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An angle is a measure of rotation in a circle. Every circle has 360 degrees. It doesn't matter how big or how small the circle is. If you stand in the center of any circle and turn completely around exactly once, you have rotated 360 degrees.
Astronomers use angles to measure the relative position of objects in the sky, because the angle between, say, two stars doesn't change and doesn't depend on how far away each of the stars are. Strictly speaking, the angle between two stars can and does change, but for our practical purposes, it will stay the same, day after day, year after year.
To locate an object in the sky at a particular time, astronomers often use two angles, called the azimuthal angle and the altitudinal angle. Sometimes, you'll hear astronomers refer to their telescopes as having an Alt-Az mount. That phrase refers to these two angles.
Azimuthal AngleImagine yourself standing at the center of this circle and facing North. This direction corresponds to an azimuthal angle of 0 degrees. If you turn and face East, you're looking in the direction that corresponds to an azimuthal angle of 90 degrees. If you had turned only halfway towards East, and faced North-East instead, you would be looking in the direction that corresponds to an azimuthal angle of 45 degrees. And so on around the circle. South is 180 degrees. West is 270 degrees. If you turn completely around and face North again, you've rotated 360 degrees.
Altitudinal AngleThe other angle that astronomers use is the altitudinal angle, which measures angles from the horizon up into the sky. The horizon (in any direction) corresponds to an altitudinal angle of 0 degrees. The point directly overhead, called the zenith, corresponds to an altitudinal angle of 90 degrees.
A pair of these two angles, one azimuthal angle and one altitudinal angle, can be used to locate any position in the sky. For example, to look at the point in the sky that is (Azi=135, Alt=45) first turn and face southeast, SE (135 deg), then tilt your head back until you're looking halfway up from the horizon to the zenith, (45 deg).
The azimuthal and altitudinal angles of particular stars or planets are usually not "nice" values like 45 degrees or 90 degrees, which are fairly easy to estimate. However, there are a few simple "rules of thumb" you can learn, to estimate just about any angle you want. The sections below show you how.
0.5 degree
The width of a full Moon, as viewed from the Earth's surface, is about
0.5 degree.
The width of the Sun, as viewed from the Earth's surface, is also about
0.5 degree.
1.5 degreesHold your hand at arm's length, and extend your pinky finger. The width of your pinky finger is about 1.5 degrees.
5 degreesHold your hand at arm's length, and extend your middle, ring, and pinky fingers, with the three fingers touching. The width of your three fingers is about 5 degrees.
10 degreesHold your hand at arm's length, and make a fist with your thumb tucked over (or under) your other fingers. The width of your fist is about 10 degrees.
20 degreesHold your hand at arm's length, and extend your thumb and pinky finger. The distance between the tip of your thumb and the tip of your pinky finger is about 20 degrees.
The Big DipperThe Big Dipper, which is part of Ursa Major, is one of the easiest constellations to find any time of year. Angles between stars of the Big Dipper happen to be very close to being multiples of 5 degrees, so they're a "handy" way to check and calibrate your hand and your technique for estimating angles in the sky.
The angle between Dubhe and Merak is about 5 degrees, or three fingers.
The angle between Dubhe and Megrez is about 10 degrees, or one fist.
The angle between Dubhe and Alioth is about 15 degrees, or one fist plus three fingers.
The angle between Dubhe and Alkaid is about 25 degrees, or one thumb-to-pinkie hand span plus three fingers.
Imagine drawing a line from Merak through Dubhe and continuing on past Dubhe for not quite 30 degrees, or three fists. (The angle is actually 28 degrees.) At that point is Polaris, the north star about which all other stars in the sky appear to revolve.
The hand sketches on this page are taken from an
American Sign Language Dictionary.
The Big Dipper sketch is taken from Chris Dolan's
Stars and Constellations pages.
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