Star positions shifted by the atmosphere
My name is Siddhartha and I live in India. I am 12 years old and am in the 8th grade.
Recently while studying about 'The Universe', our Physics teacher told us that from the earth, the stars do not appear at their original position due to refraction of light coming from the stars as it passes through the 4 layers of the atmosphere of the earth. That means, even stars like Alpha Centauri, the star nearest to the earth after sun, when viewed from earth, do not appear at their original position. That even means that the Sun also, when viewed from earth, does not appear to be in its original position.
My question is that how can the distance between the original position and the refracted position of the stars, that are in space, be measured? That means that how can the original position of the star be determined?
Reply
Twelve years old and studying physics in the 8th grade! Wow, I admire your ability and hope you will realize its full extent as you get older.
Yes, the positions of the stars and Sun are shifted by refraction in the atmosphere, but the shift is small and occurs mainly near the horizon, where sunlight and starlight slices diagonally and has a relatively long path through the atmosphere.
This was discovered by Tycho Brahe even before the telescope was introduced, but the effect is easiest to measure using a modern telescope. Telescopes (all but the simplest) have a clockwork or motor which slowly turns them to compensate for the rotation of the Earth--see picture of a telescope mounting in
http://www.phy6.org/stargaze/Scelsph.htm
Without this, the rotations of the Earth (which is greatly magnified by the telescope) causes any star to quickly drift to the edge of the field of view and be lost. If your telescope has crosshairs in the middle of the field of view and you place a star there, those crosshairs track where the star SHOULD be, taking into account only the rotation of the Earth.
Place in the crosshair a star near the horizon--for instance, the evening star Venus just before it sets. You will see that it drifts more and more away from where it is supposed to be, because as it gets closer to the horizon, refraction by the atmosphere shifts its image. You may even measure the refraction using this trick.
Astronomers of course know all this and take it into account. Before the "Hubble" telescope was sent into orbit, a huge computerized catalog of star positions was assembled for it, and since the telescope is outside the atmosphere, these all are true positions and no correction is necessary.
The Sun's position is also shifted--when you watch the Sun setting, it is really already below the horizon, and only the bending of light makes it visible. We therefore get (at any location) a little extra sunlight, our day is made a few minutes longer. Some time ago I got asked about this and you might like to read the correspondence here
.