Measuring Objects in the Sky: Angular Size & Distance

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  • 0:01 The Celestial Sphere
  • 0:50 Angles and Degrees
  • 1:19 Angular Distance & Size
  • 2:54 A Neat Trick
  • 4:02 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will teach you about the celestial sphere, angular distance, angular size and diameter, arc minutes, and arc seconds. You'll also learn some neat tricks on how to use your own hands to measure these.

The Celestial Sphere

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.

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.

Angular Distance and Size

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.

The distance between the two points is called the angular distance
diagram depicting angular distance

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.

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