What are latitudes and longitudes? What are the affects of earth movements? Why des time differ at various places?
The earth we live on is moving faster than any airplane or rocket. For it revolves around the sun at a speed of 66,000 miles an hour. And it also rotates on its axis at a speed of 1000 miles an hour at the equator. The measurements of time that we use every day are based on these two movements of the earth. You already know that the time the earth takes to revolve once around the sun is a whole year, while the time it takes to rotate once on its axis is a whole day. Each day is divided into 24 hours.
If you have ever traveled across our country, you have found that the time changes at certain places. When you travel from east to west, you must set your watch back an hour at these places. But when you travel from west to east, you must set your watch ahead an hour. Perhaps you have also noticed that a radio or television program scheduled for nine o’clock in New York comes on at eight o’clock if you live in the central states. It will come on at seven o’clock if you live in the mountain states and at six o’clock on the Pacific coast. All parts of our country do not have the same time.
As you watch the sun in the morning, it appears to rise in the east. Actually, the earth is rotating from west to east on its axis. As the earth keeps on turning, the sun appears higher in the sky. At the equator the distance around the earth is about 25,000 miles. Since the earth rotates once on its axis every 24 hours, at the equator it is turning at a speed of about 1000 miles an hour, or about 17 miles a minute. North or south of the equator, the earth is not so big around. So its speed of rotation is slower.
Distances north or south of the equator are measured in degrees of latitude. At the latitude of New York City, which is 40° N., the earth’s speed of rotation is about 13 miles a minute, or 780 miles an hour (60 minutes X 13 miles per minute = 780 miles). If the sun rises at six o’clock in New York City, it would not rise until seven o’clock in a city 780 miles farther west. In between the two cities, the sun would rise about a minute later for each 13 miles that a place is west of New York City. So each place would have a different time by the sun.
However, if each place had its own time, this would be very confusing. For this reason, our government has adopted a system of standard time. By this system, the country is divided into time zones, as shown on the map on this page. The width of each time zone is about 15° of longitude, which is the distance that the sun seems to move westward in an hour. All the places within one time zone have the same time. Actually, places on opposite sides of the same time zones would be about an hour apart by sun time.
Whenever you cross from one time zone into another, you must set your watch forward or backward an hour, depending on the direction in which you are traveling. If you travel west into another time zone, you must set your watch back one hour. But if you travel east into another time zone, you must set your watch ahead one hour.
Suppose a person travels all the way around the earth from east to west. If he set his watch back one hour each time he went into another time zone, he would set it back 24 hours. As a result, he would seem to gain a whole day. Traveling in the opposite direction all the way around the earth would have just the opposite result. He would seem to lose a whole day. To avoid confusion, the 180° line of longitude has been chosen as the International Date Line. When a person crosses this line going west, the date is set ahead one whole day. And when he crosses it going east, the date is set back one whole day.