Questions about the apparent displacement of a celestial body with respect to its background when observed along two different lines of sight.

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When is Gamma Draconis closest to the zenith in London on April 4th

I am reading Roobert Hooke's paper An attempt to prove the motion of the earth from observations (1674, faksimile). Hooke writes The principal dayes of doing which will be about the 4 of April, ...
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If the 24 microarcsec parallax precision of the Gaia spacecraft is a 1-sigma value, what is the smallest parallax…?

If the 24 microarcsec parallax precision of the Gaia spacecraft is a 1-sigma value, what is the smallest parallax that could be measured to a 3-sigma confidence? What distance, in kiloparsecs, does ...