# Earth's Rotation

Earth's Rotation and the Apparent Daily Motion of the Sky

The earth rotates about an imaginary line that passes through the North and South Poles of the planet.  This line is called the axis of rotation.  Earth rotates about this axis once each day (approximately 24 hours).  Although you most likely already knew that fact, there is a slight complication most people are not aware of.

More specifically, our rotation period (the time elapsed for one rotation) with respect to the stars  is called a sidereal day.  A sidereal day is 24 sidereal hours, or 23 hours and 56 minutes on a normal clock.  Our clock time is based on the earth's rotation with respect to the sun from solar noon to solar noon.  This is a solar day, and it is divided into 24 hours.  Because Earth travels about 1 / 365 of the way around the sun during one day, there is a small difference between solar time and sidereal time.

The earth takes about 1/365 of a day or about 4 minutes more to get into the </FONT>same
position with respect to the sun after it reaches the same position with respect to the stars. We use sun-based time because it is more important to most of us whether the Sun is up than whether a given star is up. Those who care which star is up (like astronomers) may also use sidereal time.

1) The sidereal day is shorter than the solar day because the earth rotates and revolves in the same sense - counterclockwise in the diagram above. Venus rotates in the opposite direction to its revolution - is its sidereal day longer or shorter than its solar day?

Note: Astronomers and other scientists sometimes use ordinary words but with different or more limited meaning attached.  The words "rotate" and "revolve" are examples of this:  To an astronomer, "rotate" = spin around an axis that runs through you, and "revolve" = orbit about another object.  One time around = one revolution.  In the 1500's Copernicus published a radical theory of the solar system in which he proposed that the planets revolve about the sun.  His book "on the revolutions of the planets" changed how we view our universe.  From its title we get the word "revolution" meaning "action giving rise to radical change in society".

Not quite sure why the solar day lasts 4 minutes longer than the sidereal day?  Click on Brisban's icon for
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So which way does the world turn?  One way to visualize how the earth turns is to hold out your right fist with your thumb extended and pointing straight up (the traditional hitchhiker fist).   If you visualize that your thumb points north, then your fingers are curling in the direction of Earth's rotation.  Because of this easy set up, we say that the earth rotates in a "right-handed" manner (because you are using your right hand as a model).

You can take this model a step further by visualizing that the base of your thumb is where Europe and  Africa are located (Europe is above Africa of course when North is up).  The Atlantic Ocean would be on your fingers and the flat part of your fist by your knuckles are the Americas.  The back of your hand is the Pacific Ocean, which leaves Asia to rest where your wrist is.  If you keep this simple model in mind, you will always have an Earth-globe to check directions by.  This is not so different from counting using your fingers.

For an observer at a fixed position on Earth, the rotation of the earth makes it appear as if the sky is revolving around the earth.  In other words, if you are standing for long enough in a field at night, it looks like the sky is moving, not you.  This motion is called "apparent diurnal motion."  "Diurnal" means having to do with a day, in the sense of a 24-hour period.

Please note that the word "day" has two popular meanings, which can lead to confusion in astronomical contexts.  Day can mean when it is light out, as in "day or night."  Or, it can indicate a 24-hour period.  Here we only use the second meaning.  You might want to be careful to do the same.

In the diagram to the right we show the earth as seen from space with Ames, Iowa, the home of Iowa State, at the top.  This means that a person in Ames would be standing with his or her head towards the top of the diagram.  This picture may seem confusing at first because we like to think of north as always being up, but remember that up and down are arbitrary designations in space.

People in Australia, in the Southern Hemisphere of our planet, certainly do not consider themselves as being upside down.

The earth rotates around the North – South (N-S) line, so that if we watched it rotate, the parts at the bottom left -- Indonesia, Australia -- would move up and to the right being parallel to the dashed line (the equator).