MadSci Network: Astronomy

Re: why does the presetion(like translation or rotation) happen on earth?

Date: Sat Jul 22 12:18:17 2000
Posted By: Meghan Gray, Grad student, Astronomy, Cambridge University
Area of science: Astronomy
ID: 963871181.As

Hi Juan Diego, I think that precession is the word you mean, but to be absolutely clear I'll start by mentioning a few different types of motion. First of all, the Earth rotates or spins about its own axis (the imaginary line connecting the north and south poles). This is what causes night and day, as the sun appears to move across the sky. So one rotation = one day. Secondly, the Earth revolves around the Sun. This is what causes the seasons, as the axis of the Earth is tilted with respect to the Sun so that one end is alternately pointing towards and away from the sun. So one revolution = one year. Now onto precession. If we were to spend a night outside looking at the stars, we would see that they rise in the east and set in the west, following circles across the sky (just as the Sun does). In the northern hemisphere we have Polaris, the 'pole star' or the 'north star': it appears to be special because the axis of the earth is pointing nearly directly at it, so the other stars seem to move around it while it remains stationary. Polaris itself is a pretty ordinary star, however: it is not the brightest star in the sky nor is it the closest to Earth. It is a complete coincidence that it happens to be at the spot where the northern axis of the Earth is pointing, which we call the 'north celestial pole'. In the southern hemisphere this is not the case so there is no equivalent 'south star'. In Columbia you are very close to the equator so you can see stars in both the northern and southern hemispheres -- so Polaris may be visible just above the horizon to the north. But Polaris hasn't always been at the position of the north celestial pole. Because the Earth is not completely spherical (it is a bit squashed or flattened at the poles), the gravitational pull of the Sun and the Moon causes a very tiny wobble in its spin. This means that the direction in which the Earth's axis points wanders about a bit -- exactly like a spinning toy top! It takes 25 770 years for the north celestial pole to return to the same place. You'll find lots more information about precession and some very nice pictures at the Astronomy Picture of the Day site. I hope this is helpful! Meghan

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