Time is based upon the rotation of the earth. The earth turns once on it's axis in 24 hours. This rotation is a full
360°. So, in one hour, the earth turns 15°. Every four minutes, the earth has turned one degree. In one minute of
time, the earth has turned 15 minutes of arc. So, every four seconds of time, the earth rotates one minute of arc.
Each of the infinite meridians has a different time as its position relative to the sun changes during the rotation. When the sun passes directly over a meridian is noon. Here, in this simple treatment of time, we will not deal with the equation of time or the effect of the earth's revolution and the shape of the orbit. It is impractical, though, for all longitudes to have their own time, so time zones which share the time of a central meridian were devised. In this exercise, we will also not deal with the political and cultural adjustments to the boundaries of these zones. Each time zone is 15° wide, with the Central Meridian being a multiple of 15°. This is located in the center of the belt which shares that time, so the time zones extend 7 ½° on either side of the central meridian. Since the earth rotates 15° in one hour, each time zone shares the same hour.
The Prime Meridian or Greenwich time is used as a base. Since the earth rotates in a counterclockwise direction when viewed so that the north pole is visible, the sun rises first in the east and sets in the west. Thus, noon reaches the Prime Meridian before it reaches the United States. Thus, places to the east have a later time than places to the west. At 30°W, the time would be two hours earlier than at the Prime Meridian. At 30°E, the time would be two hours later than at the Prime Meridian.
The new date first begins when midnight reaches 180° or the International Dateline. As midnight moves around the globe, the old day is disappearing at 180°. Except right at midnight at 180°, two dates always exist at any one moment.
Use the following map to show the important features for time.
1. Draw a red line on the International Dateline.
2. Draw a yellow arrow to show in which direction the time is earlier in the day. Label it EARLIER.
3. Draw a purple arrow to show in which direction the time is later in the day. Label it LATER.
4. Use a pencil and a ruler to draw the generalized borders for each of the time zones.
5. Using this map, determine the time and day for each time zone if it is 3 p.m. on Wednesday at Greenwich, England.
(Note the typographical error in the text on page 25 where a division sign should have been inserted rather than a plus sign, 360 divided by 24 = 15 degrees per hour.)