysics.phy-astr.gsu.edu The main reason for using light years, however, is because the distances we deal with in space are immense. If we stick to miles or kilometers we quickly run into unwieldy numbers just measuring the distance to the nearest star: a dim red dwarf called Proxima Centauri that sits a mere 24,000,000,000,000 miles away!

Using a longer yard stick, so to speak, helps keep the numbers at least manageable. To an astronomer, the distance to Proxima Centauri is only four light years. Put another way, it takes the light from our stellar next door neighbor four years to get to Earth. The Milky Way galaxy in which our sun and all the stars we see at night reside spans 100,000 light-years from one end to the other. Putting that into perspective, the duration of recorded human history is roughly 5,000 years. So light from a star at one end of our galaxy takes 20 times longer than all of recorded history to get to the other end. Astronomers measure the distance to the Andromeda galaxy using light-years. One light-year equals about 6 trillion miles. This galaxy is over 2 million light-years from Earth. Image credit: Adam Evans via Wikipedia Moving beyond our galaxy, it’s just over two million light years to our nearest galactic neighbor, the Andromeda galaxy . The light we currently see from that galaxy left there about the same time the ancestors of modern humans were first discovering stone tools. Our last stop takes us to the edge of the visible universe.

It’s also here where the trickiness of measuring distance in an ever-expanding universe becomes apparent. The light we see coming from the farthest depths of the universe has been traveling across the cosmos for almost three times longer than our planet has existed: nearly 14 billion years! But there’s a catch: we cannot say that the edge of the visible universe is 14 billion light-years away. Hubble Ultra Deep Field image from 2004, one of the deepest images of galaxies in our universe. Credit: S. Beckwith & the HUDF Working Group (STScI), HST, ESA, NASA Why? Because the universe has grown larger in that time! A galaxy whose light took 14 billion years to reach our little planet has, in the intervening aeons, moved even further away. The current physical distance to that remote beacon, if we stopped the universe from expanding and stretched out a really long tape measure, is just over 46 billion light years! Even in light years, measuring distances across the universe becomes unwieldy. But measuring in something familiar, like miles, is truly humbling.

From here to the edge of our vision spans a distance of approximately 276,000,000,000,000,000,000,000 miles. And it’s getting bigger every day. Bottom line: How far is a light-year? It’s the distance a beam of light travels in one year – a distance of six trillion miles. Light-year | Units of Measurement Wiki | Fandom powered by Wikia exactly 31,557,600 light-seconds . The figures above are based on a Julian year (not Gregorian year ) of exactly 365.25 days (each of exactly 86,400 SI seconds, totaling 31,557,600 seconds) [2] and a defined speed of light of 299,792,458 m/s, both included in the IAU (1976) System of Astronomical Constants , used since 1984.

[3] Other values Edit Before 1984, the tropical year (not the Julian year) and a measured (not defined) speed of light were included in the IAU (1964) System of Astronomical Constants, used from 1968 to 1983. [4] The product of Simon Newcomb 's J1900.0 mean tropical year of 31,556,925.9747 ephemeris seconds and a speed of light of 299,792.5 km/s produced a light-year of 9.460530×1015 m (rounded to the seven significant digits in the speed of light) found in several modern sources [5] [6] [7] was probably derived from an old source such as C. W. Allen 's 1973 Astrophysical Quantities reference work, [8] which was updated in 2000. [9] Other high-precision values are not derived from a coherent IAU system.

A value of 9.460536207×1015 m found in some modern sources [10] [11] is the product of a mean Gregorian year of 365.2425 days (31,556,952 s) and the defined speed of light (299,792,458 m/s). Another value, 9.460528405×1015 m, [12] [13] is the product of the J1900.0 mean tropical year and the defined speed of light. History Edit The first successful measurement of the distance to a star other than our Sun was made by Friedrich Bessel in 1838. The star was 61 Cygni , and he used a superlative Template:Convert heliometer designed by Joseph von Fraunhofer . The largest unit for measuring distances across space at that time was the Astronomical Unit (AU), equal to the radius of the Earth's orbit (1.50×108 km; 9.30×107 mi).

The use of this unit in trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, gave the distance to the star as Template:Convert . Bessel realised that a much larger unit of measurement was needed to make the vast interstellar distances comprehensible. James Bradley had stated in 1729 that light travelled 10,210 times faster than the Earth in its orbit. In 1769, a transit of Venus revealed the distance of the Earth from the Sun, and this, together with Bradley's figure, allowed the speed of light to be calculated as 3.01×108 m/s, very close to the modern value. Bessel used this speed to work out how far light would travel in a year, and announced that the distance to 61 Cygni was 10.3 light-years. This was the first appearance of the light-year as a measurement of distance, and, although modern astronomers prefer the parsec , it is popularly used to gauge the expanses of interstellar and intergalactic space.

