The moon is half covered with water

How does a lunar eclipse come about?

While the shadow of the moon falls on the earth during a solar eclipse, the opposite is true for a lunar eclipse: the shadow of the earth moves over the moon. Since the earth is larger than the moon, the consequences are very different.

When the earth's satellite crosses the earth's orbit and thus also the shadow of the earth during a full moon, a lunar eclipse takes place. This is the case about twice a year. Since the orbit of the earth's satellite is inclined by 5.1 degrees to the earth's orbit, the moon can pass by up to 37,000 kilometers above or below the earth's shadow on its orbit. Therefore, there is not a lunar eclipse with every orbit.

The umbra of the earth is about two and a half times as large as the moon itself at the height of the lunar orbit. In a total lunar eclipse, the moon is completely in the umbra of the earth. For an observer on the moon, the sun would then be completely covered by the earth. Nevertheless, it is not completely dark on the moon even during a total lunar eclipse. Because the earth's atmosphere breaks part of the sunlight and directs this light into the shadow region.

Lunar and solar eclipses

Viewed from the moon, this would look as if the edge of the earth was glowing reddish. Since blue light is more strongly scattered in the earth's atmosphere, red light in particular reaches the moon - like at sunset. Seen from the earth, the moon therefore shines in a gloomy copper red. In contrast to a solar eclipse, a lunar eclipse can be observed from the entire side of our planet facing the satellite.

In the case of a partial lunar eclipse, the earth's satellite only partially enters the umbra. The edge of the shadow cast by the earth is depicted on the surface of the moon and is visible as a circular arc. From this circular shape of the shadow, the ancient Greeks already concluded that the earth must be a sphere.

So-called penumbral eclipses, in which the moon only crosses the comparatively bright penumbra of the earth, are often barely perceptible. During such an eclipse, the sun would only be partially covered by the earth for an observer on the moon - a significant part of the sunlight still falls on the moon.

The part of the moon that is closest to the umbra appears noticeably darker when viewed from the earth. In addition, usually only part of the moon is in penumbra. Because the penumbra, which is in the shape of a ring around the umbra, is only about as wide as the diameter of the moon.

On average, penumbral eclipses are about half as common as umbra eclipses. And about half of all umbra eclipses are total lunar eclipses. In the 21st century, however, the earth's satellite crosses the earth's shadow more often than on average during a full moon. And precisely when the moon is close to the earth - where the umbra is slightly larger. As a result, there are significantly more total than partial lunar eclipses in this century: a total of 85. In contrast, there are just 57 partial lunar eclipses.