Ex. 2. The difference in longitude of two places is What is the difference in time?

46° 32′ 45".

Check by reducing 3 hr. 9 min. 11 sec. to degrees, minutes and seconds as in Ex. 1.

EXERCISE 22

1. In what direction does the sun appear to move as the earth revolves on its axis?

2. How many degrees pass under the sun's rays in 5 hr.? 3. When it is noon at Chicago, what time is it at a place 15° 15′ east of Chicago? 45° 30′ 45′′ west?

4. What is the difference in longitude between two places, the difference in time being 1 hr. 4 min.?

5. A person travels from Detroit until his watch is 45 min. fast. In what direction and through how many degrees has he traveled?

6. What is the difference in time between two places whose longitudes are 75° and 60°?

7. When it is 9 A.M. local time at Washington, it is 8 hr. 7 min. 4 sec. at St. Louis; the longitude of Washington being 77° 2′ W., what is the longitude of St. Louis?

146. International Date Line. Suppose that two men, starting from the prime meridian on Monday noon, travel the one eastward and the other westward, each traveling just as fast as the earth rotates. The

40

20

0

man who goes west as fast as the earth turns east keeps exactly beneath the sun all the time; and it seems to him to be still Monday noon when he reaches his starting point again twenty-four hours later. He has lost a day in his reckoning by traveling westward around the earth.

The other man travels eastward over the earth as fast as the earth itself turns eastward, and therefore he moves away from the sun twice as fast as the prime meridian does. After twelve hours' travel he reaches the meridian of 180°, but twelve hours rotation has carried this meridian beneath the sun, and so the traveler reaches it at noon. In twenty-four hours the man reaches his starting point on the prime

60

meridian, but twenty-four hours' rotation has brought this meridian beneath the sun again, so the traveler reaches it on the second noon after his start; he therefore supposes it to be Wednesday noon, though really it is but twenty-four hours after Monday noon. He has gained a day in his reckoning by traveling eastward around the earth. To correct such errors in their dates, navigators usually add a day to their reckoning when they sail westward across the meridian of 180°, and subtract a day when they cross it to the eastward. The line where the adjustment is made, corresponding in general with the meridian of 180°, is called the international date line.

The map represents the earth when it is noon Feb. 1 at Greenwich.

40

20

It is, therefore, one hour earlier in the day for each 15° west of Greenwich and one hour later in the day for each 15° east of Greenwich. Hence 180° west of Greenwich it is midnight of Jan. 31, and 180° east of Greenwich it is midnight of Feb. 1.

When it is 6 A.M. Feb. 1 at Greenwich, at 90° W. it is midnight of Jan. 31, and at 90° E. it is noon of Feb. 1; at 180° W. it is 6 P.M. Jan. 31 and at 180° E. it is 6 P.M. Feb. 1. In this case it is Jan. 31 in all longitudes from 90° W. westward to the date line, and Feb. 1 in all longitudes from 90° W. eastward to the date line.

When it is midnight of Jan. 31 at Greenwich, at·90° W. it is 6 P.M. Jan. 31 and at 90 E. it is 6 A.M. Feb. 1; at 180° W. it is noon Jan. 31 and at 180° E. it is noon Feb. 1. In this case it is Jan. 31 in all longitudes from Greenwich westward to the date line, and Feb. 1 in all longitudes from Greenwich eastward to the date line.

When it is 6 P.M. Jan 31 at Greenwich, at 90° W. it is noon Jan. 31, and at 90° E. it is midnight Jan. 31; at 180° W. it is 6 a.m. Jan. 31, and at 180° E. it is 6 A.M. Feb. 1. In this case it is Jan. 31 in all longitudes from 90° E. westward to the date line, and Feb. 1 in all longitudes from 90° E. eastward to the date line.

1. When it is noon February 1 at Greenwich, what date is it at Paris? at New York City? at San Francisco?

2. Imagine the midnight line of Jan. 31 as a dark line moving westward parallel to the meridian. Everywhere on this line it is midnight. Behind this line it is Feb. 1; in front, Jan. 31. On what part of the earth's surface is it Feb. 1, and on what part Jan. 31 when this imaginary midnight line has reached the prime meridian? 90° E.? 140° E.?

3. What date will be in front of this line when it reaches 180° ? What date will be behind it?

LYMAN'S ADV. AR. - 6

4. After crossing the 180th meridian and passing on to 175° E., what date is before the line and what date behind it?

5. What change must be made in the calendar of a ship crossing this line going westward? Going eastward?

147. Table of longitudes for use in solving problems:

1. Determine the time and date at Ann Arbor, Berlin, Cape Town and Peking when it is midnight May 15 at Greenwich.

2. When it is noon March 1 at Rome, what time and date is it at San Francisco? at Sydney? at Detroit ?

3. If a man were to travel westward around the earth in 121 da., in how many days would he actually make the trip by the local time of the places he passes through? In how many days would he make the trip traveling eastward?

4. When it is noon Sunday, Jan. 31, on the 90th meridian west, what part of the world has Sunday? What is the day and date on the other part?

5. When it is 3 P.M. Feb. 5 on the 45th meridian east, what part of the world has Feb. 5, and what is the date on the other part?

148. Standard Time.

In order to secure uniform time

over considerable territory, in 1883 the railroad companies of the United States and Canada decided to adopt standard time. They divided the country into four time belts, each of approximately 15° of longitude in width. The time in the various belts will therefore differ by hours, while the