Distances in light-years Edit Distances measured in fractions of a light-year (or in light-months) usually involve objects within a star system . Distances measured in light-years include distances between nearby stars , such as those in the same spiral arm or globular cluster . One kilolight-year, abbreviated "kly", is one thousand light-years (about 307 parsecs). Kilolight-years are typically used to measure distances between parts of a galaxy . One megalight-year, abbreviated "Mly", is one million light-years (about 307 kiloparsecs). Megalight-years are typically used to measure distances between neighbouring galaxies and galaxy clusters . One gigalight-year, abbreviation "Gly", is one billion (109) light-years—one of the largest distance measures used. Gigalight-years are typically used to measure distances to supergalactic structures, including quasars and the Great Wall . List of orders of magnitude for length Scale (ly) Units Used in Astronomy to Measure Distances - In astronomy, the distances are so huge that it becomes awkward to measure in terms of miles and kilometers. Below are some of the units used to measure distances in astronomy.

Astronomical Unit (AU) An Astronomical Unit (AU) is the average distance between the Earth and the Sun, and is used to measure distances within our solar system. 1 AU = 150 million kilometers or 93 million miles (approximately) Light-Year (ly) Beyond the solar system, even the Astronomical Unit becomes a relatively small distance. This is where the Light-Year (ly) comes into picture. It is the distance traveled by light in vacuum for a year. The speed of light is about 186,000 miles or 300,000 kilometers per second. This makes the distance of 1 light-year to be about 9.5 trillion kilometers or about 6 trillion miles. 1 ly = 9.5 trillion kilometers or 6 trillion miles (approximately) 1 ly = 63,241 AU (approximately) Light-years are usually used to measure distances between stars. The closest star to our Sun is Proxima Centauri and is at a distance of 4.2 light-years.

Parsec (pc) In 1838, Friedrich Wilhelm Bessel used the method of triangulation and the parallax effect to calculate the distance of a star from our Sun. The unit to describe this distance from the Sun to the astronomical object was termed as the Parsec which stands for the “parallax of one arc second”. A Parsec is about 3.26 light-years. This is equal to just under 31 trillion kilometers or just over 19 trillion miles. 1 pc = 31 trillion kilometers or 19 trillion miles (approximately) 1 pc = 3.26 Light-Years (approximately) 1 pc = 206,264 Astronomical Units (approximately) In order to understand the meaning of Parsec, it is important to understand the following terms: Triangulation: Triangulation is an indirect method of measuring distances using geometry with the help of a “baseline” and two known angles from the baseline to the object whose distance is to be calculated. Note: Establishment of one angle as a 90° angle simplifies the calculation, but it is not necessary. Parallax: Hold your finger in front of you with reference to any background object.

Now look at the finger with one eye, and then with the other. You will notice that there is a change in the position of the finger with respect to the background object. This apparent movement is called parallax. Source: Wikipedia Arc Second: If we consider one circle to be neatly divided into 360 parts, then each part is a degree. Each degree can be further divided into 60 equal parts, where one part is an arc minute. When an arc minute is further divided into 60 equal parts, each part is equal to one arc second. Calculation of one Parsec: In order to measure the parallax effect of a star with respect to background stars, we would need a big enough baseline. For this we make the first observation of the star from the earth being on one side of the Sun, and the second observation six months later, when the earth is on the opposite side of the Sun in its orbit. This would establish a distance of 2 AU between the two positions from which the star is observed. Source: Wikipedia We can assume an imaginary right triangle in space with its three points being the distant star being observed, the earth and the Sun.

We know the distance between the Sun and the Earth to be 1 AU and the angle at the Sun’s corner to be 90°. The angle at the star’s corner is the parallax angle. The distance between the Sun and the star can be found out using trigonometry. When the parallax angle at the star’s corner is 1 arc second, the distance between the Sun and the star is 1 parsec. Using the above calculation, 1 parsec is approximately equal to 3.26 lys. What is a light-year and how is it used?? What is a light-year and how is it used? Answer: A light-year is a unit of distance. It is the distance that light can travel in one year. Light moves at a velocity of about 300,000 kilometers (km) each second. So in one year, it can travel about 10 trillion km. More p recisely, one light-year is equal to 9,500,000,000,000 kilometers. Why would you want such a big unit of distance? Well, on Earth, a kilometer may be just fine.

It is a few hundred kilometers from New York City to Washington, DC; it is a few thousand kilometers from California to Maine. In the universe, the kilometer is just too small to be useful. For example, the distance to the next nearest big galaxy, the Andromeda Galaxy, is 21 quintillion km. That's 21,000,000,000,000,000,000 km. This is a number so large that it becomes hard to write and hard to interpret. So astronomers use other units of distance. In our solar system, we tend to describe distances in terms of the Astronomical Unit (AU). The AU is defined as the average distance between the Earth and the Sun. It is approximately 150 million km (93 million miles). Mercury can be said to be about 1/3 of an AU from the Sun and Pluto averages about 40 AU from the Sun. The AU, however, is not big enough of a unit when we start talking about distances to objects outside our solar system. For distances to other parts of the Milky Way Galaxy (or even further), astronomers use units of the light-year or the parsec .

