Astronomers work out the dates of the astronomical seasons by imagining that the sun moves around the Earth and that the Earth stands still. They describe how the sun appears to move to someone on Earth, even though we know that in fact the Earth does move and that it revolves around the sun. They put the Earth at the center of a huge globe called a celestial sphere, which represents the sky. The sun’s path around the Earth, which is called the ecliptic, is represented by a circle around the inner surface of this sphere. It takes the sun one year to make a trip around its ecliptic.

The Earth’s axis intersects the celestial sphere at two points, which are called the celestial poles. The celestial equator lies midway between these poles, and the sun’s ecliptic lies at an angle of 23.5 degrees to it.

At the point where the sun’s ecliptic crosses the celestial equator, its light “slices” the Earth exactly in half from pole to pole. This means that as the Earth spins, nearly every part of the planet experiences a day in which the length of daylight hours equals the length of darkness. Astronomers say that the sun is at an equinox, or “equal night,” position. As the sun continues moving along its ecliptic, it reaches points above or below the celestial equator where it seems to stand still momentarily before descending or ascending back toward the celestial equator. Each of these standstill points is called a solstice, from Latin words meaning “sun stands still.” The solstice and the equinox points mark the beginning and end of the four astronomical seasons on Earth.

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