# Calculating Local Sidereal Time [duplicate]

When calculating local sidereal time, 100.461 "is the value needed to yield the correct value of GMST" - David Hammen (left as comment on my previous question). 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?

• Does aa.usno.navy.mil/faq/docs/GAST.php help? – user21 Feb 6 '18 at 17:38
• 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 Feb 6 '18 at 17:43
• 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 Feb 6 '18 at 22:08
• 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 Feb 9 '18 at 21:38

## 1 Answer

The USNO page refers to USNO Circular 163 which in turn refers to a paper by Aoki et al. and one by Moyer. 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, 0.1 second, it is not necessary to overhaul the calculation to account for all the changes since 1984.