Sequence of the total solar eclipse on February 26, 1998, southeast of Aruba.  Photography courtesy of Dr. Mike Reynolds, copyright ©2000

A total solar eclipse (the almost exact covering up of the Sun by the Moon) is the result of an unusual set of physical circumstances. The Sun and the Moon differ significantly in actual size. But their distances from Earth almost exactly compensate, making the "apparent" sizes of these celestial bodies in our sky surprisingly similar. As seen from Earth, they each appear to be about one-half degree across, or roughly half the width of your pinky held at arm's length.

Total Solar Eclipse, February 26, 1998, southeast of Aruba.  
Photography courtesy of Dr. Mike Reynolds, copyright ©2000

The diameter of the Sun is about 400 times greater than that of the Moon. But the Sun, at an average 150,000,000 km from the Earth, is much farther away than the Moon, which is on average 384,000 km from Earth. On average, the Sun is about 390 times farther away from the Earth than the Moon. This makes the "apparent" sizes of the Sun and Moon within 2.5% of each other.

In order to determine when a solar eclipse will occur, we need to know the paths the Sun and Moon travel through the sky relative to one another. These paths are determined by the motion of the Earth about the Sun and of the Moon around the Earth. If these orbits were in the same plane, we could expect a solar eclipse at every "New Moon"--the time when the Moon passes nearest the Sun and its illuminated side is not visible to us.

However, the two orbits are not in the same plane. The Moon's orbit around Earth is tilted about 5º with respect to the Earth's orbit around the Sun. Therefore, the conditions for a solar eclipse will only occur when the Moon crosses the Earth's orbital plane at New Moon. 

The two cases that occur at New Moon are illustrated below.  In the first case the Moon is at a point in its orbit that carries it off of the Earth's orbital plane at the time of New Moon, and the Moon's umbral shadow misses the surface of the Earth.

In the second case, New Moon coincides with a time when the Moon crosses the Earth's orbital plane, and its umbra strikes the Earth, causing observers within that shadow to experience a total solar eclipse.

A solar eclipse doesn't last long. The eclipse begins as soon as the penumbra crosses an observer's location. The speed of the Moon's shadow across the Earth's surface is largely due to the Earth's rotation on its axis. At the equator, the Earth rotates at about 1660 kilometers per hour (over 1000 miles per hour) Because the Moon's umbra at the Earth's surface is never more than 270 km across, a person watching a total eclipse will never be within the umbral shadow for more than a few minutes. At higher latitudes, where the motion of the Earth's surface due to rotation is slower, total solar eclipses can last longer than at the equator.

In an annular solar eclipse, the viewer is in the Moon's shadow, but the apparent (angular) size of the Moon is slightly smaller than that of the Sun. The outermost rim of the Sun can be seen peeking out around the entire perimeter of the Moon's dark disk, giving the appearance of a ring of bright light. During an annular eclipse, the Moon's umbral shadow falls short of reaching the Earth's surface. Because the Moon's orbit is an ellipse (not perfectly circular), at different points in its orbit it is alternately closer to and farther away from the Earth. Annular eclipses occur when, during a solar eclipse, the Moon is farther away from the Earth than its average distance. In these cases the Moon's apparent size may not be large enough to completely cover the Sun, and an annular eclipse results. 

A partial solar eclipse occurs when the viewer is within the Sun's penumbral shadow, an area in which only part of the Sun's disk is eclipsed.

Image:  The partial solar eclipse on December 14, 2001, as seen through Chabot's 8" refractor, Leah.  Click on the image for a 2 MB GIF animation.