<p>The USNO page refers to <a href="http://aa.usno.navy.mil/publications/docs/Circular_163.php" rel="nofollow noreferrer">USNO Circular 163</a> which in turn refers to a paper by <a href="http://cdsads.u-strasbg.fr/cgi-bin/nph-data_query?bibcode=1982A%26A...105..359A&amp;db_key=AST&amp;link_type=ARTICLE" rel="nofollow noreferrer">Aoki et al.</a> and <a href="http://cdsads.u-strasbg.fr/cgi-bin/nph-data_query?bibcode=1981CeMec..23...33M&amp;db_key=AST&amp;link_type=ARTICLE" rel="nofollow noreferrer">one by Moyer</a>. These papers explain how the expressions created by Simon Newcomb at the end of the 19th century were updated to work with the new system of constants introduced in 1984. If you want to change the epoch, you could redo these calculations with the epoch of your choice.</p>

<p>Further changes to celestial mechanics calculations have occurred since then, but apparently the USNO has decided that for the accuracy stated in the <a href="http://aa.usno.navy.mil/faq/docs/GAST.php" rel="nofollow noreferrer">USNO web page</a>, 0.1 second, it is not necessary to overhaul the calculation to account for all the changes since 1984.</p>

CC BY-SA 3.0
2018-02-07T13:50:21.197Z
2018-02-07T13:50:21.197Z
Gerard Ashton
<p>When calculating local sidereal time, <code>100.461</code> "is the value needed to yield the correct value of GMST" - <a href="https://astronomy.stackexchange.com/users/2618">David Hammen</a> (left as comment on <a href="https://astronomy.stackexchange.com/a/24862/21446">my previous question</a>). How is this value calculated?</p>

<p>If (hypothetically) I wanted to use January 1st 12:00 (noon) 2010 as my epoch, how would I calculate this number?</p>

CC BY-SA 3.0
2018-02-06T17:43:37.360Z
It doesn't unfortunately, it just mentions that `6.697374558` (which is `100.461` in hours) is in the formula used to calculate local sidereal time.
user3574623
CC BY-SA 3.0
2018-02-06T22:08:27.617Z
I don't see how the previous question doesn't already answer this " is the value needed to make the expression yield the correct value for GMST at 0h UT on 1 January 2000." So it is observed, not calculated. If you wanted to use January 1st 2010 you would need to observe the position of the stars at that time, or you could estimate it using the 2000 formula
James K
CC BY-SA 3.0
2018-02-09T21:38:44.217Z
Can you provide a source for it being observed rather than calculated? I don't think it's clear at all that you can infer that from the answers on my other question. The other question doesn't answer _how_ that value was calculated or measured. Please could you provide a link to the duplicate question, as I wasn't able to find it.
user3574623
<coordinate>
Calculating Local Sidereal Time
user3574623
# Calculating Local Sidereal Time

(Asked by: username_0 on 2018-02-06)

When calculating local sidereal time, `100.461` "is the value needed to yield the correct value of GMST" - [David Hammen](https://astronomy.stackexchange.com/users/2618) (left as comment on [my previous question](https://astronomy.stackexchange.com/a/24862/21446)). How is this value calculated?

If (hypothetically) I wanted to use January 1st 12:00 (noon) 2010 as my epoch, how would I calculate this number?

**Tags:** coordinate

> **Comments:**
> **username_0**: It doesn't unfortunately, it just mentions that `6.697374558` (which is `100.461` in hours) is in the formula used to calculate local sidereal time.  
> **username_1**: I don't see how the previous question doesn't already answer this " is the value needed to make the expression yield the correct value for GMST at 0h UT on 1 January 2000." So it is observed, not calculated. If you wanted to use January 1st 2010 you would need to observe the position of the stars at that time, or you could estimate it using the 2000 formula  
> **username_0**: Can you provide a source for it being observed rather than calculated? I don't think it's clear at all that you can infer that from the answers on my other question. The other question doesn't answer _how_ that value was calculated or measured. Please could you provide a link to the duplicate question, as I wasn't able to find it.  

---

## 1 answers

### Answer by username_2 on 2018-02-07. Score: 1

The USNO page refers to [USNO Circular 163](http://aa.usno.navy.mil/publications/docs/Circular_163.php) which in turn refers to a paper by [Aoki et al.](http://cdsads.u-strasbg.fr/cgi-bin/nph-data_query?bibcode=1982A%26A...105..359A&db_key=AST&link_type=ARTICLE) and [one by Moyer](http://cdsads.u-strasbg.fr/cgi-bin/nph-data_query?bibcode=1981CeMec..23...33M&db_key=AST&link_type=ARTICLE). These papers explain how the expressions created by Simon Newcomb at the end of the 19th century were updated to work with the new system of constants introduced in 1984. If you want to change the epoch, you could redo these calculations with the epoch of your choice.

Further changes to celestial mechanics calculations have occurred since then, but apparently the USNO has decided that for the accuracy stated in the [USNO web page](http://aa.usno.navy.mil/faq/docs/GAST.php), 0.1 second, it is not necessary to overhaul the calculation to account for all the changes since 1984.


---

