Introductory Astronomy: Eclipses

Eclipses occur due to the alignment of the Earth, Moon, and Sun.

Solar Eclipses

A solar eclipse occurs when the Moon blocks the light from the Sun and causes a shadow to be cast on a small area of the Earth. There are three types of solar eclipses: total, partial, and annular. Due to the relative sizes of the Moon and the Sun and their distances from us, at times they appear to be the same size in the sky. If a solar eclipse occurs during this time, the Moon exactly covers the Sun and a total eclipse occurs. These are of interest because during total eclipses, outer layers of the Sun which are not usually visible can be seen. The alignment of the Earth, Moon, and Sun have to be exact for a total eclipse to occur. Even when the alignment is exact, not everyone will see a total eclipse. This is because, if the alignment is correct for the northern hemisphere, it is slightly off in the southern hemisphere (and vice versa). During total eclipses, astronomers specify zones where the alignment is exact (the umbra) and where it is slightly off (penumbra). For more information on the umbra and penumbra (and to see a simulation of the moon crossing in front of the sun, click here

If the alignment is not exact, a partial eclipse may occur. This is when the Moon only partially overlaps the Sun and blocks only part of the Sun from our view.

An annular eclipse occurs when the apparent size of the Moon is smaller than that of the Sun, and the Moon does not fully block the Sun from our view. This occurs because the Moon's orbit is elliptical, and so the Moon's distance from the Earth varies. When the Moon is farther away from the Earth, it appears to be smaller. Therefore, there are times when the Moon appears to be smaller than the Sun. If an eclipse occurs at this time, an annular eclipse results. Annular eclipses look like a "bull's eye".

Lunar Eclipses

A lunar eclipse occurs when the Moon passes through the Earth's shadow. Because the Earth is much larger than the Moon, usually the entire Moon is eclipsed. Because the full phase can be seen from anywhere on the night side of the Earth, a lunar eclipse can be seen by more people than a solar eclipse. Since the Moon is moving through the Earth's shadow, and the size of the Earth is much greater than the size of the Moon, a lunar eclipse last for about 3.5 hours (as opposed to a solar eclipse which last on the order of about 7.5 minutes).

To learn more about lunar eclipses, see pictures of actual eclipses, and find out when the next lunar eclipse will occur, click here.

Tilt of the Moon's Orbit

While the Moon is always in a new phase during a solar eclipse, a solar eclipse does not occur every time the moon is in the new phase. This is because the orbit of the Moon is tilted relative to the Earth's orbit around the Sun. This tilt is only 5 degrees, but it is enough that the alignment of the Earth, Moon, and Sun only occurs about once every six months. This holds true for lunar eclipses as well. In fact, lunar and solar eclipses generally occur together; that is, if the alignment is correct for a lunar eclipse during the full phase of the Moon, it will also be correct for a solar eclipse during the next new phase of the Moon.