Around 1 p.m. on August 21 darkness will spread across the land, here in Missouri.

Around 1 p.m. on August 21 darkness will spread across the land, here in Missouri.  This event would have been terrifying for ancient cultures, when darkness falls in the middle of the day.  But nowadays, instead of fearing the darkness as an omen of impending doom, we celebrate the eclipse, since we know it is just the moon passing in front of the sun, casting a shadow that will pass through Missouri as it travels eastward across the middle of the United States.  To see this rare event you will have to travel either a little north or a little east of Rolla.  As part of the anticipation of this celestial marvel, a number of installments of this series, over the coming weeks, will provide a little information about the eclipse, as well as some interesting information about origin and history of our moon, and why total solar eclipses will someday be a thing of the past.

Today’s installment will discuss a few questions about the eclipse itself.  First, since the moon goes around the Earth about once a month, one might wonder why there isn’t an eclipse somewhere on Earth every month, since the moon must pass between the Earth and sun every time it goes around.  The reason there isn’t a monthly eclipse is that the orbit of the moon around the Earth is tipped slightly from the orbit of the Earth around the sun.  So most of the time when it moves between the Earth and sun, the moon is either above or below the line between the Earth and sun, so the moon’s shadow misses the Earth.

The moon is pretty large, over two thousand miles across, so another question is why the width of the total eclipse path on Earth is so small?  It will only be about sixty miles across for this event.  The reason for this difference is that the sun is much larger than the moon.  You can see this effect for yourself by holding up a coin in front of a light bulb across the room.  If you hold the coin out to where it is just big enough to cover the light completely you will see that if you move your eye just a tiny bit you will see part of the light, making it a partial eclipse.  Since the moon’s orbit is not a perfect circle around the Earth, sometimes when we get an eclipse the moon is a little too far away to totally block out the sun and it leaves a ring of light around the edge of the moon.  This event is called an annular eclipse.  Luckily for us, when our upcoming event occurs, the moon will be close enough to the Earth to give us a total eclipse.

A final question for this installment is, since the sun rises in the east and sets in the west, why does the path of the eclipse move from west to east?  The answer is that the path of the eclipse is determined mostly by the motion of the moon as it orbits the Earth.  When you look up at the moon at night, it appears to just hang in the sky.  But it is actually traveling over two thousand miles an hour in its orbit around the earth and it travels east.  Since it is almost a quarter million miles from earth, we can’t tell it is moving that fast, but its shadow will cross the North American continent from Oregon to South Carolina in about an hour and a half.  With the shadow moving this fast, it means the period of total eclipse will be at most a little over two minutes.  So enjoy it while you can.