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21 The Sun and the Moon

The Sun moves along an apparent path on the Celestial Sphere called the Ecliptic. The constellations along Ecliptic – historically 12 but officially 13 – are called the Zodiac. The Moon also travels along the Ecliptic; sometimes in the evening sky, sometimes in the morning, sometimes not visible at all. And the bright planets also travel along the ecliptic.

Measurements along the Celestial Sphere

Astronomers use degrees, minutes, and seconds to measure distances across the sky or sizes. These are referred to as Angular Sizes: the angle the object appears to span or distance between objects, for example:

  • 60 arcseconds = 1 arcminute
  • 60 arcminutes = 1 degree

The Sun and Moon appear to be about ½ degree as we see them from Earth. This is not their true size; it is just how they appear to us.

←→ ½ degree = 30 arcminutes

But in the “Real Universe” the Sun is 400 times bigger than the Moon!

Image explaining the concept of angular diameter which is used in astronomy, where the angular diameter from a person on Earth to the Sun is 32 arc minutes or 1920 arc seconds or 0.5 degrees.
Image courtesy of Florida State College at Jacksonville.

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Introduction to Astronomy Copyright © by Lumen Learning is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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