A total eclipse is an event in which the moon and sun are in perfect alignment with a spot on Earth. This occurrence is much more rare than an annual eclipse, in which the sun leaves a halo of light around the moon. A random spot on Earth experiences a total eclipse approximately once every 400 years, explains the University of Virginia Department of Astronomy.
The region in which a total eclipse can be seen is in the umbra of the moon's shadow; no light from the sun can reach this area. Because the sun and moon have the same angular size in the sky, there is only a very small region on the Earth's surface where the entire sun is covered up by the moon.
From geometrical models, the Greeks determined that the sun had to be larger than the moon with a size exactly proportional to its distance from the Earth. During a total eclipse, it is not dangerous to look at the sun at all although the phenomenon is only visible for a few minutes from any given location. During a total eclipse, planets and bright stars that are normally not visible to the human eye can be distinguished in the sky.