With the help of this planisphere, find the hour angle of a star located at a given height above the horizon at a given place, if the declination of the star is known. For example, in Petrograd (j + 60) Arcturus (d = +20, a = 12h 14m) was observed at an altitude of 30њ on the western part of the horizon; asks what time of sidereal time it was.
We will install the celestial planisphere for Petrograd and carry out the almucantarat at an altitude of 30; then the point of intersection of it with the celestial parcel + 20њ will give us the position of Arcturus at the moment.
Let’s find for this position of Arcturus, its hour angle; it will be equal to 4h 15m; therefore, sidereal time at this moment will be:
14h 12m + 4h 15m = 18h 27m.
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