Expert: Courtney Seligman - 6/3/2010

Question
Good day
What cause the earth to spin?
I am aware that the earth spins at a certain speed. How was this speed measured and if i was out of space would i see the earth spinning at this speed?
How is the distance of planets and stars measured?

Regards,
Sunil Mahabir

Answer
The rotation rate of the Earth (approximately 23 hours 56 minutes 4 seconds) is determined by seeing how long it takes the stars to move once around the sky (to the West) as a result of the Earth's rotation (to the East). If you need to know the actual speed of some place on the ground, just calculate the circumference of its parallel of latitude, and divide it by the rotation period. For the Equator, this works out to a little over a thousand miles an hour. For 60 degrees north or south latitude, at which the parallel is only half the circumference as at the Equator, the speed is only half as fast. At the Poles, which are defined as the places which do not move, while the Earth rotates around an axis through them, the speed is zero, but the rate at which a stationary object turns around the vertical axis is the same as anywhere else.

As to whether you could see this motion by going out into space, the answer would be no, if you are in orbit around the Earth, and yes, if you were so far from the Earth that you had no net motion relative to it. So an observer on a distant star, if able to see the Earth's rotation, would see it turning at the correct rate. But someone on the International Space Station would see a motion (relative to the ground) which is far more due to their own orbital motion, than to the rotation of the Earth beneath them.

Measuring the distance of an object in our solar system can be done in a number of ways. Historically, observations would be made from different parts of the Earth, and compared to each other, to see how much of a difference was caused by the different locations of the observers. As an example, discussed at http://cseligman.com/text/history/braheastro.htm suppose you look at the Moon's position relative to the stars as it rises on the eastern horizon. At that time, you are on the western side of the Earth, as seen by someone on the Moon. As the Moon rises, crosses the sky and sets, what is really happening is that you are moving across the face of the Earth, to the "other" side, as seen by someone on the Moon. That causes the position of the Moon to change by about four lunar diameters, compared to where it would be if you didn't move at all. Comparing that change in the Moon's apparent position to the size of the Earth, you can calculate how far away the Moon is, as was done (as described in the aforementioned link) by Tycho Brahe, more than 400 years ago.

For the other planets, because of their greater distance, this technique is not as accurate, but still works, to a few percent accuracy, which is adequate to get an approximate idea of their distance. But in modern times, we measure their distances by bouncing radar off them and timing how long it takes the signal to return, or by sending spacecraft to them, and keeping careful track of the path of the craft, as it moves from here to there. These techniques can yield distance estimates accurate to as many as 10 to 14 decimal places.

Unfortunately, for the stars, these more accurate methods will not work, because of the vast distances involved, and we have to rely on the parallax technique. And because of the vast distances, even that technique produces very poor results. Namely, only a few hundred stars can have their distances measured to within a percent or so, using traditional techniques. Using adaptive optics or space telescopes to increase the precision of the results, we can obtain fairly accurate parallax and distance estimates for a few thousand stars, but there are errors of as much as 10% between values obtained in one way, and values obtained by different equipment. Still, by combining all the various methods, we can establish fairly accurate distances for a few thousand stars, at the current time.

For the hundreds of billions of stars for which the above techniques will not work, other techniques are used, which are based on what we know about the stars whose distances can be directly determined; but that would be another topic, entirely.

Again, I want to apologize for the lateness of this reply. I wrote a very similar answer last night, but my browser died just as I sent it, and I didn't have time to rewrite it until I got back from work, a few minutes ago. I hope that you didn't need an immediate reply, and the delay wasn't a significant problem.