Astronomy Jargon 101: Astronomical Unit

In this series, we explore the weird and wonderful world of astronomy jargon! It’s easy to measure your interest in today’s topic: the astronomical unit!

Measuring distances on earth is pretty easy with units like kilometers or miles. Your nearest town could be a few miles away. A long-haul flight will cover thousands of kilometers.

But distances in space are a whole different beast. If we were to stick to our earthbound conventions it would just get ridiculous. Our moon, the next noteworthy (natural) object in space, is nearly four hundred thousand kilometers away. On average, Saturn is easily over a billion kilometers away.

Nobody has time to put all those zeros in or keep track of all those millions, especially astronomers, so they came up with a practical measure of the solar system: the astronomical unit.

Abbreviated as “AU”, “au” and sometimes just “A”, the astronomical unit was previously defined as the average distance between the earth and the sun (I know, I know it’s a very earth-centered point of view, but you can tell us do not blame). But as we improved our measurement techniques, we found that the Earth-Sun distance was constantly changing. To make life easier, the astronomical unit is now defined exactly 149,597,870.7 kilometers.

With this new convention, typical solar system distances become much easier to manage. Mercury, the planet closest to the Sun, orbits with just under 0.4 AU. Uranus and Neptune, far out in the backyard of the system, are 20 and 30 AU, respectively. The objects in the Kuiper Belt are a few dozen AU away, depending on which one you are visiting.

Outside of the solar system, even the astronomical unit breaks down, which is why astronomers switch to light years (when talking to the public) and parsecs (when talking to each other). But for solar system scales, the AU is the favorite.

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