When is the astronomical unit used?

I always thought it had something to do with measuring light years.

AU are used mainly in the hopes of locating the Voyager messenger services currently operating outside the local solar system. Whatever the lengths its a long haul by any road. Not bad milage for a volkwagen beetle huh? Not!

It has two main uses.  To avoid writers cramp by tired astronomers who don't want to write out all the digits in planetary distances.  it also is used for calculations making use of Kepler's third law.

The Astronomical Unit (AU) is the unit of measure for distances in the solar system. Any time you express distances between objects in the solar system, it is more convenient to express it in astronomical units than in meters/kilometers.

The astronomical unit is the large semiaxix of the Earth orbit around the Sun. This is exactly the average between the smallest distance and the largest distance between Sun and Earth (or, more precisely, the Earth-Moon barycenter). Due to the non-uniform motion of Earth around the Sun, the AU does not coincide with the average Sun-Earth distance, but it is close (especially since the excentricity of the Earth orbit is small)

For any activity, it is useful to define a unit in the same order of magnitude as the distances in normal use. In the solar system, it was easy to find the distances in the solar system in in values relative to each other, so the distance between Sun and Earth became the natural etalon; the position of the planets and theit motion between the stars, as seen from Earth, was easily explained, even though the Sun and Earth (in terms of units incommon use, such as meter) was not known at all (at the time of Copernicus) or known with low precision.

It's used when talking about distances on a planetary scale.

It was also used by someone on Who Wants to Be A Millionaire to win a million dollars.  Damn I wish I was him cause I know how long an AU is.

It really isn't anymore.

100 or so years ago scientists didn't have the equipment to directly measure the distances to other planets or stars. (Red shift hadn't been discovered yet for one thing...).

They could however use Geometry to determine these distances, sort of.

What they could do was use the radius (or sometimes the diameter) of the Earth's orbit as one side of a triangle, and use astronomical observation to determine the angles involved. Then it was just a matter of doing some trig to determine the length's of the other two sides.

Problem was... they didn't have a good number for how far from the Sun the Earth was. They had some rough estimates, from things like observing the Transit of Venus and the Transit of Mercury, but it was  a pretty rough number. They could tell you that  Saturn was 8.833 times farther from the Sun than Earth was... but they didn't have a really good number for how far Earth was from the Sun, so they couldn't convert anything into miles or km.

 See the problem?

So... they invented the AU.  For one thing it made the math a lot easier (they could use 1 instead of 92,955,807 ) and it meant that as the Earth-Sun distance numbers got refined over time they didn't have to go back and re-do all those calculations. It also meant that astronomers who thought the Earth was 91,000,000 miles from the sun would get the same answer as astronomers who thought it was 94,000,000 miles from the Sun.

Now however we have really good measures of how far things are in the Solar System, so we don't use the AU much anymore.

Au's are used when measuring things in the Solar system.