Eclipse is the blocking of light, partially or completely, from one celestial body reaching to another by a third celestial body located in between them.
Eclipses are of two kinds: lunar and solar.
A lunar eclipse occurs when the Earth blocks the light of the Sun reaching the Moon.
The moon starts disappearing as it goes into the penumbra (lighter shadow) of Earth, and suddenly glows reddish-orange when it enters the umbra (darker shadow).
The Moon appears red when in the umbra, because the only light reaching the moon has to go through Earth’s atmosphere, which scatters away red light the least.
Solar eclipses occur when the Moon passes between the Earth and the Sun, thus blocking out the light of the Sun for some portions of the Earth’s surface. Consequently, solar eclipses can only occur during the phase of the moon that we call “new moon.” We thus might expect eclipses to take place every month, except for the fact that the moon’s orbit is tilted about 5 degrees with respect to the earth’s orbit. Eclipses can only take place when new moon coincides with the moon’s crossing the plane of the earth’s orbit, and on average this occurs about once every six months.
Types of Solar Eclipses
Partial eclipses occur when the Moon only covers up a portion of the Sun. These can range from very tiny eclipses where only a small portion of the Sun is covered, up to the eclipses where almost the entire Sun is obscured. Different regions of the Earth’s surface will see different amounts of the Sun covered up during a partial eclipse.
During some eclipses the Moon passes directly in front of the Sun as seen from a narrow strip, or “path,” along the Earth’s surface. Sky-watchers located within this path — usually a couple of hundred miles wide see either a total eclipse or an annular eclipse, depending upon how far the moon is away from the earth. (The moon’s orbit around the earth is not a perfect circle, and thus is farther from the earth than at other times.) Sky-watchers who are not located within the part but close to it witness a partial eclipse; the closer they are to the path, larger the partial eclipse. “Totality” or “annularity” only lasts at most a few minutes from any particular location within the path; this is preceded by an ever-increasing partial eclipse for approximately 1.5 hours prior to the main event itself, and followed by a steadily decreasing partial eclipse for 1.5 hours afterward.
Total eclipses, which occur when the moon is relatively near to the earth, are by far the most spectacular. The moon covers up the sun entirely, except for the corona, the Sun’s hot but very tenuous outer atmosphere. For the few minutes of totality day is briefly turned into night, and the brighter planets and stars become visible. Why is a total solar eclipse such a rare event? First of all, eclipses do not occur every month during a new Moon or a full Moon. This is because the orbit of the Moon is tilted by about five degrees with respect to the Earth’s orbit, so that usually the Moon passes slightly above or below the line between the Sun and the Earth. Thus at most new and full Moons, the shadows miss their mark and no eclipse occurs. Only about every six months, during an eclipse season, are the conditions right for a lunar or solar eclipse.
The total phase of a solar eclipse is very brief. It rarely lasts more than several minutes. Nevertheless, it is considered to be one of the most awe inspiring spectacles in all of nature. The sky takes on an eerie twilight as the Sun’s bright face is replaced by the black disk of the Moon. Surrounding the Moon is a beautiful gossamer halo. This is the Sun’s spectacular solar corona, a super-heated plasma two million degrees in temperature. The corona can only be seen during the few brief minutes of totality. To witness such an event is a singularly memorable experience which cannot be conveyed adequately through words or photographs.
Annular eclipses occur when the moon is relatively far away from the earth, and thus does not appear large enough to cover up the entire sun. A thin ring or “annulus” of sunlight remains around the moon.
PATTERN OF ECLIPSES
Two different cycles of the Moon determine the pattern of eclipses over time.
One cycle — the familiar monthly lunar phases — is easy to understand: a solar eclipse may occur only at a new Moon, as the Moon passes between the Earth and the Sun, casting its shadow toward the Earth. The other cycle involves the gradual shift in orientation of the Moon’s orbit. Only when these two cycles are favorably combined (about every six months) can a solar eclipse occurs.
We have a new Moon every month, but we don’t have an eclipse every month. Usually the Moon’s shadow passes completely above or completely below the Earth. This is because the Moon’s orbit is tilted at about a five-degree angle to the Earth’s orbit, so that the Moon usually passes above or below the direct line of sight between the Earth and the Sun. Only at those times when the new Moon is near one of its nodes can a solar eclipse occur. (The nodes are the two points where the Moon’s orbit intersects the plane of the Earth’s orbit, the ecliptic.)
For a solar eclipse to occur, the new Moon must be close enough to the ecliptic plane so that its shadow will touch some part of the Earth. As it turns out, when the new Moon appears within 18-3/4 days before or after the alignment of a node, a solar eclipse will take place. This creates a 37-1/2-day time window for eclipses, called an eclipse season, when the conditions are favorable for an eclipse to occur. If the lunar nodes were stationary with respect to the stars, each node would be lined up between the Earth and the Sun at the same time each year, and eclipses would occur at the same two periods of time every year, six months apart.
In fact, this is what almost happens, except that the nodes of the lunar orbit are gradually shifting their orientation in space. By the time one node is in line with the Sun again, it has regressed slightly. The alignment happens 18.6 days sooner than if the nodes were not moving, creating the shorter eclipse year (about 346.6 days). This regular regression of the Moon’s nodes is the other cycle that determines the patterns of eclipses over time
SCIENCE DURING ECLIPSES
During a total solar eclipse the light from the sun’s “surface” is completely blocked out. We are then able to see solar phenomena that are otherwise invisible the rest of the time. For example, the sun’s “corona,” or very hot and thin outer atmosphere, becomes visible. “Prominences,” or large loop-like structures of hot gas, may also be visible along the disk of the eclipsed sun. These features, along with changes in the structure of the corona, are related to the sun’s magnetic field which in turn is related to the 11-year sunspot cycle, and observations of these phenomena during eclipses — and comparing these from one eclipse to another — can provide valuable clues to the goings-on within the sun. Since these changes in the sun can affect conditions here on Earth — for example, changes in the ionosphere which can affect communications; weather and climate; and so on — observations of the solar environment during eclipses can aid our understanding of what is happening here on Earth.
The near-dark environment produced by a total solar eclipse allows us to make observations of astronomical objects near the sun at a time when they would otherwise be invisible. One of the earliest and most notable observations in this regard was made during a solar eclipse in 1919, when measurements of the positions of stars near the sun were found to be changed in the manner predicted by Einstein’s theory of general relativity.
Otherwise invisible comets have occasionally been detected near the sun during total solar eclipses. The Solar and Heliospheric Observatory (SOHO) spacecraft has recently been detecting small comets near the sun at the rate of approximately one per week.