Man's physical universe

xanabras

120 THE UNIV^ERSE A VAST SYSTEM OF PARTS

The sidereal day is about four minutes shorter than the ordinary

solar day, and it is divided into twenty-four sidereal hours. Sidereal

noon occurs at the moment the vernal equinox is on the meridian.

This time occurs at different times of the day or night at different

times of the year.

called sidereal clocks.

Astronomical clocks regulated to sidereal time are

Sidereal time is actually obtained by observing the positions of fixed

stars, called "clock stars." The transit is used in navigation in making

these observations. All sidereal days are of the same length. Inasmuch

as our activities are based largely on the position of the sun in the sky,

sidereal time is inconvenient.

The stars rise four minutes earlier from night to night, or about half

an hour earlier from week to week. For this reason the constellations

move slowly westward across the evening sky as the seasons go around.

The Sun Is a Poor Clock.

The earth moves through a complete revolution around the sun in

about 365}4: days, but this motion is not uniform, because the path of

the earth is an ellipse rather than a circle. It requires 186 days for

the earth to pass from vernal equinox to autumnal equinox, March 21

to September 22, and only 179 days to pass to vernal equinox again.

For this reason there are times of the year when the sun time runs

nearly fifteen minutes slow, and there are other times when it runs

fifteen minutes fast.

We use as our timekeeper not the real sun, but an imaginary one

whose apparent motion is uniform.

Standard Time Became a Necessity in the Age of the Machine.

With the development of transportation in countries as extensive

as the United States, it was necessary to adopt standard time, for

otherwise communities on different longitudes or meridians would

have different local times.

Standard time was of^cially adopted by the United States in 1884.

The railroads divided North America into five standard time belts,

each belt to the west being one hour slower than its eastern neighbor.

Radio broadcasts have made us quite familiar with the names of most

of these time belts, which are given as follows:

1. Atlantic, which includes Newfoundland and a small part of Canada

2. Eastern, which includes states east of a line running roughly from

Toledo, Ohio, to the western boundary of Florida

3. Central, which includes states west of this line to about the western

boundaries of the Dakotas, Nebraska, Kansas, Oklahoma, and

Texas

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