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Explanation of the Ecliptic Plane

written by: George Garza•edited by: RC Davison•updated: 5/25/2011

The plane of the ecliptic comes from the orbits of the Earth about the Sun. The Earth orbits the Sun at a particular angle and its orbit is elliptical. The Ecliptic name comes from the moon passing in front of the Sun; a New Moon creates a Solar Eclipse, whereas a Full Moon creates the Lunar eclipse

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    What is the Ecliptic Plane?

    Earth's orbit around the Sun forms an elliptical pattern. When stretched out over space, the pattern looks like a two-dimensional plate (or saucer), and it shows what the Earth's orbital path would be at different times of the year. As the Earth orbits around the Sun this determines a plane; this is the plane of the ecliptic.

    The ecliptic plane is tilted at a 23.5° angle with respect to the celestial equator. In other words, if you take the Earth, which is the celestial sphere, and point the poles straight up and down this will show four points. One is the celestial north pole, then the celestial south pole, the celestial equator will have two points of intersection, called equinox. This is where the equator of the celestial sphere intersects with the ecliptic plane. These points are the autumnal equinox (occurs in the autumn), and the vernal equinox (occurs in the spring).

    Ecliptic Plane 

    This image shows how the ecliptic plane and the celestial sphere interact with one another.

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    Planets in the Ecliptic Plane

    Almost all of the planets in the Solar System are in the ecliptic plane. They appear in what looks like a two-dimensional plane with the Sun at the center. The inclination is the angle between the plane of an object's orbit, and the plane of the ecliptic. A planet's orbit may be on the ecliptic or above or below. With most planets, the inclination is fairly constant with little deviation and matches the ecliptic. However, that said, Pluto, even though it is no longer considered a planet, has the most extreme inclination, 17° from the ecliptic. Likewise, Mercury also has an inclination of 7°. Meanwhile, the Sun also has a 7° inclination between the plane of the Sun’s equator and the ecliptic plane. For other information about Mercury and Pluto see Nothing but the Facts About Mercury and Nothing but the Facts About Pluto.

    Planetary Orbits 

    This image show how the different planets orbit around the Sun in the ecliptic plane. Pluto is the one planet that is the furthest from the ecliptic plane.

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    Odds and Ends About the Ecliptic Plane

    While Earth's orbit is elliptical. The closest distance Earth is to the Sun is 91 million miles and the farthest is 94.5 million miles. Because of this almost circular pattern of the orbit, then what causes the seasons? Well it has to do with the planet‘s axial tilt of 23.5° along the elliptical orbit. This creates variations in the amount of sunlight received at different parts of the Earth at different times of the year, and this causes the seasons. For example, there are two points when the Sun is closest and furthest distance from the Earth. These are the solstices. The Summer Solstice occurs in June 21, and the Winter Solstice occurs on December 22. The Equinox's occur when day and night are about the same length of time; this is when the Earth's orbit crosses the ecliptic plane. The Vernal equinox occurs on March 20, and the Autumnal equinox occurs on September 22. These are for the Northern Hemisphere. The dates are reversed for the Southern hemisphere.

    The axis tilt explanation goes for all the other planets, as well. Uranus, for example, rotates on its side with an axial tilt of 97.8°. This causes extreme variations in its seasons. For more information about Uranus see Interesting Facts About Uranus

    Ecliptic Plane 

    Finally, the ecliptic happens to be home to the 12 constellations of the Zodiac. The Zodiac may possibly be one of the oldest astronomical and ecliptic discoveries, going as far back to the Babylonians in the first millennium BC. They divided the sky into 12 zones and created a coordination system around the constellations. Then they added the 12 constellations to the Babylonian calendar linking each month with one of the constellations. You can find more information about the Zodiac in the article The Constellations of The Zodiac.

    Zodiac and the Plane of the Ecliptic 

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    The Ecliptic, which is the orbital plane of the Earth, can explain some features of the Solar System. The ecliptic breaks down the orbital path of the Earth by intersecting it with the celestial equator giving the equinoxes. It also explains the variations in the seasons along with the elliptical orbit that the planet takes. Almost all of the planets are on the ecliptic, with exceptions of Pluto, and Mercury. Finally, in a historical fact, the ecliptic helped establish the Zodiac, which became the basis of the 12 month year, going as far back as the Babylonians close to three thousand years ago.

    Sources and References

    What is the Ecliptic Plane?

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    Planets in the Ecliptic Plane

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    Odds and Ends About the Ecliptic Plane

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    Image: (used with permission)

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    Nasa -The Path of the Sun, The Ecliptic: