Half a Degree... That's Really Cold, Right?
You’ll often hear astronomers talk about degrees, or even half degrees. And while it’s true that many amateur astronomers brave sub-freezing temperatures to view a crystal clear starry winter sky, in this case they’re not talking about temperature. They’re talking about “angular distance”.
If you want to measure how big or how far apart objects appear in the sky, you could use a ruler, but where would you put it? Up close to your face? At arm’s length? What if my arm is shorter than yours? This sort of measuring system just doesn’t work all that well in the sky.
Instead, astronomers use angles. The sky over our heads can be divided into 180 degrees, from the horizon, to straight overhead, to the horizon on the opposite side. If an object is 90 degrees up, it’s directly overhead. If it’s 0 degrees up, it’s right on the horizon.
You can also measure the angular distance between two objects in the sky. For example, if we say that Venus and Saturn are half a degree apart, we mean that the angle between the two objects (with your head as the vertex of that angle) is half a degree. By the way, half a degree is just about the width of the full moon.
But how can you measure angles in the sky? You can make approximate measurements just by using your hand. Held out at arm’s length, the tip of your pinky finger is about 1 degree across. You can cover a Full Moon with the tip of your pinky. Next, make a fist. At arm’s length, your fist is about 10 degrees across. Again, these are only approximations, but they work. You can improve your measurements by comparing your hand at arm’s length to a known angular distance. For instance, try measuring the Big Dipper. From the end of the handle to the bottom of the bowl is about 25 degrees. Try it out!
Adapted from the June 2007 star chart
