Trace the Ecliptic Across the Sky

Trace the Ecliptic Across the Sky
Bright planets and the moon mark way stations along the ecliptic as seen from Miami, Florida, just after sunset on February 28.
(Image: © Starry Night Software)

The ecliptic is the path the sun, moon, and planets takeacross the sky as seen from Earth. It defines the plane of the Earth's orbitaround the sun.

The name "ecliptic" comes from the fact thateclipses take place along this line.

This imaginary line can best be visualized in the days justbefore full moon, especially when there are bright planets in the sky. Thisweek is a fine example.

Go out under a clear sky just after sunset. In the west,just above the setting sun, you will see the brilliant planet Venus. The moonwill be riding low in the east, just above the horizon. Mars, higher in theeastern sky, defines a third point on the ecliptic. A couple of hours later,Saturn will rise in the east, defining another point on the ecliptic.

Notice in the image that the moon and planets do not sitexactly on the line of the ecliptic. Although the orbits of the moon andplanets are almost in the same plane as the Earth's orbit, they are all alittle bit off. At the moment, Mars is slightly north of the ecliptic, and themoon and Mars are slightly below the ecliptic.

This discrepancy explains why we don't get eclipses everymonth. Because the plane of the moon's orbit is slightly different from theplane of the Earth's orbit, the moon usually passes above or below the sun orthe Earth's shadow, except close to where the two orbits cross, and an eclipseis avoided.

This year, the orbits of the Earth and moon intersect onJune 25 and Dec. 29, so eclipses can only occur within a month or so of thesedates.

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