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Can anyone tell me how to calculate date of year given a specific sun's Right ascension (RA)? I know RA at Vernox (21 March 00hh:00mm:00ss)is 0 degrees. How can I get date and time for a given RA, say 1h 29m 6.69s? Thxs vm

jaoc
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  • Most procedures that I am familiar with start with the date/time and calculate the mean longitude, mean anomaly, ecliptic longitude and obliquity of ecliptic, from which the RA can be calculated. Going in the reverse order would be difficult. Would a simple lookup table be sufficient for your purposes? I would think the position would not change by much from year to year. At the very least, it would get you close to the correct day of the year from which you could do a more precise calculation to get the real date and time. – JohnHoltz Nov 12 '19 at 15:20
  • This Wiki page gives the formulas (accurate to 36 arcseconds) for the Sun's position. https://en.wikipedia.org/wiki/Position_of_the_Sun#Ecliptic_coordinates – JohnHoltz Nov 12 '19 at 15:33
  • First, thanks vm John. To giive more context, I want to determine OPTIMAL date, time *and klocation) to observe a particular star (i.e. RA and DEC known for this given star_. – jaoc Nov 12 '19 at 19:35
  • Thanks vm John. I want to determine OPTIMAL date, time (and location) to observe a particular star (i.e. RA and DEC known). I impose 2 conditions: (i) star is 'CULMINATING' (i.e. pasing thru local meridian), so Local Siderial Time (LST) = RA (star) + HA (star) = RA (star), and (ii) it's midnight so LST = RA (sun) + HA (sun) = RA (sun) + 12 hr. Equating both LST expressions, RA(sun) = RA(star) - 12h, which I'd like to transalte in a date. I guess that knowing RA(sun) = 0 on Vernal Equinox (21 March) and that 24hr=360degrees=365days, I could infer the day of year. – jaoc Nov 12 '19 at 19:58
  • Okay, that is all right, but ... you could use the formulae @JohnHoltz provided in his comment, what exactly are you asking? – Tosic Nov 12 '19 at 20:21
  • A faster approximation would be to convert the Sun's RA to ecliptic longitude, and then use the Mean Longitude (L) to get the approximate day (n), and then get a refined date by calculating the mean anomaly (g) and ecliptic longitude. – JohnHoltz Nov 13 '19 at 13:56
  • I know RA at Vernox (21 March 00hh:00mm:00ss)is 0 degrees. Unless I'm missing something, this statement is false. The time of the vernal equinox can vary by as much as 2-3 days. Also, minor note: Vernox doesn't appear to be an accepted abbreviation for vernal equinox). –  Nov 13 '19 at 15:40

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