Mechanisms of a Solar Eclipse



All solar eclipses involve the moon passing between the Sun and the Earth, and in the process, casting a shadow upon the Earth’s surface. However, the type of eclipse that can be observed from a given location is dependable on the way the moon passes between the Earth and the Sun, whether it passes directly, or only partly between the Earth and Sun.

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Distances are not to scale, and we have idealized the illustration by fixing the Earth in space relative to the Sun. In reality, the Earth will be in motion around its orbit as the Moon moves around the Earth.


There is an amazing cosmic coincidence that the Moon is about 400 times closer to the Earth than the Sun. At the same time, the Sun is about 400 times larger than the Moon. What this means is that the size of the Sun and Moon as seen from the surface of the Earth is about the same in the sky. When viewed from the surface of Earth, both the moon and sun appear to be about one half degree in size – that is, about the size of your thumbnail when you extend your arm. In astronomical terms, the Sun and Moon have roughly the same angular size. This makes it possible for a solar eclipse to occur. No other planets in our solar system enjoy the same one-to-one ratio.


For a solar eclipse to occur, there are firstly 2 factors that must be fulfilled. Firstly, There must be a new moon. Secondly, the New Moon must be in the ecliptic, on the Line of nodes of the intersection of the ecliptic plane and the orbital plane of the moon. There are 2 situations that a solar eclipse can occur.


Case 1: Shadow misses the surface of the Earth


In the first situation, the New Moon does not lie exactly on the orbital plane of the Earth between the Sun and the Earth, but is above or below the Earth’s orbital plane. This causes the moon’s umbral shadow to miss the surface of the Earth and fall away into space.



In this case, as portrayed above, the umbra does not reach the Earth at all, resulting in a partial eclipse where the umbra will either pass North or South of the Earth.


Case 2: Shadow falls across the surface of the Earth


In the second situation, the New Moon coincides with the time when the moon crosses the orbital plane of the Earth and the umbra strikes the Earth, falling across the Earth’s surface and tracing out a path of totality (if it's a total solar eclipse) on the Earth's surface. The moon will also block off the face of the Sun totally enabling observers within the area of the umbra to experience a total solar eclipse.


The explanation above seems to suggest that solar eclipse happens every month at new moon. But of course, in reality total eclipses happen infrequently. So why doesn't the moon get between the sun and the earth every month at new moon and produce an eclipse? Because the explanation above has over-simplified matters. The real situation is a little more complicated. We need to discuss the orbit of the earth around the sun and the orbit of the moon around the earth. The orbits of both are not circles, but rather slightly oval-shaped ellipses. Also, these orbits do not lie parallel to each other in the same plane.

As the earth orbits the sun, taking one year to complete one circuit, it appears to us on earth that the sun moves around our sky once against the background of stars. Here's an analogy--If you walk around a campfire (the sun) looking at your friends on the other side (the stars), to you it would look like the campfire moves past your friends. Likewise, from earth, it looks like the sun moves against the background of stars, making one circuit of the sky in one year.


If the sun could draw a line as it moved against the stars, we would see Image: Sun & Moon cross paths a great circle called the ecliptic. If we could ask the moon to also draw a line in the sky as it orbited the earth, we'd notice that the two lines would be close to each other, but the moon's path is tilted about 5 degrees to the path of the sun.


This is why the moon doesn't eclipse the sun every month. Most of the time, the moon passes over or under the sun. An eclipse can happen only when both the sun and the moon arrive near one of the crossing points (these are called nodes). There are two of these nodes on opposite sides of the sky, one where the moon crosses from south to north, and one where the moon passes from north to south. Since there are two crossing points in the sky, eclipses happen during two "eclipse seasons" separated by about six months.

Diagram: Ecliptic


The sun does not have to be exactly on the node when the moon arrives there, only close enough for the moon to block some portion of the sun. This leaves a "window" of about 18.75 days before and after the sun gets to the nodes. During this 37.5-day period, the moon can cause an eclipse. Since the moon takes 29.5 days to go from new moon to new moon, this means that an eclipse of some kind is guaranteed about every six months.