<p>If you have a correct UT (including date) for a given GMST, adding a sidereal day yields another correct UT. So, if you have the earliest correct UT for a given date, just add a sidereal day to it. If it's the same date, it's the second solution for that date.</p>

CC BY-SA 4.0
2024-02-09T02:09:11.023Z
2024-02-09T02:09:11.023Z
John Doty
<p>Duffett-Smith, in 'Practical Astronomy With Your Calculator', says  of converting a given Greenwich mean sidereal time into the corresponding universal time.</p>
<blockquote>
<p>The problem is complicated, however, by
the fact that the sidereal day is slightly shorter than the solar day so that
on any given date a small range of sidereal times occurs twice. This range
is about 4 minutes long, the sidereal times corresponding to UT Oh to
Oh 4m occurring again from UT 23h 56m to midnight (see Figure 3). The
method given here correctly converts sidereal times in the former
interval, but not in the latter.</p>
</blockquote>
<p>What is a solution for arriving at the correct UT from GST for both cases?</p>

CC BY-SA 4.0
2024-02-01T11:54:49.550Z
Does this answer your question? [Sidereal times occuring twice a day question](https://astronomy.stackexchange.com/questions/50338/sidereal-times-occuring-twice-a-day-question)
If not, what's the difference?
Mike G
CC BY-SA 4.0
2024-02-01T13:28:09.920Z
You would solve normally for the first case, then subtract one sidereal day to get the second case.  But the problem is further complicated by the fact that Earth's rotation varies, so only an approximate solution is possible.  For the most accurate result, you will need to incorporate the UT1-UTC value from the IERS: https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html
Greg Miller
CC BY-SA 4.0
2024-02-01T22:05:22.513Z
MIke G.  The link you provided is to a page I already looked at.  The 'difference' is that the answer it gives  does not address how to convert from GST to UT.
Andalusional
CC BY-SA 4.0
2024-02-01T22:14:21.730Z
Greg, how would I conclude that I needed to subtract a sidereal day to be correct? I would need to know somehow that the first answer was incorrect.  (I can be a slow learner betimes. )
Andalusional
CC BY-SA 4.0
2024-02-08T00:28:14.910Z
@Andalusional How do you want to deal with the ambiguity? A mean solar day is ~4m longer than a sidereal day. So if the Duffett-Smith method gives a UT time in the first 4 minutes of the day there's a 2nd solution  23h56m (UT) later.
PM 2Ring
<time>
Convert GST to UT while avoiding ambiguity
Andalusional
# Convert GST to UT while avoiding ambiguity

(Asked by: username_0 on 2024-02-01)

Duffett-Smith, in 'Practical Astronomy With Your Calculator', says of converting a given Greenwich mean sidereal time into the corresponding universal time.

> The problem is complicated, however, by
> the fact that the sidereal day is slightly shorter than the solar day so that
> on any given date a small range of sidereal times occurs twice. This range
> is about 4 minutes long, the sidereal times corresponding to UT Oh to
> Oh 4m occurring again from UT 23h 56m to midnight (see Figure 3). The
> method given here correctly converts sidereal times in the former
> interval, but not in the latter.

What is a solution for arriving at the correct UT from GST for both cases?

**Tags:** time

> **Comments:**
> **username_1**: Does this answer your question? [Sidereal times occuring twice a day question](https://astronomy.stackexchange.com/questions/50338/sidereal-times-occuring-twice-a-day-question) If not, what's the difference?  
> **username_2**: You would solve normally for the first case, then subtract one sidereal day to get the second case.  But the problem is further complicated by the fact that Earth's rotation varies, so only an approximate solution is possible.  For the most accurate result, you will need to incorporate the UT1-UTC value from the IERS: https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html  
> **username_0**: MIke G.  The link you provided is to a page I already looked at.  The 'difference' is that the answer it gives  does not address how to convert from GST to UT.  
> **username_0**: username_2, how would I conclude that I needed to subtract a sidereal day to be correct? I would need to know somehow that the first answer was incorrect.  (I can be a slow learner betimes. )  
> **username_3**: @username_0 How do you want to deal with the ambiguity? A mean solar day is ~4m longer than a sidereal day. So if the Duffett-Smith method gives a UT time in the first 4 minutes of the day there's a 2nd solution  23h56m (UT) later.  

---

## 1 answers

### Answer by username_4 on 2024-02-09. Score: 1

If you have a correct UT (including date) for a given GMST, adding a sidereal day yields another correct UT. So, if you have the earliest correct UT for a given date, just add a sidereal day to it. If it's the same date, it's the second solution for that date.


---

