By using any fixed point to represent the sun, and holding a globe at some distance from it, the pupil can see the meaning of noon, twilight, and midnight. By rotating the globe from west to east he will see that the sun time of a place west of a given meridian is earlier than that of the meridian, while the time at a place east of the meridian is later than that of the meridian.

If told that it is noon at A we know that the meridian of A is directly under the sun. If B is 15° west of A, then B must turn through 15° for its meridian to be directly under the sun. This turning through 15° will require one hour. Therefore, it is one hour before noon at B when it is noon at A. In the same way it may be seen that the meridian of C, 15° east of A, has passed directly under the sun one hour before that of A. Then, if it is noon at A it is one hour past

noon at C.

The following table shows what changes in longitude correspond to given changes in time.

In 24 hours the earth turns through 360°.

In 1 hour the earth turns through

15°.

of 360°, which equals

In 1 minute the earth turns through of 15°, which equals 10, or 15'.

In 1 second the earth turns through of 15', which equals ', or 15".

The following table shows what changes in time correspond to given changes in longitude.

The earth rotates through:

360° in 24 hours.

1o in ʊ of 24 hours, which equals of an hour, or 4 minutes.

1'in of 4 minutes, which equals of a minute, or 4 seconds.

1" in of 4 seconds, which equals of a second.

Exercise 74

1. Learn the tables given above so that you can give them accurately and readily.

2. A point on the earth's surface rotates through what arc in 1 day? In 1 hr.? In 1 min.? In 1 sec.?

3. Through how many degrees does the point where you are located rotate from noon Monday to noon Tuesday? From noon to midnight? From 11 A.M. to noon? From 10 A.M. to noon? From 9 A.M. to noon? From 6 A.M. to noon?

4. If a certain meridian is under the sun, that is, if it is noon along that meridian, has a meridian 15° west of that one passed under the sun on the same day? How long before it will do so? How long since it was noon on a meridian 30° east?

5. If it is noon by the sun at Chicago, is it before or after noon at Denver? At New York? At Boston? At places east of Chicago? At places west?

6. What time is it where you are now? Is it earlier or later at places east? At places west?

7. It is noon at A. earth rotate before it Six o'clock?

Through how many degrees must the is one o'clock at A? Two o'clock?

8. If it is noon where you are, what time is it 15° west? 30° west? 45° west? 15° east? 45° east? 60° east?

9. What is the difference in time between two places if the difference in longitude is 1°? 1'? 1"?

10. When it is 9 A.M. in New York, what time is it in Denver?

SOLUTION. Denver is 30° 59′ west of New York.
For the earth to rotate through 1o requires 4 min.
To rotate through 30° requires 30X4 min. = 120 min.
= 2 hr.

104 59

74 0

30

59

To rotate through 1' requires 4 sec.

To rotate through 59' requires 59 ×4 sec. =236 sec.
=3 min. 56 sec.

Since Denver is west of New York the time is 9 2 hr. 3 min. 56 sec. earlier in Denver than in New York.

2 hr. 3 min. 56 sec. earlier than 9 A.M. is 6:56: 4 A.M.

2

3 56

6

56

4

11. When it is noon in San Francisco what time is it in London?

12. One ship is in longitude 42° 36' 10" and another is in longitude 55° 12' 10". Find the difference in longitude and the difference in time.

13. The time in New York is 4 min. 10 sec. earlier than in New Haven. Find the longitude of New Haven.

14. When it is

tain other place.

noon at Chicago it is 1:12 P.M. at a cerFind the longitude of this place.

15. If a wireless signal is sent at 4 P.M. from a ship in longitude 58° 20', at what time is it received in Boston, allowing no time for transmission?

16. The time in A is 4 hr. 25 min. 6 sec. later than in B. Find their difference in longitude and tell which is the farther west.

17. Find the time in Melbourne when it is 8:30 P.M. in Chicago.

18. Find the time in Tokio when it is 10:20 A.M. in San Francisco.

19. Making no allowance for time of transmission, a message started from New York to Manila at 11:35 a.m. would reach Manila at what time?

20. Allowing two hours for transmission, a cablegram started from London to Washington at 4:25 P.M. would reach Washington at what time?

71. Standard time. The time that has been used in the above problems is called local time. The continuous change of local time in going east or west from a given point caused

FIG. 36.-Map showing standard time belts of the United States. great inconvenience in railroad time schedules. In 1883 the railroads of the United States adopted a system of time called standard time, which is much simpler for purposes of transportation.

In making this system of time the United States was divided into standard time belts, Eastern, Central, Mountain, and Pacific. The time in the Eastern belt is the same as the local time of the 75th meridian; for example, when it is noon, local time, on the 75th meridian, it is noon, standard time, throughout the Eastern belt. The Central, Mountain, and Pacific belts take the local time of the 90th, the 105th, and the 120th meridians respectively. The lines dividing the time belts do not follow meridians but run through important railroad centers (Figure 36).

In nearly all places in the United States the time kept by the clocks is standard time.

Exercise 75

1. When it is noon, standard time, in New York what is the standard time in St. Louis? In Boston? In Denver? In Chicago? In San Francisco?

2. What standard time do you use?

3. At what hour, standard time, does your arithmetic class meet? What is the standard time then in Portland, Maine? In Florida? In New Orleans? In Utah?

4. In going from New York to Seattle what changes in time would you make?

5. Find from a map the longitude of the place in which you live, and find the difference between its local time and its standard time.

6. When it is noon on the 90th meridian what is the local or sun time on the 85th meridian? What is the standard time on that meridian? What is the difference between local time and standard time on the 85th meridian? Which is faster?

7. What is the difference between the local time and the standard time on the 100th meridian?

8. When the clock says 10:30 A.M., standard time, on the 82d meridian what is the sun time?

9. The standard time of a place in the Mountain belt is 25 minutes faster than its local time. Find its longitude. 10. What is the local time in Chicago when it is 9 A.M., standard time?

11. The standard time in London is 5 hours later than that of New York and that of Rome is 6 hours later than that of New York. By how much do these differ from the differences in local time? The longitude of Rome is 12° 29′ E.

12. When it is 11 A.M. by the standard time of San Francisco, what is the standard time of London?