The light-year we have already defined. The parsec is equal to 3.3 light-years. Using the light-year, we can say that : The Crab supernova remnant is about 4,000 light-years away. The Milky Way Galaxy is about 150,000 light-years across. The Andromeda Galaxy is 2.3 million light-years away. 10 Ridiculously Precise Units of Measurement | Mental Floss 10 Ridiculously Precise Units of Measurement Wikimedia Commons // CC BY 3.0 Like us on Facebook In October 1958, Oliver R. Smoot (future Chairman of the American National Standards Institute) repeatedly laid down on the Harvard Bridge connecting Boston and Cambridge, Massachusetts, so that some of his Lambda Chi Alpha fraternity brothers could measure the entire length of the bridge in relation to his height. At 5 feet 7 inches tall, the bridge was found to be 364.4 “Smoots” long (plus or minus an εar).

The prank quickly became the stuff of legend (to this day, graffiti on the bridge still divides it up into Smoot-based sections) until finally, in 2011, the word smoot was added to the American Heritage Dictionary , defined as “a unit of measurement equal to five feet, seven inches.” Ten more equally precise units of measurement, and the stories behind them, are explored here. 1. BARN A barn sounds enormous, right? You’d think so, but it’s actually equal to somewhere in the region of 0.000000000000000000000001 square centimeters (10^-24cm²)—which is the approximate size of the cross-section of one uranium nucleus. The name was coined by researchers working on the Manhattan Project at Purdue University in Indiana in the early 1940s, and refers both to the relatively large size of the uranium nucleus compared to other elements, and to the fact that it was the intended target—as in, “you couldn’t hit a barn door ”—for the atoms whizzing around in their particle accelerator.

As whimsical a name as it might be, however, referring to the uranium cross-section they were aiming for as a barn had the added bonus of allowing the researchers to keep their wartime work a secret. 2. SYDHARB One sydharb is equivalent to 500,000,000,000 liters, namely the approximate volume of Sydney Harbor. Why would you ever need such an enormous measurement? Well, just like using the relative sizes of countries or regions to compare one against another (as in “Brazil is the same size as five Alaskas”), the volume of Sydney Harbor can be used to give context to otherwise incomprehensibly vast quantities like the annual water consumption of a city or country, the size or impact of a flood, and the capacities of lakes and dams. In comparison, it takes two full days (49 hours to be precise) for 1 sydharb of water to flow over Niagara Falls. 3. BEARD-SECOND If a light-year is the distance traveled by light in one year (i.e. approximately 6 trillion miles), then a beard-second is the length that a beard hair grows in one second—or, according to Google’s unit converter , 5 nanometers.

4. MICKEY Mickey was a mouse of course, and so is that thing attached to your computer. Used by computer scientists and programmers, 1 mickey is the smallest measurable movement of a computer mouse, typically equal to 1/200th of an inch, or just over 0.1mm. The sensitivity of a computer mouse is likewise measured in mickeys-per-inch, while its speed is measured in mickeys-per-second. 5. CANDLEPOWER In the 19th century, long before the candela took over as the standard unit of luminous intensity, the relative luminosity of different types of gas- and oil-powered lamps and lights was measured in comparison to one spermaceti candle weighing one-sixth of a pound (76 grams) and burning at a rate of 120 grains (just under 8 grams) per hour. A candle of this size and burning rate, ultimately, would be said to produce 1 candlepower of light. This standard was first introduced in Great Britain by the Metropolitan Gas Act in 1860 and adopted, with some changes, in 1909 by the U.S., the UK, and France.

But as technology progressed, the definition of one candlepower changed several times over the decades, before it was finally replaced altogether by the candela in 1948. In modern terms, one candlepower is equal to 0.981 candelas. 6. MICROFORTNIGHT The Furlong/Firkin/Fortnight or FFF System is a humorous alternative to more standardized, decimal-based measuring systems like SI and the centimeter-gram-second system. Although FFF isn’t really meant to be used in real-world situations (and is instead intended to show just how impractical older systems can be, as well as to test the conversion skills of math students), some of its measurements have nevertheless slipped into wider use: One microfortnight, equal to 1.2 seconds or 1/1,000,000th of two weeks, for instance, is used in the VMS computer operating system . 7.

MILLIHELEN If Helen of Troy had “ the face that launch’d a thousand ships ,” then 1 millihelen—following the correct system of prefixes in the SI system—is the precise quantity of beauty required to launch one ship, or 1/1000th the number of ships Helen is said to have launched. Although the term is credited to a number of different writers and journalists, it was probably originally coined by Isaac Asimov . 8. SWATH According to the Oxford English Dictionary, 1 swath is “a measure of grass land [or] a longitudinal division of a field,” equal to the breadth of one sweep of a thresher’s scythe. The term has been in use since medieval times but has seemingly never been standardized, and it’s highly likely there were numerous local variations over the centuries. Nevertheless, according to one 19th century agricultural textbook , a single sweep of a scythe should thresh an area roughly 7 feet long by 14 to 15 inches wide—which would make one swath roughly 8 square feet. 9.

DRAUGHT As a unit of measurement, draught can be used to refer to the distance a standard bow can shoot an arrow (also called the bow-draught or arrow-shot), or to the quantity of fish taken in by one drawing of a fishing net (which is also called a take). Based on that second definition, in the 19th century one draught was a measurement of eels that came to exactly 20 pounds. 10. MUGGESEGGELE One Muggeseggele is equal to 0.22mm , or just under 1/100th of an inch. Not the most useful of measurements you might think, but that’s the point: In Swabian German, Muggeseggele is used as a byword for any proverbially tiny distance, length, or measure, like “a hair’s breadth” or “a cat’s whisker” might be used in English.

