EXERCISE 3 Time To learn to calculate time and day differences around the world.

Question

EXERCISE 3 Time To learn to calculate time and day differences around the world. Objective: Reference: Hess, Darrel. McKnight's Physical Geography, 1Ith ed. pp. 24-27 LOCAL SUN TIME Although few people today are concerned with the local Sun time, it is a useful starting point for a discussion of time. Local Sun time is based on the position of the Sun in the sky. The local Sun time "noon" for a given location is the moment in the day when the Sun reaches its high- est point in the sky. However, at the same moment that it is local Sun time noon at our location, at locations east or west of us the local Sun time is different Earth rotates from west to east (looking down at the North Pole on a globe, Earth would appear to be spinning counterclockwise). This means that at the same moment the Sun is low in the morning sky in Honolulu, it is high in the sky at noon in Denver, and low in the afternoon sky in New York. In other words, as we travel to the east, the time becomes progressively later. STANDARD TIME Rather than having people continually adjusting clocks to local Sun time when moving east or west, 24 standard time zones have been established by international agreement. Each time zone is a band of longitude, within which it is the same standard time (although, of course, the local Sun time varies slightly within the time zone). When moving from one time zone to the next, we adjust our watches by 1 hour. The time zones are based on central meridians spaced 15° of longitude apart. Earth rotates through 360° of longitude in 24 hours, and so rotates through 15° of longitude in 1 hour (360° ÷ 24 = 150). Although a standard time zone is 15° of longitude wide, the actual time zone boundaries have been adjusted over most inhabited areas of Earth (Figure 3-1). TIME ZONE CALCULATIONS The map in Figure 3-1 shows standard time zones around the world. If we remember that it is always later in New York than in San Francisco, it is easy to calculate time differences. It becomes I hour later for each time zone we cross moving from west to east (from San Francisco toward New York), and 1 hour earlier for each time zone we cross moving from east to west. New York is 3 time zones to the east of San Francisco, so New York time is 3 hours later than San Francisco time. To avoid confusion, it is usually best to refer to “12:00 midnight" and "12:00 noon" rather than to 12:00 A.M. and 12:00 P.M., respectively.

Explanation / Answer

PART I

There are 360 degrees of longitude around the Earth. One full rotation of the Earth through 360 degrees happens in 24 hours (1 day). Thus, in 1 hour we have 360/24 = 15 degrees. 15 degrees corresponds to 1 hour- this is the basic concept for all the calculations.

1. If in DENVER 105 degree W, the time and day are 10:00 A.M. MONDAY, then in NEW YORK 75 W, time and day are 12 NOON MONDAY. Difference in longitude = 105-75 = 30 degrees. If 15 degrees corresponds to 1 hour, 30 degrees will correspond to 2 hours.

2. Seattle - 120 W. Thursday 11 A.M.

Seoul in South Koea- 125 E

Difference in longitude = 120+125 = 245 since one is in Western Hemisphere and the other in Eastern Hemisphere.

245 degrees correspond to 245/15 = 16 hours 5 minutes.

So, time in South Korea - 3:05 A.M. Friday

3. It is 3:00 A.M. at Chicago if satellite image is taken at 0900z. Chicago is at 90 degree W. Chicgo is 6 hours behin Greenwich Mean Time.

4. Quito at Ecuador and Kansas City are located in the same time zone although not in the same longitude Thus, it is the same time in both the cities. This is because day light saving is not encountered at Quito while it is done at Kansas.

PART II

1. The difference in longitude = 120-75 = 45.degrees.

If 15 degrees corrsponds to 1 hour, then 45 degrees corresponds to 3 hours.

If it is 7 A.M. Saturday at Boston, then it is 10 A.M. at Los Angeles. The flight takes 5 hours. Thus, we arrive at Los Angeles when the time in Boston is 12 noon at Saturday. The corresponding time at Los Angeles is 3 P.M. at Saturday as Los Angeles in 3 hours ahead of Boston.

2. Los Angeles= 120 W

Taipei = 120 E.

Difference in longitude = 120+120=240

Taipei is thus 240/ 15 = 16 hours ahead of Los Angeles. When it is 1 P.M. at Los Angeles on Thursday, it is 5 A.M. at Taipei on Friday. The flight takes 11 hours. Thus, it reaches Taipei when it is 12 midnight at Saturday at Los Angeles on the same day and 4 P.M. at Saturday at Taipei.

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