Man's physical universe

xanabras

TIME AND THE CALENDAR 121

4. Mountain, which includes all states from this line west through the

Rockies

5. Pacific, which includes all states west of the Rockies to the Pacific

Ocean.

These lines are roughly about 1/24 of the circumference of the earth,

or about 1040 miles apart on the equator.

are irregular,

Actually these time belts

because changes have been made at different points to

suit local business conditions.

A traveler going westward sets his watch back an hour at certain

intervals. Obviously, this process involves a day's change at some

point. By international agreement a line was chosen as the "international

date line"; it is in the Pacific Ocean, and ships crossing it

westward must skip a day. If it is Monday at 3 p.m. when the ship

reaches the line sailing westward, it will be Tuesday at 3 p.m. when the

ship has crossed it. When traveling eastward, the same day is counted

twice. Thus one may read in a Monday newspaper about events in

Japan dated Tuesday.

Daylight-saving Time Provides More Daylight Living in the Summer

Months.

Daylight-saving time is usually established by advancing the standard

time one full hour.

Knoxville, Tennessee, is 750 miles east of Amarillo, Texas. Both

cities use central standard time, but the sun actually rises in Amarillo

nearly an hour and a half later than it does in Knoxville. Amarillo

may be said to have daylight-saving time while it uses standard time.

On the other hand, the people living in Knoxville would lose an hour of

out-of-door daylight recreation in the summer if they did not adopt

daylight-saving time. The cities in the eastern half of a standard time

zone are the ones which would profit from daylight-saving time.

Many Instruments Have Been Used in Reckoning Time.

The Sundial. The earliest sundial, dating back as far as 2000 B.C.,

was based on the principle of a vertical rod or gnomon casting a shadow

on a flat plane. The surface of the plane was marked, so that the time

of day could be read from the position of the shadow.

Such sundials were not accurate because they failed to take into

account the apparent motion of the sun from north to south, which

changed the position of the shadow with the advance of the seasons.

The Modern Sundial. The modern sundial is based on the same

principle as its predecessors, but the gnomon which casts the shadow

is inclined from vertical to an angle equal to the latitude of the place

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