Be careful when you drop this one into everyday conversation, however—Muggeseggele literally means “a house fly’s scrotum.” Online Unit Converters • Common Unit Converters • Length and Distance • Unit definitions micrometer A micrometer (μm) is a decimal fraction of the base SI unit of length, the meter, which is defined as the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second. The term micron and the symbol µ, representing the micrometer, were officially revoked by the International System of Units (SI) in 1967. micron A micron (μ) is an obsolete name of a micrometer, which is a decimal fraction of the base SI unit of length, the meter. The term micron and the symbol µ, representing the micrometer, were officially revoked by the International System of Units (SI) in 1967.

However, in practice, “micron” remains a widely used term, which is preferred to “micrometer” in many English-speaking countries because it helps to differentiate between the unit of length and the “micrometer”, a device used for precise measurement of small distances. megaparsec A megaparsec (Mpc) is an decimal multiple of the astronomical unit of length parsec, which is equal to about 3.26 light-years, which is about 30.9 trillion (3.09×10¹³) kilometers. A parsec is the distance from the Sun to an astronomical object which has a parallax angle of one arcsecond. kiloparsec A kiloparsec (kpc) is an decimal multiple of the astronomical unit of length parsec, which is equal to about 3.26 light-years, which is about 30.9 trillion (3.09×10¹³) kilometers. A parsec is the distance from the Sun to an astronomical object which has a parallax angle of one arcsecond.

parsec A parsec (pc) is an astronomical unit of length, which is equal to about 3.26 light-years, which is about 30.9 trillion (3.09×10¹³) kilometers. A parsec is the distance from the Sun to an astronomical object which has a parallax angle of one arcsecond. light year A light-year (also light year or lightyear, ly), is a non-SI unit of length equal to just under 10 trillion kilometers or about 6 trillion miles or 0.306 parsec. It is defined as the distance that light travels in a vacuum in one Julian year. pole A pole is a unit of length or distance in the British imperial Units equal to 5½ yards or 16½ feet. Since 1959 it has been equivalent to exactly 5.0292 meters. A pole is the same length as a rod or a perch. Its length varied from 2 to 5 meters at various times and in various European countries. fathom A fathom (ftm) is a unit of length or distance in the British imperial and the U.S. customary systems.

It is equal to 2 yards, 6 feet or 1.8288 meters and used especially for measuring the depth of water. Originally, it was based on the distance between the man’s outstretched arms link A link (l., li. or lnk.) is a unit of length or distance in the in the US Customary Units and British Imperial Units. Prior to the 20th century, links were commonly used for in surveying real property. A link is exactly 33⁄50 of a survey foot. Twenty-five links make a rod. One hundred links make a chain. One thousand links make a furlong. Eight thousand links make a statute mile. link (US survey) A US survey link (l., li. or lnk.) is a unit of length or distance in the US Customary system. Links were commonly used for in surveying real property. A link is exactly 33⁄50 of a survey foot. Twenty-five links make a rod. One hundred links make a chain. One thousand links make a furlong. Eight thousand links make a statute mile.

cubit (UK) A cubit is a traditional unit of length equal to approximately 50 cm and based on the length of the forearm: from the elbow to the tip of the middle finger. Cubits of various lengths were employed in many parts of the world from classical antiquity, into early modern times. hand A hand (h) is a non-SI unit of measurement of length, now used only for the measurement of the height of horses in some English-speaking countries. In ancient Egypt, it was originally based on the breadth of a human hand. It is today equal to four inches or 10.16 centimeters. span (cloth) A span is the distance measured by a human hand, from the tip of the thumb to the tip of the little finger. In ancient times, a span was considered to be half a cubit. It is today equal to 9 inches or 0.2286 m. finger (cloth) A finger is any of several units of measurement that are approximately the width of an adult human finger. It is sometimes also called fingerbreadth or finger’s breadth.

It is used for measuring short lengths of fabric and in medicine and related disciplines (anatomy, radiology, etc.) where the fingerbreadth is often informally used as a unit of measure. nail (cloth) A nail is an obsolete unit of cloth measurement generally equal to a sixteenth of a yard or 2¹⁄₄ inches (5.715 cm). The nail was apparently named after the practice of hammering brass nails into the counter at shops where cloth was sold. inch An inch (in or ″) is a unit of length or distance in a number of systems of measurement, including in the US Customary Units and British Imperial Units. One inch is defined as 1⁄12 of a foot and is therefore 1⁄36 of a yard. According to the modern definition, one inch is equal to 25.4 mm exactly. angstrom The ångström (A) is a unit of length equal to 10⁻¹⁰ m or 0.1 nm. Its symbol is the Swedish letter Å. The ångström is used in the natural sciences and technology to express very small sizes comparing to that of atoms, and molecules.

