Expert: Philip Stahl - 8/30/2007

Question
Thank you ahead of time for helping a 58 year old lover of astronomy.
As long as I have loved watching the stars at night and staying up for hours watching the many events that occurred during my lifetime, one thing still eludes my reasoning that no one can explain to me logically.
I know that the earth rotates on it's axis as it goes around the sun. but if the earth it rotating at such a great speed why :
1.) don't we feel it rotating as well as see other objects rotating or spun off the planet)
2.) doesn't night and day occur much faster
3.) when we look at the sun moving across the sky, it should appear to be moving faster due to the earth's rotation?
4.) when you see video's of the earth rotating in space it is rotating very slow (like on Nova) is that the actual speed?
5.) standing on the earth it just seems to not spin at all.
I never was able to understand the concept easily in grammar school. Please understand this is no joke. I want to put this to rest once and for and and hopefully have an answer that I can explain to my grandson someday.

Answer
Hello,

The problem for a human trying to detect the Earth's rotation, is that he must use extremely clever means to do it. (I will get to those). Otherwise, relying on mere "common sense" and first hand experience, he will be misled into thinking the Earth is either stationary, or rotating much slower than his ordinary perceptions allow.

Why is this?

Mainly because the planet relative to the human trying to make the determination is ENORMOUS! Not only does it have vast dimensions relative to any puny observer - it also has vast mass (and *inertia*) to go along with it. That is - a mass of nearly 6 x 10^24 kilograms.

This combination ensures you will not easily detect the rotation by first hand experience. (Well, you can IF you are patient. For example, if you stayed out all night for the recent Perseid meteor shower you would have seen the stars markedly change their positions).

But there is no way - given the Earth's mass and dimensions- you can detect a change from ordinary perceptions on the scale of seconds, minutes! (Never mind the Earth's rotational speed of ~ 1000 miles/hour. That is neither here nor there given the Earth's 25,000 mile circumference! Nothing will "seem" to move faster or "ought to move faster" (including the Sun), given the combination of Earth's size and inertia- which latter you happen to share at your location)

So how can we be clever enough to detect it? One way is by the use of time exposure photography. One sets up a suitable camera - and sets if for a long enough time exposure - then aims it at the Pole star. (Polaris). One then returns, develops the film and will find perfect concentric circles of light centered on the North celestial pole! (There are a number of basic astronomy texts that feature such a classic photo).

If Earth were stationary, the concentric circles of light would never be formed. Obviously, they are the result of the Earth's rotation about its axis - which will cause stars to form circles in the course of the film exposure.


Another way to show the Earth's rotation is through the use of the Foucalt Pendulum (which you can "google" to learn more about) This is named after the French physicist Jean Foucalt who (in 1851) suspended a 200 ft. pendulum weighing 60 lbs. from the Pantheon's domed ceiling in Paris.

Beneath the pendulum stood a table, with a ring of sand atop it. The sand recorded the progressive change in the direction of the swing - since at the end of each oscillation- a pointed stylus (attached to the pendulum bob) cut a notch in the sand.

After only a few minutes it was clear the plane of oscillation of the pendulum was slowly changing - with respect to the sand on the table, and hence with respect to Earth.

Now, the only force acting on the pendulum is gravity (downwards) between it and the Earth. If Earth were stationary, no force would exist to cause the plane of oscillation of the pendulum to alter. (In accord with Newton's first law of motion, the pendulum would merely swing on and on in the same mode and direction)

There are a number of observatories (open to the public) and some planetariums that have Foucalt pendulums - you might want to google - to see if one is near you. It is a terrific illustration of Earth's rotation.

The bottom line from all this, is there are NO rough and ready ways to detect the rotation. You cannot, from your senses alone, perceive it on any kind of short time scales. However, as I have shown, there *are* ways to get round this provided you have the necessary tools!