What is an eclipse of the Sun? What causes eclipses and why?

If the Moon's inner or umbral shadow sweeps across Earth's surface, then a total eclipse of the Sun is seen. The track of the Moon's umbral shadow across Earth is called the Path of Totality. It is typically 10,000 miles long but only about 100 miles wide. It covers less than 1% of Earth's entire surface area. In order to see the Sun become completely eclipsed by the Moon, you must be somewhere inside the narrow path of totality.

The path of a total eclipse can cross any part of Earth. Even the North and South Poles get a total eclipse sooner or later. Just one total eclipse occurs each year or two. Since each total eclipse is only visible from a very narrow track, it is rare to see one from any single location. You'd have to wait an average of 375 years to see two total eclipses from one place. Of course, the interval between seeing two eclipses from one particular place can be shorter or longer. For instance, the last total eclipse visible from Princeton, NJ was in 1478 and the next is in 2079. That's an interval of 601 years. However, the following total eclipse from Princeton is in 2144, after a period of only 65 years.