<p>For calculating the Right Ascension of the moon I need the GMST (Greenwich Mean Sidereal Time) in Degrees.
I have found the formula
GMST = 100.4606184 + 0.9856473662862 * D + H * 15,
D = number of days, H = Universal Time in hours
I think the unit of GMST is days.
RA is then calculated GMST(degrees) + Longitude(degrees) converted to Hours Minutes Seconds.
In the example to the above formula D = 305.209, H = 17, that results GMST(days?) = 656.288 but GMST(degrees) = 296.288° For the actual date the values are 8750.95 days? and it should be -11.54° for the actual RA.
How can I convert GMST(days) to GMST(degrees)?
Or do I think wrong?</p>

CC BY-SA 4.0
2023-03-28T13:05:38.960Z
Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.
Community
CC BY-SA 4.0
2023-03-28T14:09:22.287Z
The formula you found **is** in degrees. You can tell this is the case via the $15*H$ term.
David Hammen
CC BY-SA 4.0
2023-03-28T14:32:36.850Z
BTW, JPL have an equation for GMST [here](https://ssd.jpl.nasa.gov/horizons/manual.html#greenwich-mean-sidereal-time). The result is in seconds, so divide by 240 to get degrees. Note that 3155760000 = 36525 * 86400, the number of seconds in a Julian century. I don't know why they include that term...
PM 2Ring
CC BY-SA 4.0
2023-03-28T15:20:39.810Z
"RA is then calculated GMST(degrees) + Longitude(degrees)".  This is the equation for comuting local sidereal time, not RA.
Greg Miller
CC BY-SA 4.0
2023-03-28T16:58:21.337Z
You didn't mention which epoch your D is measured from. It looks like it's midnight at the start of 2000-Jan-1. The GMST at the [J2000 epoch](https://en.wikipedia.org/wiki/J2000) is ~280.46062°.
PM 2Ring
CC BY-SA 4.0
2023-03-28T21:45:01.200Z
The first calc that gives and answer of 656.288 is in degrees. You need to subtract (or add) multiples of 360 to put the "time" in the proper range of 0 to 360. That gives GMST = 296.288 degrees. Not sure how you are doing the second calc that gives 8750.95 degrees which does not equal -11.54 degrees unless you have included the longitude but did not provide that value to us.
JohnHoltz
<the-moon><right-ascension><sidereal-period>
How to convert GMST to GMST° (degrees) for calculating RA
Ulrich Rosin
# How to convert GMST to GMST° (degrees) for calculating RA

(Asked by: username_0 on 2023-03-28)

For calculating the Right Ascension of the moon I need the GMST (Greenwich Mean Sidereal Time) in Degrees.
I have found the formula
GMST = 100.4606184 + 0.9856473662862 \* D + H \* 15,
D = number of days, H = Universal Time in hours
I think the unit of GMST is days.
RA is then calculated GMST(degrees) + Longitude(degrees) converted to Hours Minutes Seconds.
In the example to the above formula D = 305.209, H = 17, that results GMST(days?) = 656.288 but GMST(degrees) = 296.288° For the actual date the values are 8750.95 days? and it should be -11.54° for the actual RA.
How can I convert GMST(days) to GMST(degrees)?
Or do I think wrong?

**Tags:** the-moon, right-ascension, sidereal-period

> **Comments:**
> **username_1**: Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.  
> **username_2**: The formula you found **is** in degrees. You can tell this is the case via the $15*H$ term.  
> **username_3**: BTW, JPL have an equation for GMST [here](https://ssd.jpl.nasa.gov/horizons/manual.html#greenwich-mean-sidereal-time). The result is in seconds, so divide by 240 to get degrees. Note that 3155760000 = 36525 * 86400, the number of seconds in a Julian century. I don't know why they include that term...  
> **username_4**: "RA is then calculated GMST(degrees) + Longitude(degrees)".  This is the equation for comuting local sidereal time, not RA.  
> **username_3**: You didn't mention which epoch your D is measured from. It looks like it's midnight at the start of 2000-Jan-1. The GMST at the [J2000 epoch](https://en.wikipedia.org/wiki/J2000) is ~280.46062°.  
> **username_5**: The first calc that gives and answer of 656.288 is in degrees. You need to subtract (or add) multiples of 360 to put the "time" in the proper range of 0 to 360. That gives GMST = 296.288 degrees. Not sure how you are doing the second calc that gives 8750.95 degrees which does not equal -11.54 degrees unless you have included the longitude but did not provide that value to us.  

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## 0 answers

