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Astronomical Units are a device that astronomer Robert Burnham Jr came up to held define mind blowingly HUGE distances in the universe. SPACE writers Bruce McClure and Christopher Crocket have defined it as follows:  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.

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.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. For general reference, we can say that one astronomical unit (AU) represents the mean distance between the Earth and our sun. The AU is approximately 150 million kilometers or 93 million miles. It is approximately 8 light-minutes.  One light-year = 63,240 AU.

Bottom line: Astronomers like to list the distances to objects within our solar system (planets, dwarf planets, asteroids, comets, spacecraft, etc.) in terms of the astronomical unit, or AU. One astronomical unit is the approximate mean distance between the Earth and sun. It’s about 150 million kilometers or 93 million miles.

More exactly, one astronomical unit (AU) = 149,597,871 kilometers = 92,955,807 miles.

Earth’s orbit around the sun isn’t a perfect circle. So Earth’s distance from the sun changes throughout the year. Astronomers give the Earth’s changing distance throughout the year relative to the astronomical unit, too. For instance, when the Earth is at perihelion – its nearest point to the sun for the year, in January – it’s about 0.983 AU from the sun. When our planet swings out to aphelion – its farthest point, in July – we’re about 1.017 AU away from the sun.

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.

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