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When you look up at the sky, it looks as if someone took a really big bowl, turned it upside down, and placed it over where you stand. In the wintertime, I feel like we're encased in a really big snow globe of sorts.
This upside-down bowl represents one half of an imaginary sphere that surrounds the Earth that's called the celestial sphere. At night, it looks like someone drew stars on the inside of the bowl, stars that appear much closer together than they actually are.
Although we now know that the stars are really much farther away than they appear, we can use the concept of a celestial sphere to conveniently relate important astronomical concepts as they appear to us. Namely, we do this through angles and degrees.
The most familiar way to measure angles is through the use of degrees. A degree is 1/360th of a circle. That is to say, if you were to spin in a circle one time, you have spun around 360 degrees. If you turn directly to your left or right, you've made a 90 degree turn. You get the idea. Astronomers can use angles to measure the distance between stars as they appear to us in the sky and express these angles with degrees.
The angle that is formed by two imaginary lines starting at an observer's eye and ending at two objects is known as the angular distance. The illustration on your screen depicts this concept well. You can see how the person is looking out into the sky at two different celestial objects. There are two lines drawn from their eye to the objects. The distance between those two lines is known as the angular distance.
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Such a distance is expressed in degrees. When an angle is too small to express in degrees, it is then expressed either in arc minutes (aka minute of arc), which is 1/60th of a degree, or arc seconds (aka second of arc), which is 1/60th of an arc minute.
Therefore, distances between celestial objects, as they appear to us on the sky, can be measured with angles and degrees. For example, you might say something like: 'The moon was three degrees from a certain star in the sky.'
This same concept of angles and degrees is used to measure the angular diameter or angular (apparent) size of a celestial object. The angular diameter is the angle that's made by two lines starting at an observer and ending on the opposite sides of an object. For instance, the angular diameter of the full moon is about half a degree, some planets have one that's almost an arc minute, while the stars have an angular size of less than one arc second.
You can actually approximate many of these things on your own at night tonight. If you hold your arm in front of you, fully outstretched, your finger (depending on which one you use) is about 1-2 degrees across and your fist is about 10 degrees across. If you form a 'radical man' sign with your hand or a 'call me' sign where the thumb and pinky are outstretched, that's 20 degrees from tip to tip. Furthermore, your index finger can be broken up into three segments at each joint, with distances of 3, 4, and 6 degrees as shown on screen.
This way, the next time you go outside with a friend and tell them there's a cool star you want them to look at, you won't have to point it out for half an hour while they search aimlessly throughout the night sky. And you won't have to stand behind them, pointing your finger in front of their eyes trying to get them to see it.
Instead, you can measure the distance from an object both of you can see, like the moon, and have them measure out the same distance to get to the star you want them to see! It's much more convenient and faster that way.
The celestial sphere is an imaginary sphere that surrounds the Earth. It's used to help us measure objects in the night sky in terms of how they appear to us.
Imaginary angles are drawn and are measured in degrees. A degree is 1/360th of a circle.
The angle that is formed by two imaginary lines starting at an observer's eye and ending at two objects is known as the angular distance. When an angle is too small to be expressed in degrees, it is then expressed either in arc minutes (aka minute of arc), which is 1/60th of a degree, or arc seconds (aka second of arc), which is 1/60th of an arc minute.
This same concept of angles and degrees is used to measure the angular diameter or angular (apparent) size of a celestial object. The angular diameter is the angle that's made by two lines starting at an observer and ending on the opposite sides of an object.
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