But it is the beauty of an eclipse that lures most observers across oceans.

When the Moon moves in front of the Sun on June 21, day will turn rapidly to night, resembling twilight about a half our after sunset, according to Fred Espenak and Jay Anderson, who develop eclipse information for NASA. Observers along the path of totality will have a rare chance to see stars and planets in the middle of the day.

Jupiter will be the brightest planet visible, according to Espenak and Anderson, and some observers will be able to spot Saturn and Venus.

The total eclipse begins in the Atlantic Ocean at 10:35 UT (6:35 EDT). The Moon's full shadow, known as the umbra, will create this narrow path called totality.

For two hours, the Moon's umbral shadow will migrate across the ocean, making landfall in Angola at 12:36 UT (8:36 EDT). To a viewer at this location and in the center of the 120-mile-wide (193 kilometers) path, the eclipse will last 4 minutes 36 seconds. A partial eclipse, caused by what's called the Moon's penumbral shadow, will occur along a broader path covering all of sub-Saharan Africa.

The total eclipse will then move through Zambia, Zimbabwe and Mozambique before heading out into the Indian Ocean, making a brief touchdown in Madagascar.

While weather can ruin viewing of any eclipse, this one will occur during the winter dry season in southern Africa, among the sunniest regions of Earth at that time. Solar photographers are optimistic.

The mechanics behind a total solar eclipse are simple, though unlikely, and make the event all the more remarkable.

The Sun is about 400 times larger than the Moon, and also about 400 times farther away from Earth. So the apparent size of both the Sun and the Moon, when viewed from Earth, is sometimes the same.

But for a total eclipse to occur, certain things need to fall into place. It's all based on angles and minor changes in distances, which are based on orbits that are elliptical instead of circular. The distances between the Earth and the Sun changes as Earth orbits around its host star once each year, and the same thing happens as the Moon revolves around our planet every month.

To further complicate matters, the Moon's orbit around Earth is tilted, so much of the time the Moon is above or below our line of sight to the Sun, making a total eclipse impossible. When the three bodies line up just right, which only happens at a new Moon and when the lunar orbit is in the proper point of its orbital plane, a total eclipse can occur.

There are at least two solar eclipses per year somewhere on the Earth, researchers say, and the maximum number of total, partial or annular eclipses (when the Moon is too small to block the Sun's whole disk) that can occur in a year is five. Total solar eclipses happen about once every 1.5 years. But the Earth is a big place, and these total eclipses can be rare in any given location.

And one day, total solar eclipses will be a complete thing of the past.

Researchers say the Moon is moving gradually away from Earth, its orbit growing by about a centimeter each year. In a billion years or so, by some estimates, the Moon will be far enough from Earth that its apparent size will be too small to block the Sun out entirely.

Editor's note: While a total solar eclipse can be viewed with the naked eye, you should never look directly at the Sun before or after totality, or at any other time. Severe eye damage will result.