sidereal

tähti- Of or relating to the stars.
1919, Hector Macpherson, S.Chapman (series editor), s:Herschel/Chapter 5|Men of Science: Herschel, Chapter V,

The field of sidereal astronomy, therefore, was virtually untrodden when, shortly after the beginning of his telescopic work, Herschel began his first review of the heavens.

Then, from a sufficient number of observations of synodic periods to give their mean, we obtain the sidereal period, or period with reference to the stars.