One of the perennial star party questions is "How far is it?" where "it" is the thing in the telescope field. This summary information can be used to get a feel for astronomical units and distances.

Astronomical distances vary by many orders of magnitude. The units used to measure the distances are tailored to the distances being measured, just as we use feet to measure our houses and miles to measure our cities here on Earth.

An AU, or Astronomical Unit, is the distance from the Earth to the Sun or about 93 million miles. It is a convenient unit of measure for things in and around the solar system.

A light year is the distance that light travels in one year, or 5.87 * 10^12 miles. It is a convenient unit of measure for nearby stars. Even this unit is small once one begins to talk about the distances to the galaxies.

To relate these two units to each other, consider the following quote from Burnham's Celestial Handbook, volume 1 by Robert Burnham Jr. By a fortunate circumstance, the number of inches in a mile is very nearly equal to the number of AU in one light year, a perfect arrangement for the purpose of constructing mental scale models. Let us imagine a scale model of the solar system, with the Earth represented as a speck one inch away from the Sun. Pluto is then about 3.5 feet away from the Sun. The nearest star on this model will be nearly 4.5 miles away. And all the stars are, on the average, as far from each other as the nearest ones are from us.

The Solar System

Distances are given from the Sun.
 

The Moon is 407,000 KM from the Earth and is 3400 KM in diameter.

Nearby Stars

Distances are given in light years

Note on star names: The bright stars are named by constellation and a Greek letter. The brightest star is usually Alpha, the second brightest Beta, etc. So Vega is also known as Alpha Lyra, the brightest star in the constellation Lyra.
 

Bright Stars

Distances are given in light years
 

Some Messier Objects