The ångström is not formally a part of the International System of Units (SI) and its use is officially discouraged in SI. The closest SI unit, the nanometer (10⁻⁹ m), is recommended for use instead of ångström. a.u. of length An atomic unit of length (au or a.u. or a0) is a part of a system of natural units (Hartree atomic units) which is convenient for atomic physics calculations. The atomic unit of length is the Bohr radius, which is a physical constant, approximately equal to the most probable distance between the proton and electron in a hydrogen atom in its ground state. The Bohr radius has a value of 5.2917721092(17)·10⁻¹¹ m or approximately 53 pm. X unit An x copper unit (xu) is a unit of length approximately equal to 0.100207 pm (1.00207⁻¹³ m). It is used to quote the wavelength of X-rays and gamma rays.

There are two separate and slightly different x units, which are defined in terms of the wavelengths of the two most commonly used X-ray lines in X-ray crystallography — the copper x unit and the molybdenum x unit. In this converter the copper x unit is used. fermi A fermi (fm) is another name of a femtometer, which is a decimal fraction of the base SI unit of length, the meter. It is used in nuclear physics because typical dimensions of a nucleus are in the order of several fermi. For example, the diameter of a nucleus of uranium-238 is equal to 16 fermi. arpent An arpent is a pre-metric French unit of length and a unit of area. It is used in Quebec as well as in some areas of the United States that were part of French Louisiana. 1 arpent = 180 French feet (of approximately 32 centimeters) = about 192 English feet = about 58.47 meters. point A point (pt) is the smallest unit of measure in typography. It is a subdivision of the larger pica.

The contemporary desktop publishing point (also called the PostScript point) was defined as 72 points to the inch (1 point = 1⁄72 inches = 25.4⁄72 mm = 0.3527 mm). There are 12 points to the pica. twip A twip (abbreviated from “twentieth of a point”) is a typographical unit of measurement, defined as 1/20 of a typographical point. One twip is 1/1440 inch or 17.639 µm when derived from the PostScript point at 72 to the inch (used in this converter), and 1/1445.4 inch or 17.573 µm based on the printer’s point at 72.27 to the inch. long cubit A long cubit is a traditional unit of length equal to approximately 53.3 cm and based on the length of the forearm: from the elbow to the tip of the middle finger. Cubits of various lengths from 60 cm to slightly over one meter were employed in many parts of the world from classical antiquity, into early modern times.

handbreadth The handbreadth or handbreadth, also called a palm, is an obsolete non-SI unit of length. In English usage the handbreadth was originally based on the breadth of a human hand without the thumb, and has origins in ancient Egypt. It is distinct from the hand, the breadth of the hand with the thumb. It is usually taken to be equal to four digits or fingers, or to three inches (7.62 cm). fingerbreadth A fingerbreadth is any of several units of measurement that are approximately the width of an adult human finger. It is also called a finger or finger’s breadth. In medicine and related disciplines (anatomy, radiology, etc.) the fingerbreadth is often informally used as a unit of measure. It is sometimes used in the measurement of distilled spirits where two fingers of whisky refer to the amount of whiskey that would fill a glass to the level of two fingers from its bottom. Bohr radius An Bohr radius (b a.u.) is a part of a system of natural units which is convenient for atomic physics calculations. The Bohr radius is an atomic unit of length.

It is a physical constant, approximately equal to the most probable distance between the proton and electron in a hydrogen atom in its ground state. The Bohr radius has a value of 5.2917721092(17)·10⁻¹¹ m or approximately 53 pm. Earth’s equatorial radius In geodesy, a reference ellipsoid is a mathematically-defined surface that approximates the geoid, the truer figure of the Earth. Reference ellipsoids are used as a surface to perform computations. The difference between Earth’s equatorial and polar radii is about 21 km. Earth’s polar radius In geodesy, a reference ellipsoid is a mathematically-defined surface that approximates the geoid, the truer figure of the Earth. Reference ellipsoids are used as a surface to perform computations. The difference between Earth’s equatorial and polar diameters is about 42 km. Sun’s radius The solar radius is a unit of distance used to express the size of stars in astronomy equal to the current radius of the Sun.

The solar radius is approximately 695,500 kilometers (432,450 miles) or about 109 times the radius of the Earth. light-nanosecond A light-nanosecond is a non-SI astronomical unit of length equal to 10⁻⁹ light-seconds or just under 0.3 meters or just about 1 feet. It is defined as the distance that light travels in a vacuum in one nanosecond. This unit is useful for planning telecommunications networks. light-microsecond A light-microsecond is a non-SI astronomical unit of length equal to 10⁻⁶ light-seconds or just under 300 meters or just over 0.186 miles. It is defined as the distance that light travels in a vacuum in one microsecond. This unit is useful for planning telecommunications networks. light-millisecond A light-millisecond is a non-SI astronomical unit of length equal to 10⁻³ light-seconds or just under 300 kilometers or just over 186 miles. It is defined as the distance that light travels in a vacuum in one millisecond. This unit is useful for planning telecommunications networks.

light-hour A light-hour is a non-SI astronomical unit of length equal to 60 light-minutes or 3600 light-seconds or just over 1 billion kilometers or just under 0.7 billion miles. It is defined as the distance that light travels in a vacuum in one hour. light-day A light-day is a non-SI astronomical unit of length equal to 24 light-hours or 86,400 light-seconds or just over 25 billion kilometers or just over 16 billion miles. It is defined as the distance that light travels in a vacuum in one day. light-week A light-week is a non-SI astronomical unit of length equal to 7 light-days or 604,800 light-seconds or just over 181 billion kilometers or just under 113 billion miles. It is defined as the distance that light travels in a vacuum in one week.

