How does a solar eclipse occur?

When the sun darkens in the middle of the day it feels like the end of the world has arrived. During solar eclipses, the moon moves between the earth and the sun and partially or totally obscures it.

It is probably the most dramatic spectacle that the sky has to offer us: the sunlight becomes pale, the crystal clear daylight transforms into twilight. Then, as if someone turns off the light on the dimmer, everything happens very quickly. The brightest stars flare up and where the blazing bright sun was just visible, there is now a pitch-black circle in the sky, surrounded by a fiery, finely structured ring of light. A total solar eclipse!

There are three kinds of solar eclipses:

Total eclipse
Partial eclipse
Annular eclipse

In a total solar eclipse, the moon moves exactly between the earth and the sun, so that the sun is completely covered. The solar eclipse is only total from places that lie along a very narrow strip on Earth. This path of totality (track of the Moon’s shadow across Earth’s surface) is a maximum of a few hundred kilometers wide and many thousand kilometers long.

During partial solar eclipses only part of the sun is covered by the moon. An annular solar eclipse occurs when the moon appears smaller than the Sun as it passes centrally across the solar disk. During such an eclipse a bright ring of sunlight remains visible.

How is total eclipse possible if the moon and sun are not the same size?

The fact that eclipses can be total is due to a unique coincidence in the planetary system: the sun and moon appear practically the same size in the earth’s sky. The diameter of the sun is actually about 400 times larger than that of the moon. But since the sun is about 400 times further away from the earth than the moon, both appear to be about the same size.

There are up to four solar eclipses per year (two of which are total), but a specific location on Earth only benefits from a total solar eclipse once every few hundred years. The last total solar eclipse occured in July 2019 and was visible from the Pitcairn Islands, central Argentina and Chile, Tuamotu Archipelago of French Polynesia. The next total eclipse will take place in December 2020 and will be visible to the residents of Southern Chile and Argentina, Kiribati and Polynesia.

Partial solar eclipses are much more common: only part of the sun is covered by the moon. The sun is not completely “swallowed” by the moon, but only “bitten”.

Attention! The observation of solar eclipses should only be done with special glasses. Never look directly into the sun with binoculars or a telescope. The consequences would be severe eye damage up to blindness.

The sun’s corona

During a total eclipse of the sun, the dark moon disk is surrounded by a bright, spectacular ring of rays. That is the corona, the atmosphere of the sun. The gas, which is a million degrees hot, is normally outshone by the bright sun. Only when the moon covers the sun disk can the corona be seen with the naked eye.

Solar eclipses are largely of no interest to astronomers today. The sun can always be observed very well in telescopes and with satellites – also with the help of “artificial” eclipses in telescopes. However, some solar researchers are still examining the areas of the corona close to the sun’s surface during total eclipses. These deep areas of the sun’s atmosphere can hardly be observed otherwise. Ancient records of solar eclipses are of great importance for geophysics. Because knowing in which regions on earth which eclipses could be seen can be used to determine the gradual slowdown of the earth’s rotation.

The moon during solar eclipse

A solar eclipse can only occur on a new moon. However, not every new moon gives us an eclipse. Because the moon’s orbit is inclined by about 5 degrees to the Earth’s orbit around the sun. Therefore, the new moon usually runs from above or below the sun. We only experience a solar eclipse when the new moon is exactly in the plane of the earth’s orbit.

In optimal conditions, total darkness lasts a maximum of seven and a half minutes. The longest eclipse that people currently living on Earth could see took place in China and the West Pacific. It lasted 6 minutes and 39 seconds.

Solar eclipses 2020

Calendar Date	Eclipse Type	Central Duration	Geographic Region of Eclipse Visibility
2020 Jun 21	Annular	00m38s	Africa, se Europe, Asia (Annular: c Africa, s Asia, China, Pacific)
2020 Dec 14	Total	02m10s	Pacific, s S. America, Antarctica (Total: s Pacific, Chile, Argentina, s Atlantic)