Our Sun and Our Moon

My friend Cynthia’s house is on a north-south ridge in Poway, California. From her front yard she can look east over the mountains. From her back yard she can see far into the distance over the Pacific, on a clear day. She posted these two pictures a couple of days ago when the moon was full. At the same time she saw the rise of the full moon and the sunset over the ocean. Here are her pictures:

Cynthia's Moon and Sun
[click for picture credit]
Seeing this picture I marveled about the amazing beauty of our world. Especially our moon. We on Earth have a great, big moon that we can enjoy, very unlike any other moon in the solar system. It reminds me of the great old Jerry Jeff Walker song about the Luckenbach Moon:

They won’t believe we have such a big moon for such a small town!

We don’t just have a big moon for a small town in Texas, we have a big moon for a small planet. Scientists have speculated that the fact that we have a big moon is one of the most important contributing factors to life having started on Earth. A big moon causes strong tidal forces. Tides cause tidal pools. And tidal pools, for a billion years early in the life of our planet, provided just the right environment exposing microscopic amino acids to alternating periods of dry and light and darkness and wetness – an environment conducive to the formation of life.

And then there is the size of our moon as it relates to the sun. The moon and the sun on Earth look about the same size, and they are. The moon appears sometimes just a bit smaller than the sun. That occurs when it’s farther away in its elliptical orbit. If a solar eclipse happens at that time, there is a ring of fire around the dark disk of the moon. This type of eclipse is called an annular eclipse. The English word annular comes from the Latin word anulus, which means ring. At other times, when the moon is closer to the Earth in its orbit, it appears slightly bigger than the sun, and if an eclipse occurred then, the moon would cover the sun completely.

The fact that the sun and the moon appear the same size in our sky is completely and amazingly coincidental. The moon just happens to be 400 times smaller than the sun and the sun is 400 times farther away from us than the moon. So they just happen to look the same.

This wasn’t always the case in the history of our Earth. Since the Earth spins faster than the moon orbits, the tidal bulge raised on the Earth pulls on the lagging moon. This gradually raises its orbit and it slows down our day. Every year the orbit of the moon grows by about 3.8 centimeters and our day lengthens by about 0.000015 seconds.

This means that when the velociraptors, thought to be one of the most intelligent dinosaurs, ruled about 70 million years ago, when they sat on their back porches sipping their Cretaceous cocktails and watching the moon, the moon appeared much bigger. It was 2,660 kilometers closer to the Earth than it is now. Velociraptors would never have experienced an annular eclipse.

Likewise, in a few million years hence, the moon will be much farther away and it will therefore appear smaller. The days of the full solar eclipses on Earth are numbered.

We humans are lucky to be here on Earth just during this short astronomical time window where the moon and the sun appear to be the same size. The odds of that happening in our solar system are – well – astronomical. The odds of a big moon on another small planet, where the moon appears the same size as the star, are – well – astronomical astronomical.

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