rack unit A rack unit (abbreviation RU or U) is a unit of measure, which describes the height of equipment designed to install in a 19-inch or 23-inch rack where 19 or 23 inches is the width of the equipment including its edges or ears that protrude on each side and allow the module to be fastened to the rack. One rack unit equals to 1.75 inches or 44.45 mm. horizontal pitch A horizontal pitch (abbreviation HP) is a unit of length defined by the Eurocard printed circuit board standard. It is used to measure the horizontal width of rack mounted electronic equipment, similar to the rack unit (U) used to measure vertical heights of rack mounted equipment. One HP is 0.2 inches or 5.08 mm wide. cicero A cicero is an obsolete unit of measure used in typography equal to 1/6 of the historical French inch and is divided into 12 points, known in English as French or Didot points. It equals to 4.5 mm. It is similar in length to an English pica.

pixel In computer technology and digital imaging, a pixel or picture element is a physical point in a raster image, or the smallest addressable element in a display device. Its length is different for different monitors. The address of any pixel corresponds to its physical coordinates on the screen. In this converter the size of the pixel is 0.265 mm and approximately corresponds to the pixel size of a 17-inch 1280*1024 monitor. liniya (line) A liniya (or line) is an obsolete Russian unit of length abandoned in 1918 (Soviet Russia) and 1925 (USSR) in favour of the metric system. Since XVIII century it is equal to 1/280 of an archin, or 1/10 of an inch, or 2.54 mm. Caliber of firearms was measured in liniyas. For example, the caliber of the Mosin-Nagant three-line military rifle (Russian: трехлинейная винтовка Мосина) was three liniyas (Iines) or 2.54*3=7.62 mm.

The AK-47 (Kalashnikov) assault rifle has the same caliber. Learn Technical English with Our Videos! You can use this online converter to convert between several hundred units (including metric, British and American) in 76 categories, or several thousand pairs including acceleration, area, electrical, energy, force, length, light, mass, mass flow, density, specific volume, power, pressure, stress, temperature, time, torque, velocity, viscosity, volume and capacity, volume flow, and more. Note: Integers (numbers without a decimal period or exponent notation) are considered accurate up to 15 digits and the maximum number of digits after the decimal point is 10. In this calculator, E notation is used to represent numbers that are too small or too large. E notation is an alternative format of the scientific notation a · 10x. For example: 1,103,000 = 1.103 · 106 = 1.103E+6. Here E (from exponent) represents “· 10^”, that is “times ten raised to the power of”.

E-notation is commonly used in calculators and by scientists, mathematicians and engineers. If you have noticed an error in the text or calculations, or you need another converter, which you did not find here, please let us know ! Distances to the Sun and Stars Distances to the Sun and Stars Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay First-time Visitors: Please visit Site Map and Disclaimer . Use "Back" to return here. It is no accident that we call enormous numbers "astronomical". Astronomers deal with quantities so vast, distance, mass, and energy, that they tax our imaginations and mathematical skills. But such vast quantities also provide some of the most exciting ideas the human mind can encounter. Distances In The Solar System We can determine the distances to objects in the Solar System, and to nearby stars, by Triangulation. If we observe an object from two stations a known distance apart, we can find the distance to the object. For objects in the Solar System, we can measure the position of a the object from different sides of the Earth. Observations of Venus crossing or transiting the face of the Sun, or asteroids passing very close to the Earth, were once used.

Once we know the actual distance of any object, Kepler's Third Law allows us to determine the distances of every object in the Solar System. [Nowadays we can actually bounce radar signals off nearby planets and measure the distance more accurately this way] Distances To Nearby Stars The stars are so far away that they look the same all over the Earth. The farthest apart we can ever make observations at present is from opposite sides of the Earth's orbit, on opposite sides of an ellipse 186 million miles in diameter. When we photograph the stars six months apart, some of them shift very slightly because of the Earth's motion. You can demonstrate this shift, or parallax, easily. Hold up your finger at arm's length and alternately open and close each eye. Your finger will appear to move relative to objects in the background. For the stars, the parallactic shift is extremely tiny but it can be measured. The distances to the stars turn out to be so great that we need a new unit of distance: the light year . The light year is not a unit of time; it is the distance light travels in a year, or about 6 trillion miles (10 trillion km) Measuring parallax involves measuring very tiny angles.

