Cumberland Times-News

Bob Doyle - Astronomy

January 19, 2013

We are spinning at 795 miles per hour

Most of us know that the Earth’s rotation or spinning causes our progression of day and night. If asked how long it takes the Earth rotate, most would quickly say 24 hours, the length of our day/night cycle.

But this 24 hour period is based on the appearance of the sun. As we orbit the sun, the sun appears to drift eastward relative to the stars about a degree each day.

So to bring back the sun (to the same relative place in the sky), the Earth has to turn for about 4 more minutes. The Earth’s rotation period is 23 hours and 56 minutes and 4.2 seconds.

Here’s how this period can be verified. Suppose on a clear evening, you have a powerful telescope with crosshairs. You put a star on the center of the cross hairs and note the time.

Because of the Earth’s rotation, the star would leave the field as the telescope magnifies our spinning. If you left the telescope in place out all through the next day, you would see on the next evening that same star on the cross hairs nearly four minutes earlier than the previous night.

This time is called the Sidereal day. Observatories have special clocks set to the Earth’s actual motion that shows the Sidereal time.

Over the course of one year, this difference between our solar day and sidereal day would add up to 23 hours and 56 minutes and 4 seconds.

So during the course of a year, the Earth actually spins around roughly 366 and a quarter times, one more spin than the actual number of days in our calendar.

How fast are we rotating? The rotation speed varies with latitude. The highest rotation speed is at the equator with a speed of 1,038 miles per hour. The speed is proportional to the cosine of the latitude.

The Tri-State area is close to latitude 40 degrees north. Cosine of 40 degrees is 0.766. This gives us a rotation speed of 1038 x 0.766 = 795 miles per hour.

Hawaii with a latitude of 21 degrees north has a rotation speed of 969 miles per hour. London at latitude 52 degrees north has a rotation speed of 639 miles per hour. A jet plane flying over these areas due west with the same speed as the rotation speed could “keep” the sun from setting.

Of the regular planets with solid surfaces, the Earth has the fastest rotation speed. Mars’ speed of rotation at its equator is 537.4 miles per hour. Mercury spins only 163.5 miles per Earth day while Venus creeps along with a speed of 97.22 miles per Earth day backwards (relative to the spin of the other inner planets). (All of these speeds are for the planets’ equators.)

  A number of moons have rotation periods that’s the same as their orbital period. For instance, our moon both rotates and orbits the Earth in 27.32 days.

This means that one side of the moon faces the Earth while the other side we don’t see. This condition is due to the tidal pull of the Earth on the moon. The tidal pull of the Earth on the moon is much stronger than the moon’s pull on the Earth.

What if the Earth’s rotation period matched the Earth’s year? Then there would be a side of the Earth that always faced the sun and a dark side of the Earth.

There would be a great flow of heat from Earth’s sunlit side to its dark side, creating very strong winds in the twilight zone.

What if the Earth didn’t rotate at all? Then in most parts of the world, there would be one sunrise a year, followed by months of daylight.

When the sun set, a night lasting months would begin. As above, there would be very strong winds as day turned into night or as night turned into day.

In his book, “Guesstimation 2.0,” author Lawrence Weinstein considers the question, “What is the shortest day the Earth could have without flying apart?”

Consider a rock on the Earth’s surface. This rock is pulled downward by its weight or the Earth’s gravity pull on the rock.

Any object moving in a circle has to supply an inward or center seeking force. When the gravity pull is less than the center seeking force, then objects not tied down could float away!

At our equator, the pull of gravity and the center seeking force are equal at a rotation speed of 10 kilometers per second or 6.2 miles per second.

Presently, our equatorial rotation speed is 1,038 miles per hour or 464 meters per second. Increasing our rotation speed by a factor of 21.55 times to reach 10 kilometers per second or a speed of 22,369 miles/hour would nullify our weight at the equator.

Our day/night cycle would then be 67 minutes long! At this speed, the equatorial waters could slosh upward as well as side wards. As the rotation speed increased beyond this point, more of the Earth’s surface would be unstable.

SKY SIGHTS AHEAD: Tomorrow evening, the moon will appear close to the bright planet Jupiter.

On Wednesday evening, the moon would appear above the star group Orion. The moon will be full next the evening of Jan. 26.

Bob Doyle invites any readers comments and questions. E-mail him at rdoyle@frostburg.edu . He is available as a speaker on his column topics.

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Bob Doyle - Astronomy
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