One of the best ways to find a faint star or a deep-sky object, such as a galaxy or star cluster, is to look for it in relation to a bright, well-known star. But when you look at the bright star, which is easy to see, how do you tell someone how far away the fainter object is from it? You could say that object X is five 'inches' north of star Y, but that assumes that everyone agrees on how big an inch appears on the sky. Such a method of judging apparent distance is not only awkward but inaccurate.

Astronomers measure apparent distances and separations on the sky using the angular scale: degrees, minutes of arc, and seconds of arc. In this book, we will be mostly concerned with degrees. In the above example, then, we might say that object X is '5°' north of star Y. Fine. But how much is a degree?

Everyone knows that a circle contains 360°, and a half circle 180°. A circle traced across the celestial dome (essentially half a sphere) from one point on the horizon to the point opposite would equal 180°. From the horizon to the zenith would be half that amount, or 90°. Half that distance again, midway up into the sky from the horizon, is 45°; half again is 22.5°, and so on.

This gives us an approximate scale that can be applied to quadrants of sky, but what about smaller areas in and around constellations? We need something familiar to help us gauge smaller chunks of sky. That 'something' is your fist. A fist at arm's length covers about 10° of sky; 12° if your hands are large. (For more on calibrating fist size, see 'Spring arrives,' March 15 - 21.) From horizon to zenith, you should be able to mentally stack nine fists. An even finer gauge is the Moon. The Moon's apparent diameter is about half a degree (as is the Sun's). So two full Moons side by side equal one degree.

We can go to a still smaller scale. Consider that a single degree consists of 60 minutes of arc, and each minute of arc consists of 60 seconds of arc. If the Moon's apparent diameter is half a degree, then it can also be described as being 30 minutes of arc, or 1,800 seconds of arc, in apparent diameter. (We say 'apparent' diameter as opposed to 'true' diameter to describe the size of these bodies as they appear on the celestial sphere. Obviously the actual sizes of the Moon, Sun, and planets are much greater than their angular sizes.)

The planets, too, exhibit tiny disks in the sky that are measured in seconds of arc. When nearest Earth, Mars has an apparent diameter of 25 seconds of arc; Jupiter, 50 seconds of arc; and Venus nearly a minute of arc.

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