The Ecliptic and the Celestial Equator

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The Ecliptic and the Celestial Equator As you can see on the diagram below, the axis of the Earth (the North and South line) is tilted 23.5 degrees away from the plane of the Earth's orbit around the Sun. Imagine the equator line on the globe extending out into space to the stars. Imagine that you are that little person standing in the northern hemisphere in the wintertime. You will see that the Sun would appear to be below the line of the equator. Now imagine that the Earth has gone half way around the Sun in its orbit. It is summertime. Again, extend the equator line out into space and you will note that the Sun now appears to be above that line. So, you can see that as the Earth moves around the Sun, the Sun appears to us to be moving up and down in the sky. When you project this up and down path that the Sun takes in one year onto a map of the sky, it appears as a curved line and is called the ecliptic. On the map below, the equator that the Sun appears to cross is a projection of the Earth's equator onto the sky and is called the celestial equator. Above illustration from www.nakedeeplanets.com This may sound confusing, but the plane of the Earth's orbit around the Sun is also called the ecliptic. One word is used for two things -- but when you think about it, these two things are related to each other because it is really the Earth's movement around the Sun that creates the illusion of an up and down pathway of the Sun in the sky! Note: The plane of the Earth's orbit around the Sun is called the ecliptic because eclipses of the Sun and the Moon can only occur when the Moon crosses it. .

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