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Aberration or aberration of light (also called astronomical aberration or stellar aberration) is the apparent displacement of the stars in the sky, due to the motion of the Earth‘s revolution and the fact that the speed of light is finite.
The first observations of the aberration of starlight we are English astronomer James Bradley, in 1728. Bradley compared the apparent positions of a fairly bright star in the space of a year.
The aberration due to the annual revolution motion of the Earth in its orbit around the Sun is called δ annual aberration: it leads across the sky in an apparent annual movement of the stars, which in the course of a year on that describes a small ellipse.
The importance of this phenomenon is that the measurement of constant aberration can be traced, it is known that the speed of light, the distance Earth-Sun, ie the astronomical unit. To better understand what is in effect, imagine walking in the rain. If, as we move, we look to our right or left the rain to come down, we will notice that the drops describe an oblique trajectory and apparently originate from a point in the sky in front of us. If we stop the other hand we see the trajectory straighten up and the rain falls perpendicularly. So when we move the rain seems to be received from an area of the sky in front of us, when in fact comes from our perpendicular. The same effect occurs for the light, which falls perpendicularly on the earth, but being the earth moving around the sun seems to come from a slightly different sky area. Basically, when we see a star at a certain point in time, in fact, the star is not quite there, but slightly back along the axis of revolution of the earth around the sun. The angle between the point at which apparently is a star, the observer and the real position of the star takes the name of “aberration angle”.
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