Solar and Lunar Eclipse

An eclipse is the partial or total blocking of the light of one object by another.  In the solar system, relative positions of the Sun, Moon, and Earth create solar eclipses and lunar eclipses.

Frequency of Eclipses

Perfect alignments of the Sun, Moon, and Earth are relatively uncommon, because the plane of Earth’s orbit around the Sun (ecliptic plane) is not the same as the plane of the Moon’s orbit around Earth.  Thus, during the new moon or full moon phases when an eclipse might be possible, the Moon is usually located just above or below the straight line that runs between Earth and the Sun, so no eclipse occurs.  All three bodies viz. Earth, Moon, and Sun line up just right about twice a year.

Lunar Eclipse

A lunar eclipse occurs when Earth passes between the Sun and the Moon in such a way that the Moon moves into Earth’s shadow. When a partial lunar eclipse is going on, the curved shadow of our planet is apparent on the Moon’s face; the Moon looks kind of like it is in a crescent phase, but the terminator line (the line between light and dark) is not curved the same way. When a total lunar eclipse is happening, the entire Moon is in Earth’s shadow, and the Moon looks full, but glows only faintly red. Why?

The reason is as follows: Earth’s atmosphere is dense enough to act a little bit like a lens, so it refracts a small amount of sunlight shining through it toward the Moon. This small fraction of light, which is mostly red because that is the color of light that refracts best, bounces off the Moon’s surface and comes back to Earth. Before and after totality, the direct sunlight reflected off the Moon is so strong by comparison that it drowns out this refracted light, so we normally cannot see it with our unaided eyes. During totality, however, the Earth-atmosphere-refracted light is quite visible as a soft reddish glow.

Solar Eclipse

A solar eclipse happens when the Moon is directly in line between Earth and the Sun. The Moon’s shadow sweeps across Earth’s surface; at those places where the shadow lands, an eclipse is seen.

Like Earth’s shadow, the Moon’s shadow consists of two parts: a dark, central region called the umbra, and a lighter region called the penumbra that surrounds the umbra. Under the penumbra, a partial solar eclipse occurs. Under the umbra, a total eclipse or an annular eclipse is seen.

Since the Moon travels in a slightly elliptical orbit around Earth, rather than in a perfectly circular path, its distance from Earth is not always the same.  If the Moon’s umbra falls on Earth’s surface when the two bodies are at a closer point in the Moon’s orbit, there is total solar eclipse. But if the Moon happens to be too far away from Earth at that time, the Moon does not cover enough of the sky to block the Sun’s rays entirely. In that case, the Sun is seen as a ring, or annulus, of light glowing around the silhouette of the Moon.

During totality of a solar eclipse, the Sun looks like a perfectly black disk surrounded by glowing light. This light is actually the Sun’s corona, which is invisible under normal circumstances because the Sun is so bright. Away from the corona, the sky is dark, so planets and stars that ordinarily could be seen only at night become visible.

Frequency of Solar Eclipse at a particular location on earth

The entire process of a solar eclipse, from the beginning of partial coverage until the end, usually takes about an hour. However, the totality of solar eclipse lasts at most only a few minutes. Most total solar eclipses last between 100 and 200 seconds— just about two to three minutes. Furthermore, total solar eclipses can be observed only from narrow bands on Earth’s surface, and these bands change with each eclipse. In any given location on Earth, therefore, a total solar eclipse may appear only once every few centuries.

Why Moon blocks Sun so perfectly during solar eclipse?

The Moon’s diameter is just under 400 times smaller than the diameter of the Sun. Coincidentally, the Moon’s distance from Earth is also just under 400 times smaller than the Sun’s distance from Earth. That is why the Moon covers almost exactly the same amount of sky, when viewed from Earth’s surface, as the Sun. We are able to see only Corona during Total solar eclipse.