When measuring distances to stars or distances between stars in the sky, normal units are not going to work. First off, there are two different concepts when measuring distances between stars in the sky. Do you mean the distance I would need to travel when moving from one star to another? Or do you mean the distance between the stars as I am standing here on Earth looking at them in the night sky?

Let's approach the first question. When I am talking about the distance to travel from one astronomical object to another, I am talking about a long distance. So long, in fact, that standard units of measure are not appropriate.

Consider the distance from New York to Los Angeles. This is sensibly measured in miles. We could however measure the distance in inches as well. We would come up with a number, but the number would mean nothing to us. Which of the following is closest to the distance from New York to Los Angeles? Is it

• 2,000,000 inches?
• 20,000,000 inches?
• 200,000,000 inches?

None of these answers are easily chosen. This is because we are not familiar with the use of inches for such long distances. Is it

• 300 miles?
• 3000 miles?
• 30,000 miles?

Its much easier in this scenario to conclude that 3000 miles is the closest answer. The other two are too small or too large to be sensible.

Astronomy has the same problem. When measuring the distances between planets, it does not make sense to measure in miles. The numbers are too large to handle. So astronomers have come up with a unit for the task, the Astronomical Unit (or AU). Its defined as the average distance between Earth and the sun. For measuring distance between stars, we use the light year. Its defined as the distance light travels in one year. We also use a unit of distance called the parsec, or also megaparsec. You know from scientific notation how many parsecs in a megaparsec, right? We will discuss these units later.

So I might say that star A is 5 light years away and star B is 10 light years away. How far is 10 light years? It doesn't really matter. The number of miles is too large to be useful to us. Suffice it to say its a long distance. What we can say is that star B is farther from us than star A. In fact, twice as far from us.

We have seen that astronomers work with very large distances. They also work with very small distances as well. For these distances we will use the standard of scientific notation (discussed above).

The other distance we discuss is the distance between stars as we view them from Earth. This is also how we discuss planetary positions and the planet's motion through the sky. These distance are better referred to as angular separations. We cannot measure these distances in feet or inches or miles. They must be measured in angular units, degrees.

Keep in mind that there are 360 degrees in a full circle. (In fact this is based on 365 days for the sun to circle the Earth. At least that's what the Greeks thought.) There are 90 degrees in a right angle. A good right angle is the angular separation between a point straight ahead of you and a point directly above your head. How many degrees between a point straight ahead of you and a point directly behind you? I hope you said 180.

So angular separation in astronomy is measured in degrees. Sometimes when we are talking about astronomical measures, we measure in units smaller than a degree. When we do this we talk about units of arcminutes (not minutes of time). There are 60 arcminutes in one degree (just as there are 60 minutes in one hour). The moon is approximately 30 arcminutes in diameter as we view it from the Earth. That's about one half of a degree. We will also break down the arcminute into smaller units. Any idea what they are? We won't use them much for now, but we might later.

As a simple rule of measure, the width of your finger when your arm is stretched away from you is about 1 degree. The width of a clenched fist (thumb out to the side) in the same position is about 10 degrees.

Now that we have some idea of science and numbers, let us begin our study of astronomy as a science.

This page was last updated on 06/06/01.