Stellar Aberration

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Stellar aberration is the apparent shift of stars about their actual positions, depending on the direction Earth is moving in its orbit around the sun. It occurs because the speed of light is finite; it takes time for light to reach the observer.

Stellar aberration was discovered in 1727 by James Bradley. It was the first direct proof of heliocentrism, that Earth is in orbit around the sun.

An analogy to explain stellar aberration is the apparent direction of falling rain. If rain is falling vertically, to a person running in the rain, the rain will arrive at an angle. The rain will appear to originate not from straight up but slightly tilted toward the direction the person is running.

Stellar aberration is different from stellar parallax, which is the shift of nearby stars relative to more distant stars due to the change in Earth’s position around the sun. The magnitude of stellar parallax depends on the distance of the stars, while stellar aberration affects every star by the same maximum shift. Both phenomena are direct evidence of heliocentrism.

Stellar aberration has the same annual cycle as Earth’s orbital period. It causes stars at the ecliptic poles to move in circles, those in the ecliptic plane to move in lines, and other stars in between to move in ellipses.

The maximum shift of stars due to stellar aberration is the speed of Earth’s motion around the sun divided by the speed of light, which is 0.00099365 radians or 20.49552 arcseconds.