# On stellar aberration in the context of ether drag hypothesis

In Resnick's book on introduction to special relativity he mentions two effects that contradict the ether drag hypothesis. Fizeau convection coefficient and stellar aberration. He says that if ether was to be moving along with the earth then the light coming from a star directly 'overhead' wouldn't appear to move or in other words we would not have to correct the telescope angle due to earth's motion around the sun. In his words:

ether is not dragged around with the earth. If it were, the ether would be at rest with respect to the earth, the telescope would not have to be tilted and there would be no aberration at all.

This wiki animation shows what he means. What I'm doubtful about is that since the earth is changing its position with respect to the sun all stars should appear to move in the night sky regardless of the presence of an ether medium. So how does it make sense to argue using this point? Does stellar aberration tell us that no star should be visible directly overhead due to earth's constant motion? Does the star appear to move during the time you're seeing it through the telescope?

Note: I have studied Michelson-Morley experiment and am quite convinced that ether is an unnecessary hypothesis. I'm only seeking a clarification on the phenomenon of stellar aberration.

• Did you mean to say "...since the earth is changing its velocity with respect to the sun..."? Oct 10 '16 at 16:12
• No i meant earth's motion around the sun at 30 km/s (approx) Oct 10 '16 at 16:15
• What I'm confused about is why is Stellar aberration inconsistent with the ether drag hypothesis in the first place. The stars should trace a circular path in the night sky anyway throughout a year. I'm misinterpreting something I guess. Oct 10 '16 at 16:18

Since the earth is changing its position with respect to the sun all stars should appear to move in the night sky regardless of the presence of an ether medium. So how does it make sense to argue using this point?

Stellar aberration is a result of the changing motion of the the earth rather than its changing position. The changing position of the the earth does result in apparent change in position of the stars due to parallax. This is what Bradley was looking for when he observed aberration though the parallax effect was smaller than aberration and too small for him to see. This parallax does not affect "all stars" equally; close stars appear to move more than distant ones.

Consider also that if some how the earth were changing position at a constant rate (straight line motion) there would still be some tiny motion of stars due to parallax but not due to aberration. Aberration would shift the apparent positions of the stars but this shift wouldn't change over time.

Does stellar aberration tell us that no star should be visible directly overhead due to earth's constant motion?

No. Think of the animation you linked to in your question; if the telescope was pointed straight up and moving to the right light from a star would be able to make it down to the bottom of the tube if the star were located somewhere up and to the left, slightly behind the telescope. In that way the light would have a forward component of its motion that matches the forward motion of the telescope.

Does the star appear to move during the time you're seeing it through the telescope?

Here I'm not sure what you mean. Stellar aberration amounts to about 40 seconds of arc over six months. How long are you thinking of looking through the telescope?

• So when you say the parallax effect is smaller than the aberration what exactly do you mean? What I'm guessing is that if there was no aberration the stars would move in ellipses(in a year) and aberration somehow distorts that elliptical path? And disregard my last question it's dumb I realise that now. Oct 10 '16 at 18:57
• The parallax of 61 Cygni (one of the closest stars at about 11.4 light years) is about one third of a second of arc. It was first measured about 35 years after Bradley's work. 61 Cygni Wikipedia page Oct 10 '16 at 19:16
• Without aberration, the position of near stars would appear to move in ellipses over the course of a year due to parallax. The observed motion due to aberration has a phase difference of a quarter year compared to that of parallax. This accessible article might be useful. Oct 10 '16 at 19:26
• It's not that the observed effect is too large to be explained by parallax but that 1) it affects all stars equally when parallax should closer stars more than others and 2) the direction you have to point your telescope to sight the star at various points of the year does not agree with the parallax prediction.... Oct 10 '16 at 21:01
• ...Picture the earth orbiting the sun the way it's often drawn in a horizontal plane and picture a distant star "above" the sun. To correct for parallax you would have to point you telescope up and slightly towards the sun - perpendicular to the earths motion. To correct for aberration you have to always point your telescope forwards, in the direction of the earth's motion. Oct 10 '16 at 21:05