Solar eclipses happen several times a year. During a Solar eclipse the Sun darkens and in some occasions it is totaly blocked by the Moon. In those cases reduction of visible light is great and can look like twilight. In ancient mithology solar eclipses were believed to be a bad sign. It was believed they were sign of the end of world.
The reason why solar eclipses happen is movement of the Moon around the Earth. A solar eclipse can happen only during New Moon phase and when the Earth passes through Moon's shadow. It happens when the Moon and the Sun are on opposite sides of the Earth.
The sun shines on the Moon and it produces a shadow on the opposite side of the Moon, just like a shadow on the street. There are two types of shadows: penumbra (light shadow) and umbra (dark shadow). These two types of shadows are responsible for different types of the solar eclipses. Also, the Moon's path around the Earth is not circular but eliptic. This causes the Moon to be at a different distance from the Earth producing different types of eclipses.
There are three types of solar eclipses: partial eclipse, annular eclipse and total eclipse.
Partial eclipses happen when the Moon covers only part of the Sun. This happens when the Moon is at it's greatest distance from the Earth. The picture below shows what we can see during this type of eclipse. The moon covers part of the Sun leaving the rest visible. During this type of eclipse the reduction of incoming light is not very big.
Annular eclipses happen when the Moon covers almost the entire Sun but there is still a ring around the Moon that is left uncovered. This happens when the Moon is at it's average distance from the Earth. The picture below shows what we can see during this type of eclipse. The Moon covers almost the entire Sun leaving a ring of light visible. During this type of eclipse the reduction of incoming light is very big.
Total eclipses happen when the Moon covers the entire Sun. This happens when the Moon is at it's smallest distance from the Earth. The picture below shows what we can see during this type of eclipse. The Moon covers the entire Sun greatly reducing incoming light. Visible light is the same as during twilight. During the total eclipse we can see corona, Sun's atmosphere, and prominences, active parts of the Sun's surface.
During a total eclipse the shadow is typically 200km wide and a total eclipse can last up to seven minutes. These numbers are lower most of the time. Outside of the path of the total eclipse, the partial eclipse can be observed.
The next total eclipse visible from the USA will happen on August 21. It will be visible from many states as a total eclipse and as a partial eclipse from the other states. After that there will be no total eclipse visible from the USA until April 8, 2024. The same states as those during August 21, won't see an eclipse for the next 400 years.
It is important to follow safety tips given by proffesionals to watch an eclipse. The best and safest way to watch an eclipse is by using special glasses. They reduce light coming to the eye by a great amount leaving enough light to see the eclipse. Also, it is possible to use some devices (binoculars, telescopes...) to project an image of the Sun onto paper and to watch an eclipse this way. The most important thing to remember is: NEVER LOOK DIRECTLY AT THE SUN WITH NAKED EYES OR THROUGH ANY TYPE OF EQUIPMENT!