BellaOnline Forums Education Astronomy ABC of Astronomy – E Is for Ecliptic

Star maps show you where the ecliptic is. That's because it's where you find the planets and the zodiac constellations. But what is the ecliptic plane on which the planets orbit? What shape are their orbits, and what do we mean by an eccentric orbit?

ABC of Astronomy – E Is for Ecliptic

The ecliptic is the projection onto the Sun of Earth's path around the Sun. Although it's Earth that's moving, for earthly observers it's less complicated to view it as the Sun moving. The zodiac constellations gained their importance because they are the constellations through which the Sun seems to move during the course of a year. The ecliptic is the center line of the zodiac and the Sun's progression acted as a calendar to ancient peoples.

Why don't we have a lunar eclipse every full Moon?

We only have eclipses when the Moon is on the ecliptic. (The ecliptic is where eclipses happen!) That's the only time when the Moon, Earth and Sun all line up nicely. (It's one kind of syzygy.) The Moon's orbit around Earth is tilted slightly to the ecliptic, but there are two places where the Moon's orbit crosses the ecliptic. If the Moon is full when this happens, we get a lunar eclipse.

Here is a diagram showing the ecliptic. Earth orbits the Sun in the ecliptic plane - if you imagine stretching the ecliptic plane out, the other planets also orbit close to the ecliptic.

When we look at the sky, everything we see is tilted just over 23 degrees to the ecliptic. That's because Earth's axis is tilted by that much. At the equinoxes the Sun seems to cross the celestial equator.