Why is sun time different from clock time?

Sundials tell local sun time ("time as it is" ) at your location. The time when the sun is highest in the sky is 12 o'clock noon. The difference between two successive noons is usually not exactly 24 hours, but can be up to 30 seconds longer or 22 seconds shorter, depending on the position of the earth in its orbit round the sun. This leads to cumulative differences with the time shown on our watches. This difference is known by its historical name of the Equation of Time.

The Equation of Time is fairly small (under 5 minutes during the months from May to September, but reaches peaks of nearly 15 minutes in mid-February and nearly 17 minutes in early November.

Our watches tell Standard Time ("time as we would like it to be") or Summer/ Daylight Saving Time. Standard Time in Great Britain originated as Railway Time in the 1830s, and was established as Greenwich Mean Time by Act of Parliament in 1880. "Mean time" indicates that each day is assumed to be exactly 24 hours long to the second. "Greenwich" means that it is based on noon at the Greenwich Observatory in south-east London°. Standard time in other parts of the world is based on standard time zones, typically centred on lines of longitude at a multiple of 15 degrees east or west from Greenwich. (15 degrees of longitude is 1/24 of the earth's daily rotation, and thus equivalent to 1 hour of time)

Thus, Eastern Standard Time is centred on 75 degrees East (GMT-5), and the time in the Philippines and Western Australia is centred on 120 degrees East (GMT+8)

visitors have come to this page since 29 May 2011