A degree can be divided into 60 minutes and each minute into 60 seconds. A quarter spans one degree at a distance of 1.5 meters. The Sun and Moon span about half a degree in the sky. A quarter spans one minute at a distance 60 times greater, or 90 meters. Venus at its nearest and Jupiter both span about a minute of arc in the sky. A quarter spans one second at a distance of 5 kilometers. The moons of Jupiter appear about one second of arc across. Astronomers routinely measure the positions of objects to about 0.1 second of arc and the Hubble Space Telescope, above the distortions produced by Earth's atmosphere, can see details that span a few hundredths of a second of arc. Astronomers also use another distance term, the parsec. A parsec is the distance at which a star would have a parallax of one second of arc, or 3.26 light years. Parsecs are convenient because the distance in parsecs is simply one over the parallax, making it easy to convert the two units. Hipparcos Hipparcos of Nicaea lived in the second century B.C.

and left us the first star catalog. On August 8, 1989, a satellite named in his honor, the High Precision Parallax Collecting Satellite, was launched by the European Space Agency. Despite an engine malfunction that left the satellite in a less desirable orbit than planned, the spacecraft gathered data on stellar positions and magnitudes until March 1993. The data were processed and finally released in catalog form in the summer of 1997. The data are of unprecedented accuracy and have completely revolutionized the determination of stellar distances. Hipparcos worked by observing stars through two telescopes aimed 58 degrees apart. The light from the two telescopes was merged into a detector with a fine grid of wires. As the satellite rotated, different stars passed through the field of view of each telescope and blinked on and off as the stars passed across the grid of wires. These observations allowed extremely accurate relative positions of the stars to be determined. The relative positions of all the stars could then be combined into an extremely accurate catalog of star positions across the entire sky. For 118,000 selected stars, Hipparcos measured their parallax accurate to .001 second of arc.

That's the apparent diameter of a quarter at a distance of 5000 kilometers, or putting a quarter in New York and viewing it from San Francisco. It's also the amount the hair on a person a meter away appears to grow in one second. A secondary mission named Tycho measured another million stars to an accuracy of "only" 0.01 second. How Accurate Are Stellar Distances? Earth-Based Measurements Distances in older star catalogs look so authoritative that many people think the distances to the stars are known very exactly, but this is not true. For parallax measurements made from Earth: The nearest star is some 4.2 light years away, and that distance was known to within an accuracy of better than 0.1 light year. For a star measured as 20 light years away, the distance estimate is accurate to within a light year or so. For stars 50 light years away, the margin of error is perhaps 5 to 10 light years. Beyond 100 light years, the parallax is so tiny it cannot be measured accurately, and astronomers resort to more indirect methods to find the distances to the stars. We will examine some of these methods later.

Hipparcos Measurements The Hipparcos catalog of star distances has made older distance estimates obsolete and all new astronomical publications will use the Hipparcos figures. Even more accurate observational systems are in the planning stages. However, until there is a distance measuring spacecraft on continuous duty, capable of being aimed at new targets on demand, there will still be earth-based parallax measurements and these will be subject to the same limitations as the older measurements The following table shows how Hipparcos has improved our knowledge of stellar distances: Accuracy Level 28,000 stars 300 light years The earth-based data are approximate. Much of the improvement in Hipparcos is due to its being completely automatic, whereas earth-based data were combined by laborious measurements of individual stars. Thus, earth-based data were incomplete and of variable accuracy. Travel Times to the Stars Films like Star Wars create the impression that travelling from star to star is only a bit more complex than driving down to the corner store for a loaf of bread. In fact, distances in the Universe are so vast it is hard to comprehend them. Everyday speeds will not begin to cover the distances in the Universe.

For example, walking nonstop at a brisk pace, a person could cover about 75 miles (120 km) in a 24-hour day. If he or she could keep up the pace indefinitely, it would take about 40 days to cross the 3000-mile (5000 km) width of the United States, and about 11 months to travel the 25,000 (40,000 km) miles around the Earth. It would take about 8 1/2 years to travel the 235,000 miles (380,000 km) to the Moon, and 95 years to cover the distance to Venus at its closest. In an automobile travelling a steady 55 miles (90 km) per hour, it takes about 2 1/2 days to cross the United States and about 19 days to circle the Earth. It will still take six months to travel the distance to the Moon. Venus, however, is 100 times farther away at its closest, and would be a 54-year trip. We are not even out of the inner Solar System yet.

At 55 miles an hour, just under half a million miles (800,000 km) a year, the Sun is two centuries away. If we had started out from the Sun at that speed in 1 A.D. we would now have covered about 960 million miles (1.54 billion km) and would be just beyond Saturn, only a third of the way out of the Solar System. In a jetliner, at 550 miles (900 km) an hour, we cover distance 10 times as fast as in a car. The United States is 5-1/2 hours wide, and two days will take us around the world. The Moon is three weeks away, Venus over 5 years, the Sun 20. Pluto, the outermost planet, is still 750 years away. The nearest star is still so far away as to be unimaginable: 52 million years. At 18,000 miles (29,000 km) an hour, the Space Shuttle crosses the Unites States in 10 minutes and circles the Earth in an hour and a half.

In 13 hours it covers the distance to the Moon, but even so would take two months to cover the distance to Venus and seven to cover the distance to the Sun. Pluto is still over two years away. The nearest star is still 160,000 years away. Light, at 186,300 miles (298,000 km) per second, could circle the Earth seven times in one second and reach the Moon in 1-1/2 seconds. The Sun is about 8-1/2 minutes away at the speed of light. It takes light about 5-1/2 hours to reach Pluto from the Sun, but 4.3 years to reach the next star. % Scale Models of the Universe It is impossible to make a physical model that shows Man, the planets, and the stars on the same scale. If we make the Earth a quarter (one inch or 2.5 cm in diameter), the Moon becomes a pea 29 inches (74 cm) away. The Sun is 9 feet (2.7 m) across and 1000 feet (300 m) away.

