The Moon revolves around the Earth once each month. Since the Moon's orbital plane is inclined by five degrees, we do not have lunar eclipses each month because the Moon passes either above or below the Earth's shadow. However, every few months the Moon passes either partially or entirely into the Earth's shadow, resulting in a partial or total lunar eclipse. This can only happen when the Earth lies between the Moon and the Sun when the Moon is in its full phase.

How an Eclipse Occurs

The dark inner portion of the Earth's shadow is called the umbra, and it is shaped like a cone that is pointing away from the Sun. The tip is about 870,000 miles from Earth. At the Moon's distance from Earth, the diameter of the umbra is about 5,700 miles across.

The outer shadow is called the penumbra. At the Moon's distance from Earth, the diameter of the penumbra is approximately 10,000 miles. The penumbra is not dark enough to have a noticeable effect on the Moon's brightness, so penumbral eclipses are usually ignored.

During a partial lunar eclipse, the umbra of the Earth's shadow advances onto part of the Moon's surface and then moves off the other side.

During a total lunar eclipse, the umbra hits the Moon¹ on the western limb and then gradually covers the Moon until its entire surface is covered².

Blood Red Moon

For a few hours, the Moon is immersed in the Earth's shadow. Though no sunlight falls directly on the Moon's surface during the total eclipse, some sunlight is bent around the Earth's atmosphere. From this refracted sunlight, the blue light is removed when the light is scattered in the Earth's atmosphere³, so only red light gets through to the Moon.

This is why a totally eclipsed Moon appears a bloody, red colour. The amount of volcanic dust and pollution suspended in the atmosphere can make the Moon appear darker and less reddish and even large weather systems have an impact, making the shadow appear less uniform. The Moon will also appear less evenly illuminated if it passes closer to the sides of the umbra rather than through the centre.

Normally when we see the full moon it appears flat and round, like looking at a silver coin from above. The totally eclipsed Moon offers us a rare glimpse of the true shape; it has a noticeably spherical bulge, like we are looking at a giant ball.

As the shadow begins to pass from the lunar surface⁴ the western limb is the first to appear. When the eclipse is over⁵, the Moon often remains at least partially in the penumbral shadow, but this shadow is hard to detect with the naked eye and is usually ignored.

Unlike solar eclipses, everyone in the night hemisphere of Earth sees exactly the same thing at the time of a lunar eclipse. Because they can be seen by everyone on half the planet when they occur, lunar eclipses are not as sexy as solar eclipses. Lunar eclipses can last for several hours, and totality can last for an hour or more.

Upcoming Lunar Eclipses

The following table shows the upcoming lunar eclipses for the next few years:

Past Lunar Eclipses

The partial lunar eclipse on New Year's Eve 2009 happened during the second full moon of the month, ergo a 'Blue Moon'. Such events happen only 11 times per millennium. The image was preserved for posterity by Astronomy Picture of the Day and at the spaceweather website.