光行差

光行差本质是由于光速有限以及光源与观察者存在相对运动造成的，类似于运动中的雨滴：下雨的时候，站在原地不动的人感觉到雨滴是从正上方落下的，而向前走的人感觉雨滴是从前方倾斜落下的，因此需要把伞微微向前倾斜。走得越快，需要倾斜得越厉害。光行差的成因与此相似，只不过不符合经典的速度叠加法则，而是需要考虑相对论效应带来的修正。

地球上的观测者与天体之间的相对运动可以分解为各种成分，分别对应下面几种相应的光行差：

周年光行差——地球绕太阳公转造成的光行差，最大可以达到20.5角秒。天文学中定义周年光行差常数（简称光行差常数）为 k=v/c，其中 c 是光速， v 是地球绕太阳公转的平均速度；

• 周日光行差——地球自转造成的光行差，比周年光行差小两个数量级，约为零点几角秒；
• 长期光行差——太阳系在宇宙空间中的运动造成的光行差，包括：
• 太阳自行造成的光行差，约为13角秒，但方向不变；

太阳系绕银河系自转造成的光行差，约为100多角秒，但周期很长。

在一般问题中，长期光行差可以不必考虑。

地球的公转速度约为30公里／秒，光速为30万公里／秒，由此可以估算出光行差带来的角度变化约为20.49551角秒。这个角度对于小型的天文望远镜来说非常微小，因此尽管历史上很早就有人认为存在光行差，但直到1725年才由英国天文学家詹姆斯·布拉德雷在观测恒星视差时意外发现。

In astronomy, aberration (also referred to as astronomical aberration, stellar aberration, or velocity aberration) is a phenomenon where celestial objects exhibit an apparent motion about their true positions based on the velocity of the observer: It causes objects to appear to be displaced towards the observer's direction of motion. The change in angle is of the order of v/c where c is the speed of light and v the velocity of the observer. In the case of "stellar" or "annual" aberration, the apparent position of a star to an observer on Earth varies periodically over the course of a year as the Earth's velocity changes as it revolves around the Sun, by a maximum angle of approximately 20 arcseconds in right ascension or declination.

The term aberration has historically been used to refer to a number of related phenomena concerning the propagation of light in moving bodies.

Aberration is distinct from parallax, which is a change in the apparent position of a relatively nearby object, as measured by a moving observer, relative to more distant objects that define a reference frame. The amount of parallax depends on the distance of the object from the observer, whereas aberration does not. Aberration is also related to light-time correction and relativistic beaming, although it is often considered separately from these effects.

Aberration is historically significant because of its role in the development of the theories of light, electromagnetism and, ultimately, the theory of special relativity. It was first observed in the late 1600s by astronomers searching for stellar parallax in order to confirm the heliocentric model of the Solar System. However, it was not understood at the time to be a different phenomenon. In the 1720s Italian astronomer Eustachio Manfredi carried out several observations of the phenomenon. He was one of the first to realize that aberration was not the effect of parallax, but he still interpreted it within a geocentric framework. It was Manfredi who coined the term aberration. In 1727, James Bradley provided a classical explanation for it in terms of the finite speed of light relative to the motion of the Earth in its orbit around the Sun, which he used to make one of the earliest measurements of the speed of light. However, initially Bradley's theory was incompatible with 19th-century theories of light, and aberration became a major motivation for the aether drag theories of Augustin Fresnel (in 1818) and G. G. Stokes (in 1845), and for Hendrik Lorentz's aether theory of electromagnetism in 1892. The aberration of light, together with Lorentz's elaboration of Maxwell's electrodynamics, the moving magnet and conductor problem, the negative aether drift experiments, as well as the Fizeau experiment, led Albert Einstein to develop the theory of special relativity in 1905, which presents a general form of the equation for aberration in terms of such theory. However the results of the Michelson Morley experiment were misunderstood at the time and since then it has been realised that their experiment did not rule out an aether medium for a wave only model of light. But rather showed only that the aether, now referred to as the vacuum, did not impede or deflect the direction or velocity of light in the source frame. Ruling out the necessity to invoke a relativistic explanation of aberration.