Meridians of longitude can be used to tell time at any place in the world. Most nations start counting longitude east and west from an imaginary line running through Greenwich, London. These countries have agreed Greenwich lies at oo longitude. The Earth rotates from west to east at the rate of 15 degrees of longitude an hour (360 degrees in 24 hours). There has to be some place on the globe for an old calendar day to end and a new one to begin. This place is the international date line, a line roughly following the 18oth meridian. If you cross the international date line heading west, "today" becomes "tomorrow If you cross the line heading east "today" becomes "yesterday." A single time zone covers about 15 degrees of longitude. There are 24 time zones in all, so at every hour of the day, it is a different time in each time zone.