An annulus is the region between the smaller and the larger of two circles that share a common center. The area of an annulus is the difference in the areas of the larger circle of radius R and the smaller one of radius r :

 

    A = πR ² – πr ² = π(R ² – r ²).

 

The word "annulus" is borrowed from the Latin anulus or annulus meaning "little ring". Washers, for example, take the form of an annulus. An annular eclipse occurs when the Sun and Moon are exactly in line with Earth, but the apparent size of the Moon is smaller than that of the Sun, so that the Sun appears as a bright ring surrounding the dark disk of the Moon.