<p>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 </p>

CC BY-SA 4.0
2019-11-12T15:20:09.493Z
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
CC BY-SA 4.0
2019-11-12T15:33:54.033Z
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
CC BY-SA 4.0
2019-11-12T19:35:18.413Z
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
CC BY-SA 4.0
2019-11-12T19:58:01.303Z
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
CC BY-SA 4.0
2019-11-12T20:21:39.343Z
Okay, that is all right, but ... you could use the formulae @JohnHoltz provided in his comment, what exactly are you asking?
Tosic
CC BY-SA 4.0
2019-11-13T13:56:14.930Z
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
<right-ascension>
Date and Time of Year given Sun's Right Ascension
jaoc
# Date and Time of Year given Sun's Right Ascension

(Asked by: username_0 on 2019-11-12)

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

**Tags:** right-ascension

> **Comments:**
> **username_1**: 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.  
> **username_1**: 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  
> **username_0**: 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_.  
> **username_0**: 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.  
> **username_2**: Okay, that is all right, but ... you could use the formulae @username_1 provided in his comment, what exactly are you asking?  
> **username_1**: 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.  

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