- The time of a place is related to the position of the sun in the sky, which depends on the longitude of the place
- The earth rotates from west to east and completes a full circle of 360 degrees in about 24 hours
- This means that the earth rotates through 15 degrees of longitude in one hour and one degree of longitude in four minutes
- The meridian that passes through Greenwich, a place near London, is taken as the Prime Meridian or 0° longitude
- The time at the Prime Meridian is called the Greenwich Mean Time (GMT) or Universal Time (UT)
- The places that are east of the Prime Meridian will have sunrise earlier and will be ahead of GMT
- The places that are west of the Prime Meridian will have sunrise later and will be behind GMT