Annular Eclipse: The Changing Size of the Moon

Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

The annular eclipse is a type of solar eclipse that involves the visibility of the sun's photosphere as a bright ring around the moon. Explore a comparison of sun and moon size, the changing size of the moon, types of solar eclipses, and why annular eclipses occur. Updated: 10/26/2021

Comparing the Size of the Moon and Sun

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.

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  • 0:01 Comparing the Size of…
  • 0:40 The Types of Solar Eclipses
  • 2:05 Why Does an Annular…
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The Types of Solar Eclipses

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.

Why Does an Annular Eclipse Occur?

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.

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