Take a picture of Milky Way stars with 6 month intervals - you will see some stars in the same locations (=far away), but some moved on the background of those which did not move. The larger the shift, the closer the star.

Parallax works for short distances (Earth orbit is too small), so distances beyond 10^3 light years are measured approximately by star luminosity yet distances to more distant objects like galaxis (>10^6 light years at which individual stars are no longer resolved) are measured by the amount of red shift (space is expanding quite uniformly).

Speed of light c (as well as electron charge e and plank constant h) determines the position of energy levels in hydrogen (=color of spectral lines) as well as many other observable effects. Spectra of hydrogen (and other elements) were found to be the same all over the Universe - that is why we can conclude that c, e, h, (and G) are the same all over the visible Universe (and thus in distant past, by the way) with very high accuracy.

Thus the speed of light is considered to be constant all over the Universe and back to the very distant past (almost to the Plank time 10^-44 sec).

• thank you both - isaiaha41 - June 2, 2001 - 11:37 UTC
  • It is not a premise - yanniru - June 2, 2001 - 12:46 UTC