Gmt Zero

A few nice Gmt images I found:

GMT ZERO




Image by OneRandomMonkey

This is the GMT line in Greenwich

GMT explained!




Image by kenjonbro

This Polar sundial has intervals marked in Roman figures for winter time and Arabic figures for British Summer Time. The time is indicated by the shadow of the oxhead gnomon on the dial plate.

Sundials measure time by the sun. Noon is when the sun is highest in the sky here at the Millennium Village. The time between two successive noons varies by as much as 30 seconds from from 24 hours, due to the earths orbit being in a different plane to the equator of the earth, and also to the earth moving at different speeds as it goes around the sun in its elliptical orbit.

Our watches measure "mean time", so called because all these variations have been averaged out to make a standard day of exactly 24 hours. So sundial time and watch time are different by an amount which is fairly small for the most of the summer, but which reaches a maximum of 16 minutes in the autumn. The table below shows, for the 1st, 11th and 21st of each month, the time (GMT) when the sundial will be exactly indicating 12 noon, with the shadow directly under the gnomon. (When British Summer Time is in operation, you will need to add 1 hour to the figures in the table) Other hours are indicated the same number of minutes away from the hour on your watch.

Our watches are standardised on time at the Greenwich meridian. The Millennium Village is 0 degrees 6 minutes east of the Greenwich meridian. The sun appears to the move around the earth at 15 degrees per hour (and thus 360 degrees per day).
Noon at the Millennium Village is therefore 3 seconds earlier than noon at the Greenwich Meridan. This difference has also been taken into account in the table below.