# relativity and aberration of light

I am dealing with math and physics only at amateur level and I am writing regarding my question on relativistic aberration of light. Reading "Realtivity and Common Sense" by Hermann Bondi and some other texts I discovered that the following aberration formula is being used

$$\cos a' = \frac{\cos a + v/c}{1+ (\cos a) v/c}$$

However in other physics websites the following aberration formula is being used

$$\cos a' = \frac{\cos a - v/c}{1 - (\cos a) v/c}$$ I am sure that both they are correct and something I have missed. Could you help me?

-

The two formulae obviously differ by the convention for the sign of $v$. If you rename $v$ as $-v$, you switch from the first equation to the second and vice versa. Changing the sign of the velocity depends on the question which system is moving respect to the other, whether you consider passive or active transformations, etc. One must be careful about the sign when doing detailed calculations but at the general level, the forms of the equations are the same.
Apologies, I probably don't understand this question of yours. If the two equations - regardless of the physical interpretations - differ by a sign, it either means that $x$ in one language means $-x$ in the other, or $c$ means $-c$ in the other, or $a$ means $\pi\pm a$ in the other, or all these three things combined. Depending on the context, some of the 4 options may be impossible. – Luboš Motl Jan 19 '12 at 19:42