Bright planets and the moon mark way stations along the ecliptic as seen from Miami, Florida, just after sunset on February 28.
Credit: Starry Night Software
The ecliptic is the path the sun, moon, and planets take across the sky as seen from Earth. It defines the plane of the Earth's orbit around the sun.
The name "ecliptic" comes from the fact that eclipses take place along this line.
This imaginary line can best be visualized in the days just before full moon, especially when there are bright planets in the sky. This week 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 moon will be riding low in the east, just above the horizon. Mars, higher in the eastern 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 sit exactly on the line of the ecliptic. Although the orbits of the moon and planets are almost in the same plane as the Earth's orbit, they are all a little bit off. At the moment, Mars is slightly north of the ecliptic, and the moon and Mars are slightly below the ecliptic.
This discrepancy explains why we don't get eclipses every month. Because the plane of the moon's orbit is slightly different from the plane of the Earth's orbit, the moon usually passes above or below the sun or the Earth's shadow, except close to where the two orbits cross, and an eclipse is avoided.
This year, the orbits of the Earth and moon intersect on June 25 and Dec. 29, so eclipses can only occur within a month or so of these dates.