Pluto is 7 miles (11 km) away, and the nearest star is still off the real Earth: 49,000 miles (79,000 km) away. If we let the Sun be a quarter, the Earth is a speck 1/100 inch (1/40 cm) in diameter and ten feet (3 m) away. Pluto is more than a football field away: 350 feet (110 m). And at last we can begin to show stars in our scale model. The nearest one is about 500 miles (900 km) away, and it, too, is the size of a quarter, with a pea-sized companion star about 200 feet (60 m) away. Placing a single coin in each State capital covers the U.S. with coins more densely than space is filled with stars. Beyond The Nearest Stars Most of the stars we see with the naked eye at night are within a few hundred light years. A few are as far away as 2000 light years, only about 1/50 the diameter of our Galaxy.

We can only use parallax to determine the distances of stars within about 50 light years, a realm that bears about the same relationship to our Galaxy as a period bears to the size of this page. How can astronomers measure the distances of distant stars? The City Lights Analogy There are many ways to explain how we know the arrangement of the universe, but perhaps the clearest analogy is the City Lights Analogy. Imagine being on the roof of a tall building with no way to leave. At night you can see lights in all directions. How can you map your "universe"? For nearby lights, you can use triangulation. By observing from different locations on the roof, and measuring changes in the relative positions of the lights, you can determine how far away the lights are. Once you know how far away the lights are, you can determine their true brightness. In nearby towns, the lights are all too far away to measure their distances by triangulation, but you can recognize lights of the same types as those you see nearby. Since you know how bright these nearby lights are, you can calculate how far off the lights in the town are.

For distant towns, you cannot even see individual lights, but you know how far away the nearby towns are, and how much total light the towns emit, so you can use this information to estimate how far the distant towns are. At even greater distances, only clusters of towns are visible, and finally only great urban complexes, but at each stage you can use what you already know to discaover facts about the more distant universe. We build our picture of the Universe in much the same way. Spectroscopic Distances Within the range we can measure parallax accurately, there are thousands of stars of many spectral types. With the apparent brightness and distances of these stars known, it is possible to determine their absolute magnitudes. Distant stars of the same spectral types will probably have the same absolute magnitudes, enabling us to estimate the distance to the unknown star. For isolated stars, this is virtually the only method of determining distance. When we say that the bright star Deneb is 1600 light years away, for example, the distance is an estimate based on its brightness and spectral type. The distance could easily be 25% larger or smaller. Deneb is still too far away for even Hipparcos to measure accurately.

The Cluster Method Another technique for finding the distances of faraway stars involves star clusters. On photographs of clusters taken many years apart, the stars move. Some stars will move at higher speeds than others, but the velocity will show a characteristic statistical pattern. We can also measure the Doppler shift of stars in the cluster to find out how fast the stars are moving toward or away from us. The star cluster has no connection to Earth at all, so the velocity pattern of stars along our line of sight should be the same as that across our line of sight. Radial velocity can be determined in kilometers per second. By assuming the range of velocities across our line of sight is the same, we can determine what angle in the sky corresponds to a known distance and thus determine the distance to the cluster. Cepheid Variables The cluster method works out to a thousand light years or so. Fortunately, within that distance are stars that give us a yardstick to distant galaxies. These are the Cepheid Variables. Cepheids are named for a star in the constellation Cepheus, the first star of this type discovered, but the most famous Cepheid, and also the nearest, is Polaris, the Pole Star.

Polaris is about 300 light years away and varies in brightness too slightly to be obvious to the unaided eye. Before Hipparcos, the distance to Cepheids had to be determined indirectly by the cluster method, but now the distances to several have been determined directly. Cepheid variables are yellow-white giant stars that pulsate and vary in brightness, but in a regular way. The brighter the absolute magnitude of a Cepheid, the faster it pulsates. It is easy to measure the period of a Cepheid variable, and armed with this information, we can determine the absolute brightness of the star. Instead of using distance and apparent magnitude to find absolute magnitude, we compare the absolute and apparent brightness of the star to determine its distance. Light year - definition of light year by The Free Dictionary Light year - definition of light year by The Free Dictionary Related to light year: black hole light year n. 1. The distance that light travels in a vacuum in one year, approximately 9.46 trillion kilometers (5.88 trillion miles). 2. often light years Informal A long way: a small town light years away from the action of the capital.

light year n (Units) a unit of distance used in astronomy, equal to the distance travelled by light in one year, i.e. 9.4607 × 1012 kilometres or 0.3066 parsecs light year - Not a length of time but a distance, the distance a pulse of light travels in one year (about 5.88 trillion miles). See also related terms for pulse . light year 1. (ly) The distance traveled by light in a year, equal to 5.878 trillion mi (9.4605 trillion km) or 63,290 au. Defined in 1888. 2. A unit of length (distance) representing the distance traveled by electromagnetic waves (light) through space in one year. 1 light year = 9 trillion miles. ThesaurusAntonymsRelated WordsSynonymsLegend:

Question: What unit of distance is approximately 6 trillion miles?
Answer: Light Year