Adirondack Sky Center
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Astro Quiz 32: What is an AU?

Question #32 – What is an Astronomical Unit (or AU)?


Astronomical distances are so large, astronomers need to come up with new ways to measure them. Even within our own solar system, distances to the outer planets measured in miles or kilometers are very large numbers. So astronomers developed a new measurement, the astronomical unit or AU. This is the average distance from the Earth to the Sun. Astronomers have defined that distance as 92,955,807.3 miles or 149,597,870.7 kilometers.

You can learn more about the history of the AU at this Wikipedia page.

This artist's painting shows distances in our solar system - and beyond - in astronomical untis.
Note that the Earth is 1 AU from the Sun, and the distance of Voyager 1 is over 100 AU!
Image credit: NASA/JPL-Caltech

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