I have this picture:
I need to determine what sidereal time it is (in hours), or how long has it been since the upper culmination of the point of the vernal equinox. How can I do that?
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3$\begingroup$ Why do you need this and what have you tried so far? $\endgroup$– Dr ChuckFeb 4, 2022 at 9:32
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1$\begingroup$ It a task in astronomy class, I see Ursa Major constellation so I found Polar star and calculated the latitude of the place. It's approximately 41 degrees. Declination of point of vernal equinox is 0, so I can determine the height of culmination, it's 90 - $\phi$ = 49. But I'm not sure what to do next. $\endgroup$– ALiCe P.Feb 4, 2022 at 10:13
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2$\begingroup$ I'd probably use a different star than Polaris; It's less than 0.75° from the North Celestial Pole, making determining whether it's exactly on the meridian require a very accurate measurement on the image. $\endgroup$– notovnyFeb 4, 2022 at 12:53
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3$\begingroup$ In other words, Polaris is nice for letting you know where the meridian is, it's almost worthless for finding out local sidereal time, because it will be in almost exactly the same place on the sky from that location regardless of local sidereal time. $\endgroup$– notovnyFeb 4, 2022 at 13:08
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2$\begingroup$ You need to establish a North-South line, the RA of any star on that line is the sidereal time. The further the star you pick is from Polaris, the more accurate you'll be. $\endgroup$– Greg MillerFeb 4, 2022 at 14:26
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1 Answer
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I've drawn in the position of the zenith, a line from the pole star to the zenith and the position of the autumnal equinox (RA=12)
I notice that the "pointers" are on the meridian, and the pointers have a right ascension of 11hr. (I don't know the RA of all the stars, but it is easy to check with a star chart)
It is therefore 11 hours after the culmination of the vernal equinox.
