What are the reference objects used by Gaia to establish an effectively motionless celestial sphere for measurements?
Answer
Very distant quasars whose light travels billions of light-years
Astrometry requires measuring minute positional changes against a background assumed to be stationary. For the highest precision, particularly in space, this reference frame is established using extremely distant quasars. Because these objects are billions of light-years away, their own movement across the celestial sphere is negligible over human timescales. Any error in the assumed position or motion of these background sources directly compromises the accuracy of the detected stellar wobble, making the creation of a perfect reference frame a monumental task.
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