Online Astronomy eText: The Sky
The Rotation of the Earth Link for sharing this page on Facebook
(also see Coriolis Effects)
      Each day the Earth rotates about an axis through its poles. As it does, every place on Earth save the poles themselves, which are defined as those places which remain fixed in position while the Earth rotates, moves about a circle which is identical to its parallel of latitude. In fact, although we usually think of such parallels as simply a convenient way of showing how far North or South a place is, they actually represent the paths that are followed by the people who live on them each time that the Earth rotates.

Diagram showing the rotation of the Earth
The rotation of the Earth.
Everyone is moving eastward along their parallel of latitude as the Earth rotates.

      Of course, since everyone is going in the same direction (eastward) at the same rate on a given parallel of latitude, no one actually "gets" anywhere, and as long as you only look at your Earth-bound surroundings you can't even tell that you are moving (as will be discussed in more detail below). However, if you look up at the sky, everything in the sky will seem to move to the west in a mirror image of your eastward rotation, showing that either you are moving or the stars are moving, or both, and in ancient times it was generally assumed that the Earth was stationary and the sky was moving, rather than the other way around.
      We will discuss the apparent motions of the sky and the effects of the Earth's rotation on the weather (day and night, and seasonal variations in temperature) in another place. Here we merely introduce the physical effects of the Earth's rotation, which are referred to as Coriolis Effects.

Galilean Relativity
      Everyone has heard of Einstein's Theory of Relativity, although not everyone understands it. But not everyone has heard of Galilean Relativity, which is a much simpler and almost obvious concept (though it took thousands of years for it to become "obvious"). Basically, the idea is that all motion is "relative", and you cannot tell if you are moving unless you are moving relative to something, and even then all that you can tell is that you are moving relative to the other object, and not which object is really moving. So if you see something moving toward you, you can know that either you are moving toward it or it toward you or some combination of the two, but not which of these possibilities is correct. In this sense, the apparent motion of the stars discussed above is an example of Galilean Relativity. We can tell that we are moving relative to the stars, but not whether it is we or the stars that are really moving.
      In fact it is a basic principle of modern physics (and modern in this sense means for the best part of a century or more) that there is absolutely NO experiment that you can do to tell whether you are moving, unless something is moving relative to you, and even when you do detect relative movement you cannot, if the motion is uniform (constant in direction and speed), tell whether you are moving or the other object.
      So suppose you look around the room that you are sitting in, and try to decide whether you are moving. If you don't see anything moving, why should you presume that you are moving at all? Why should you believe that you aren't really stationary, and the Earth is not rotating? For after all, if it is rotating, then depending upon your latitude you might be moving eastward along your parallel of latitude with a speed of hundreds of miles per hour.
      The answer is, strictly speaking, that as long as you don't look outside and see the Sun or Moon or stars moving across the sky, you can't tell. But as it turns out there is a way, albeit one which is not entirely obvious, which can be used to tell that something strange is going on. And that is by looking at the Coriolis Effect, and trying to decide what in the world it represents.

The Coriolis Effect
      The Coriolis Effect is, depending upon how you look at it, one of two separate but closely related phenomena which are caused by the fact that the Earth is rotating, so that as you sit quietly reading these words, you truly are moving around a circular path centered on the axis of rotation of the Earth, which is the same as your parallel of latitude.