A question about GMT

Can anybody tell me how many minuets
Limerick is behind London in terms of
rotation?
Eg.
if you hit a stopwatch over grenwitch
at 12:00 how long would it take for Limerick
(or its line of longitude) to pass under you.

24 hours = 360 degrees (ignoring the Earth's movement around the Sun which drops out of the equation in this case), so 1hr = 15 deg, and in the case of Belfast, 6 deg W means 24 mins behind GMT - Limerick will be a bit more.

Paul.

Thanks Paul!

well think of it as a simple ratio,
world time -> 24hours=86400seconds
world latitude -> 360degrees=1296000seconds of arc

1296000/86400=15, so 1 second of time = 15 arcseconds of latitude.

so Limerick is 8degrees 37mwest=31020 seconds of arc
giving us, 31020/15=2068seconds=34mins 28 seconds

I didnt use an exact latitude, the more accurate you have it, the more accurate a result you get

Dave_Lillis wrote: well think of it as a simple ratio,
world time -> 24hours=86400seconds
world latitude -> 360degrees=1296000seconds of arc

1296000/86400=15, so 1 second of time = 15 arcseconds of latitude.

so Limerick is 8degrees 37mwest=31020 seconds of arc
giving us, 31020/15=2068seconds=34mins 28 seconds

I didnt use an exact latitude, the more accurate you have it, the more accurate a result you get

also the result will vary depending where you are on the earths surface you are located. the earth is an oblate spheriod not a perfect sphere. this will also affect the result as the lines of latitude / longitude are not perfectly equal.

I believe that effects of geodesy only come into play when dealing with distances between lines of latitude.

Only the angular seperation between local meridian and any other reference meridian is needed to calculate local time.

Think about it - angular seperation from centre of Earth is equally spaced between lines of longitude - as is the corresponding distance on the ground (at any given constant latitude). Of course this distance is shorter as latitude approaches the pole - but this doesn't affect time measurement as a ring of latitude still rotates once per day.. However, the spacing on the ground *between* lines of latitude must differ slightly (at any given constant longidtude) as curvature is changing in the north-south direction. It doesn't affect the calculation of local time.

That is as per my limited understanding.

Mark C.