Solar Eclipse

Most people probably know what a solar eclipse is. Every now and then, the moon moves between the earth and the sun, and blocks the light from the sun, so it gets kind of dark in the middle of the day. The area on earth where this phenomenon can be observed is usually not that large, and a total solar eclipse does not happen all that often either.

There's a nice picture for you. Now, as far as I know, you always see this corona around the eclipsed sun. I have never seen a picture where the light of the sun was completely blocked, i.e., no corona. Why would that be?

Let's list some facts about the sun and the moon. All units are metric, by the way, no inches, miles, or furlongs here.

The distance from the moon to the earth is approximately 384,000 kilometers.
The diameter of the moon is approximately 3,476 kilometers.

The distance from the sun to the earth is approximately 149,600,000 kilometers.
The diameter of the sun is approximately 1,390,000 kilometers.

We can dive into trigonometry here, but it's not really necessary. If we calculate the ratio of diameter to distance, we find for the moon: 3,476 / 384,000 = 0.00905, and we find for the sun 1,390,000 / 149,600,000 = 0.00929.
That's pretty close. The ratio is slightly smaller for the moon than for the sun, therefore the moon does not cover the sun completely (there may be some optical effects here as well like rays bending around the moon, but I can't be bothered to look into that).

Let's look at another picture. No, it is not to scale, not even close.

The point is that the apparent diameter of the moon and sun are roughly the same, so that's why the moon can eclipse the sun almost completely.

But, that is a coincidence! There is no law of physics that dictates these ratios, it is all by accident. As a matter of fact, the moon is moving away from the earth, so if you wait long enough, there will be no more total eclipses. It'll take a while, but still...

Not so obvious, I think, although we rarely stop to think about it.

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