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Can somebody provide derivation of the relativistic explanation of (apparent) superluminality in astronomy? The derivation on wikipedia seems to be a bit confusing.

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up vote 6 down vote accepted

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You are at 'O' the object is moving from A,B and is further from you at A than it is at B (ie O-A is longer than O-B)

Imagine it flared when it was at A, the light from that would be on it's way to you along A-O, meanwhile the object moves to B and flares again.

That light left later but has a short distance to go to you. Imagine for example the two light beams arrived at O a day apart.

If you measure their angular separation between the two flares, and you estimate the distance when it is at 'B' you calculate that it has moved a distance C-B in one day - which might appear to be faster than light.

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