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Did you know that the Moon is about the same size as the Sun? Well, it is, but only when viewed from the Earth. In reality, the Sun is actually about 400 times bigger than the Moon.
But because the bigger Sun is much farther away from us than the Moon, it appears to be as big as our Moon in the sky. This is what allows for a total solar eclipse to occur on Earth in the first place.
However, these sizes are only a rule of thumb as viewed from Earth. This lesson will explain why and how that influences the kind of solar eclipse you can see, namely the annular eclipse.
There are three different types of solar eclipses. To demonstrate them, get a round piece of black cardboard paper and a flashlight. The paper should be the same diameter as the flashlight's round end, where the light is coming out of.
Turn the flashlight on and turn it towards you, but be careful not to look directly into the flashlight. You are like the Earth, the flashlight is like the Sun, and the round cardboard paper is like the Moon. If you take the cardboard paper and move it slightly over the flashlight's lit end, it will represent a partial solar eclipse.
If you take the piece of paper, the Moon, and move it so it completely covers the flashlight, that's a total solar eclipse. During a solar eclipse, the period in which the Sun's photosphere is entirely hidden by the Moon is known as totality, and the photosphere is the visible surface of the sun. Those are the two types of solar eclipses you have surely heard of before.
But, did you know there's another kind of solar eclipse? It's called the annular eclipse. The annular eclipse is a kind of solar eclipse where the Sun's photosphere is visible around the edge of the Moon as a bright ring.
This is like taking a cardboard circle that is smaller than the diameter of a flashlight and centering it on the flashlight. You'll be able to see a lot of light protruding around the edge of the cardboard.
The answer to why the annular eclipse occurs is pretty interesting. To explain why, you need to know that there's something called the angular diameter, which is the angle that's made by two lines starting at an observer and ending on the opposite sides of an object. The Moon's angular diameter is about half a degree. This angular diameter, or apparent size of the Moon on our sky, will change depending on where the Moon is located in its slightly elliptical orbit.
If the Moon is at perigee it will appear larger than if it is at apogee. Perigee is the point at which the Moon is closest to the Earth, and apogee is the point where the Moon is most distant from the Earth during its orbit. Remember, 'a' as in away is for 'a' as in apogee, and proximate, the 'p' in proximate, is for the 'p' in perigee.
In addition to all of that, don't forget that the Earth also orbits the Sun and its orbit is also slightly elliptical. This will change the sun's angular diameter by a bit as well.
Thus, if the Moon is at apogee during totality, its angular diameter will be smaller than the angular diameter of the Sun. This is like putting a smaller cardboard disk over the flashlight and you get yourself an annular eclipse. In an annular eclipse, the ring of photosphere that is visible around the edge of the Moon is known as the annulus.
During a solar eclipse, the period in which the Sun's photosphere is entirely hidden by the Moon is known as totality, and the photosphere is the visible surface of the Sun. These are two important concepts to keep in mind with respect to an annular eclipse.
An annular eclipse is a kind of solar eclipse where the Sun's photosphere is visible around the edge of the Moon as a bright ring. This occurs because the Moon's angular diameter, which is the angle that's made by two lines starting at an observer and ending on the opposite sides of an object, is smaller at apogee than at perigee. Perigee is the point at which the Moon is closest to the Earth, and apogee is the point where the Moon is most distant from the Earth during its orbit.
If the Moon is at apogee during totality, its angular diameter will be smaller than the Sun's and an annulus will appear during this type of eclipse. An annulus is the ring of the photosphere that is visible around the edge of the Moon.
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