What is the difference between Right ascension in Simbad and Gaia Archive, please? How to convert the values? For instance, HD 37468: 05h 38m 42.0s vs 84.68652242854718 deg. Should not be these values nearly the same? How to convert in astropy, please?
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1 Answer
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5
1 hour = 15 degrees. 60 min = 1 hour and 60×60=3600 sec = 1hour.
So to convert: (5 + 38/60 + 42.0/3600)*15 = 84.675
There is a small discrepancy, which is due to the fact that your quoted value is the simbad location of the sigma ori cluster, and not the bright central star, which is at 05 38 44.76538 according to simbad and identical to the Gaia value, as calculated below.
Reverse the process:
84.68652242854718 / 15 = 5.6457681619
0.6457681619 * 60 = 38.746089714
0.746089714 * 60 = 44.76538284
so 84.68652242854718 = 05h 38m 44.8s
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2$\begingroup$ Actually the descrepacy is probably due to the fact that sigma ori is not a point star, but a tight cluster with multiple sources very close to each other. Simbad gives the ra of the main star as 05 38 44.76538 and the ra of the cluster as 05 38 44.0 $\endgroup$– James KCommented Sep 11, 2022 at 19:31
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1$\begingroup$ I never understood the advantage of using hh:mm:ss instead of degrees. Do astronomers really find hh:mm:ss more convenient? $\endgroup$ Commented Sep 12, 2022 at 11:52
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1$\begingroup$ @EricDuminil Some of them apparently (still) do. I do not. I guess in the days of computer processing it is just a relic of the past, much like using sexagesimal angles DDD°mm'ss" instead of plain decimal degrees. $\endgroup$ Commented Sep 12, 2022 at 12:28
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$\begingroup$ I have vague recollections of having to do time based math to use the setting circles on an old telescope in a class I took about 20 years ago. $\endgroup$ Commented Sep 12, 2022 at 12:56