What is GMT (Greenwich Mean Time)?

GMT stands for Greenwich Mean Time. Greenwich Mean Time is the mean solar time at the Royal Observatory in Greenwich, London. When the sun is at its highest point exactly above the Prime Meridian, it is 12:00 noon at Greenwich.

Solar time is a calculation of the passage of time based on the position of the Sun in the sky. It varies throughout the year, as the time interval between the Sun crossing a set meridian line changes.

But each day measured by a clock has the same length, equal to the average (mean) length of a solar day. It’s a way of standardizing and regularizing time so we can all know exactly what time it is for our (or anyone’s) location.

Every 15° longitude represents one hour's difference in time: (24 x 15 = 360, the degrees of a circle). So we can work out the time at every location on earth if we know how many degrees it is east or west of Greenwich.

GMT was originally set up to aid naval navigation when travel around the globe started to open up, with the discovery of the “New World” (America) in the fifteenth century. Accurate time keeping was first developed and utilized in sea voyages.