Carol Elliott, age 15, of Ottawa, Ontario, for her questions

What is Greenwich mean time?

Keeping track of time is about the hardest job in the world. You verify it and the next moment it changes. This is because we track time by the sun and the sun seems to travel clear around the world every 24 hours. The key point of the time day is noon, when the sun roaches the peak of its arch over the : ky. t'lt this point we say that the sun crosses the: meridian.

We think of the sun’s  journey around the earth as a circle and we divide the circle into 360 degrees. On the globe, these degrees are the lines of longitude and the noon sun crosses 15 of these degrees every hour. Noon reaches New Brunswick two hours before it reaches New Orleans.

For this reason we need a mean, or average, time for each calender day in the year. The nations of the world agreed to make this mean time tally with longitude 0 degrees, the Prime Meridian, which runs through Greenwich, England.