A total eclipse of the Sun, also called "totality" happens rarely, only about twice per decade, and only in those parts of the world touched by the Moon's shadow as it speeds across Earth's surface.
The term eclipse refers to the complete or partial blocking of a celestial body by another body and can be used to describe a wide range of phenomena. Solar and lunar eclipses--occur any time the Sun, the Moon, and Earth are all positioned in a straight line. This arrangement is uncommon because the plane of Earth's orbit around the Sun lies at an angle different than the angle of the plane of the Moon's orbit around Earth. Thus, the Moon usually is located just above or below the imaginary plane of Earth's orbit.
An eclipse may be partial, total, or annular (where one object covers all but the outer rim of another); and it may be barely noticeable or quite spectacular. Only twice per year, do the planes of Earth's orbit and the Moon's orbit coincide, signaling an eclipse season. In addition, only during a small percentage of eclipse seasons do total eclipses occur.
A solar eclipse takes place when the Moon's orbit takes it in front of Earth, blocking the Sun from view. A lunar eclipse is different in that Earth passes between the Sun and the Moon, casting a shadow on the Moon. Another way to remember the difference between the two kinds of eclipses is that one can witness a solar eclipse only during the day and a lunar eclipse only at night.
Partial and Total Eclipses
During a solar eclipse, the Moon's shadow sweeps across Earth. The shadow has two parts: the dark, central part called the umbra, and the lighter region surrounding the umbra called the penumbra. If one is standing in a place covered by the umbra, the Sun will be blocked completely from view, meaning that a total eclipse is occurring. If one happens to be in the penumbra, only a part of the Sun will be visible, a partial eclipse.
The type of solar eclipse also depends on the distance of the Moon from Earth. The Moon's orbit, like Earth's orbit, is elongated (stretched out). Thus, at some points along its orbit, the Moon is closer to Earth than at others. In order for the umbra to reach Earth and block out the Sun, the Moon must be at a close point on its orbit. If the Moon is too far away, meaning that it appears smaller than the Sun, one of two things may happen. First, only the penumbra may reach Earth, creating a partial eclipse. The other possibility is that the Moon appears to be centered within the Sun. In this case, called an annular eclipse, the Sun is seen as a ring of light around the silhouette of the Moon.
One may wonder how the Moon, a relatively small object, can block out something as massive as the Sun. The reason is that, although the Sun is four hundred times larger than the Moon, it also is four hundred times as far away. Thus, to people on Earth, the Sun and the Moon appear to be about the same size.
The first stage of a solar eclipse is called first contact. At that point in time, the Moon just begins to cover one edge of the Sun. As the Moon shifts across the Sun's face, the sky begins to darken. At the same time, bands of light and dark called shadow bands race across the ground. Just before second contact, when the Moon completely blocks out the Sun, a final flash of light can be seen at the edge of the Sun. This effect is called the diamond ring.
Then, at totality, all sunlight is blocked, the sky turns dark, and some of the neighboring planets and brighter stars are visible. During this period, the Sun's corona is visible. The weak light emitted by the corona (about half the light of a full Moon) normally is not visible because it is overpowered by the light of the Sun's surface. During a solar eclipse, however, the corona can be seen shining around the edges of the Sun. A few short minutes later the Moon passes to the other side of the Sun and the eclipse is over.
Observing a Solar Eclipse
Never look directly at the Sun--even during an eclipse! Many people are tempted to stare at the thin crescent of sunlight visible during an eclipse, but even that level of radiation can cause permanent eye damage. Unless one has a special filter for viewing the Sun (improper filters also have resulted in eye damage), one can safely watch an eclipse using the method described here.
All that is needed are two cards (about the thickness of index cards)--at least one of them with a white surface. Make a small hole in one card by piercing it with a pin and wiggling the pin around a little to enlarge the hole. Hold this card up so the sunlight enters the hole. Hold the other card (with the white surface facing up) below the first, so the image of the Sun falls on it. Adjust the distance between the two cards to bring the image into focus. Watch the bottom card to follow the progression of the eclipse.