With the help of the celestial planisphere, determine the hour angle and height of the star at the time of its passage through the first vertical of a given place. For example, determine the height and hour angle of Arcturus at the moment of crossing the eastern part of the first vertical in Petrograd.
It is necessary to establish the celestial planisphere for Petrograd and measure the elevation of the celestial parallel + 20њ along the C – Zenith line, since the declination of Arcturus is + 20њ, and then determine the hour angle of Arcturus at this moment.
The counting on the planisphere gives us in this case that the required height of Arcturus is h = 23њ, and the hour angle at this moment: t = 18h 50m.
Knowing right ascension of Arcturus a = 14h 12m, we get sidereal time at the moment of crossing of the eastern part of the first vertical by Arcturus; it will be: 18h 50m + 14h 12m = 9h 2m.
The hourly angle of the intersection of the western part of the first vertical by Arcturus will be 5h10m, and the sidereal time at this moment is equal to:
14h 12m + 5h 0m = 19h 22m
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