When the Sun is partially or totally occluded by the Moon, we speak of a solar_eclipse. Another eclipse that is interesting for the Earth is the lunar eclipse.
An early report of a solar eclipse dates back to 2136 BC in China. Numerous reports from later times all describe the occurrences as overwhelming events. However, astronomers have calculated that most of the 200 ancient reports are false. More often than not they were made to coincide with battles that occurred around the same time.
One particularly conspicuous story is that of Mohammed, the founder of Islam. An eclipse is said to have occurred when he was born, a second when his son died. When Mohammed's remains were to be taken from Medina to Damascus, the sky became dark once again as the grave was opened. This was seen as a heavenly sign and the bones were left where they were.
How does a solar eclipse happen? Certain conditions must be fulfilled. The Sun, Moon and Earth must be in line. Astronomers distinguish between total, partial and annular eclipses.
In a total solar eclipse the core_shadow of the Moon falls on the Earth, i.e. the entire disc of the Sun is occluded by the Moon. A ring called the corona appears around the now hidden Sun. This is the outer layer of the Sun. Total eclipses last no longer than seven minutes. Most eclipses last only about three minutes. The observer must be in the core shadow of the Moon to see them. The area affected by the core shadow of the Moon is never larger than 264 kilometers in diameter.
A partial eclipse is when the Moon occludes only a portion of the Sun's disc. The observer must be located in the partial shadow of the Moon, which affects an area of several thousand kilometers in diameter.
During an annular eclipse, the Moon is at the point on its eccentric path, which is farthest from the Earth. At this time it appears one tenth smaller than when it is at the point nearest the Earth, making it smaller than the disc of the Sun. A ring of the Sun remains visible; the rest is occluded by the Moon.
If the orbits of the Earth and the Moon were on the same plane, we would see a lunar eclipse at every full Moon and a solar eclipse at every new Moon. However, because the orbits are inclined relative to each other, we only see an eclipse when the Sun or the Moon is only a few degrees away from the points where the orbits intersect. These points are known as nodes.
Sun and Moon both return to the same points at certain times. For this reason eclipses occur at regular intervals. This period is known as the saros and is 18 years and 10 days in duration.
A solar eclipse is only ever visible from areas located in a narrow strip on the Earth's surface, as the shadow of the Moon cannot cover the entire Earth. In contrast, lunar eclipses can be observed from all locations where it is night at that time. With 147 lunar and 228 solar eclipses in the 20th century, the latter were the more numerous. However, these were only visible from certain areas of the Earth.
