# Astronomy question - Sidereal time calculation

## Homework Statement

(a) Calculate sidereal time at 11am on December 8th of any year.
(b) Calculate the Greenwich Sidereal Time when the local sidereal time on longitude 20 degrees (West) is 20h 45m.

## Homework Equations

I know from my notes that a Sidereal day is 4 minutes shorter than a solar day, so it's 23h 56 m.
I don't know if it's relevant but I know that Julian Day is the number of days since noon at Greenwich on 1st Jan 4713 BC.
At the December Solstice, the RA of the sun is 18h 00m.

## The Attempt at a Solution

(a) At the December solstice, the RA of the sun is 18h 00m.
Rate of change per day at the same time is 4 minutes.

I don't understand my astronomy notes at ALL, so I'm very very shaky on my understanding of these concepts.

Thank you :)

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gneill
Mentor
Part (a) is rather open-ended and vague since LST (local sidereal time) depends upon the location (longitude) on the planet and hasn't been specified. Also, they don't specify what time scale the 11am is given in; Is it UT, Local Time, or something else?

You can start by deciding on the answers to those loose ends.

Your thought about the Julian Day number (JD) has merit; You should be able to locate an algorithm that uses the JD corresponding to the given date at 0h UT to find the mean sidereal time at Greenwich at 0h UT for that date (GMST). That will serve as a starting point for finding the GMST for the given instant of time, and then the LST.

Yeah, I thought myself the question was missing something.
This is an Irish exam paper given in Ireland so I guess it assumes an Irish location, which is 53 degrees North and 7 degrees West.

As for the 11am time scale, I can't be sure, but I think it's LCT.

I'm still not sure where to go next though... I don't really understand the Julian Number.

In my notes it gives an example of a Julian Day calculation but it doesn't show the steps and it leaves me completely stumped.
It says '6am 17th February 1985 ≡ JD 2446113.75'