# How stellar aberration was quantified in the early days

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I am working through "Spacetime Physics" and encountered exercise 3-9, which concerns aberration of starlight. They ask the following question: "Since the background of stars also shifts due to aberration, how can the effect be measured at all?"

I got part of the answer. You measure the angle between the north celestial pole and the position of the star. It shifts depending on what time of year it is. That takes care of one component (I think). But I am puzzled as to how you quantify the aberration in the direction perpendicular to this.

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The whole sky "moves" towards the direction Earth is moving to (as seen from the Sun), and this direction changes over time. This distorts the pattern of the stars with a yearly cycle.

The motion in the north-south direction can be detected by reference to celestial north. How is the motion in the East-West direction detected. What is the absolute reference for east-west direction?

The whole sky "moves" towards the direction Earth is moving to (as seen from the Sun), and this direction changes over time. This distorts the pattern of the stars with a yearly cycle.

The aberration affects the stars around the star you are observing, so you cannot simply compare the star to the stars around it.

With stars nearby you won't see a strong effect, with stars at larger angles you will see one. The angle between stars at 90 degree angles (e.g. one in "forward"/"backward" direction, one in "outwards"/
inwards" or "upwards" or "downwards" direction 6 months apart) varies by up to 40 arcseconds over a year.

Thank you