Koula Hatzimichael, age 14, of Somerset, Mass., for her question:

What does the International Date Line do?

This long line on maps and g1obes zigzags down the middle of the wide Pacific Ocean. It is the International Date Line, and it is not marked on the watery face of the Sea. It is merely part of the man made system of lines of latitude and longitude. Its purpose is to mark the dawning of each new calendar day.

The international date line is based on the lines of longitude that girdle the Earth from pole to pole. You may wonder what lines of latitude or longitude have to do with the date and why the line that marks the international date for all the world should zig and zag down the center of the lonely Pacific Ocean. These puzzling questions, however, have very sensible answers.

The man made system of latitudes and longitudes mark off the face of the globe somewhat like the streets that nark off the blocks of a city. When you know the streets and cross streets you can find the intersection you want, and from there it is no problem to reach the building you want. Lonely oceans and uninhabited wastelands have no streets, so travelers must depend on latitudes and longitudes. These lines tell pilots and mariners not only the place, but also the time. And the 12 hours on the face of a clock are directly related to the 365 dates on the calendar.

Wherever you live, each day dawns in the east and sets with the sun in the west. Your noon is the moment when the sun reaches its highest point in the sky, and your midnight is exactly 12 clock hours later. The city of Boston stands on longitude 71 degrees, and its day begins and ends an hour earlier than places situated 15 degrees farther west. Its day begins later than places farther to the east. To allow for these different local days the globe has been divided into 24 time zones, one for each hour of the calendar day.

The Date Line marks the midnight between one calendar day and the next. On the East side of this imaginary line it is still Wednesday, but on its west side it is already Thursday. It would be most inconvenient if the calendar date line sliced through your city, for you would lose or gain a day every time you crossed it. This is why the date line is down the thinly populated Pacific Ocean. Here and there it jogs to avoid slicing neighboring islands into different days of the week.

The date line runs from pole to pole; and for most of its distance it stays with longitude 180 degrees. This half circle is exactly halfway around the glob from the Prime Meridian from which the world takes Greenwich time. Each new calendar day is born at the International Date Line. Hour by hour it swings around the world across 24 time zones. It ends on the east side of the Date Line as the next new day starts on its way on the western side of this imaginary line.