Aberration
Aberration
Another problem inquired in the pre-relativistic period is the phenomena of aberration of fixed stars (which are remote from the earth). As you can see from the Figure, even if the line connecting the earth with a star is perpendicular to the orbit of eath, the apparent direction of the star is a little different. This is what is called "aberration", and since the angle of aberration is generally very small, observation can be made easier by comparing two data with a half year interval.
Since the velocity of light were already known in the 19th century, aberration could be calculated on the Newtonian theory, but some speculation was necessary in order to get a good match between theoretical predictions and observational data. Some people thought that ether may be dragged by the movement of earth, but Fresnel explained aberration by assuming that ether is unaffected by the motion of the earth (Stachel 1995, 258; translation, 267-8).
The relativistic calculation (by no means elementary) gives a simple answer, but its difference from the Newtonian calculation cannot be easily detected by observational data.
Reference
Stachel, John (1995, Japanese tr., 1999) "History of Relativity" in Twentieth Century Physics, vol. 1;「相対性理論の歴史」『20世紀の物理学』vol. 1、pp. 257-366、丸善。
Last modified Sept. 15, 2007. (c) Soshichi Uchii
