Passing the Bus

That\'s what that yellow line is for!

Passing busses and trucks on a Chinese highway, the modern silk road, reminds me of the speed of the shadow of the moon during an eclipse.

There is a lot of traffic on two-lane Chinese highways, including many slow trucks. I was surprised to find that the highways are actually three lanes; traffic in both directions passes by blowing a horn then pulling out and driving down the yellow line. I’m glad we have a professional driver!

I notice that when a truck is going 30 miles per hour and we are passing it at 45 miles per hour it appears, when we look at the truck, that we are moving past it at 15 miles per hour, the difference in the speeds.

The moon orbits the earth at a speed approximately 2000 miles per hour. The shadow of the moon travels along with the moon at the same speed. But the shadow falls upon the surface of the earth which is also moving. At the equator the earth rotates one 24,000 mile circumference in one 24-hour day so the surface moves along at 1000 miles per hour. The surface of the earth and the orbit of the moon travel in the same direction, so just like our bus the shadow of the moon during an eclipse moves across the surface at a relative velocity of about 1000 miles per hour. 

As we stand on the desert waiting for the shadow of the moon to create a total solar eclipse at our position it is exciting to realize that it is traveling toward us at a thousand miles per hour, much faster than the jet aircraft that brought us to China. Luckily a direct hit by a thousand mile per hour shadow doesn’t hurt; in fact, being hit by a total solar eclipse is a wonderful thing!

This explains why the shadow of the moon will take its first bite out of the bottom of the setting sun. The sun appears to move toward the west because the earth surface is rotating toward the east; the moon is also moving toward the east faster so it overtakes and passes the sun in the sky.
Note about the length of totality: Even at a thousand miles per hour it still will take 2 minutes for the 30-mile diameter shadow of the moon to cross over me, and that is why this eclipse will last only 2 minutes.


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4 Responses to “Passing the Bus”

  1. Michael Says:
    January 7th, 2010 at 10:39 am

    So, is the 1000 mph speed primarily from the speed of earth’s rotation to the east, or from the moon’s rotation around the earth. Which body is traveling faster wonder? Fascinating!

  2. Shadows Moving Says:
    August 1st, 2008 at 12:29 am

    hi paul,

    your description made me wonder: can a shadow move faster than the speed of light?

    (propeller head)

  3. Jason Says:
    July 31st, 2008 at 11:06 am

    thanks guys for the hard work!!! another 16 hours to totality!!!

  4. Yeats Jackalope Says:
    July 30th, 2008 at 9:01 pm

    Wow; those buses passing in between two lanes of traffic sounds so Harry Potterish (The Knight Bus). It was cool in the magic world; it sounds downright scary in the real world.