Man's physical universe

xanabras

UNIT II

SECTION 9

ASTRONOMICAL MEASUREMENTS ARE USED TO

MEASURE TIME, TO FIX THE CALENDAR, AND

TO AID IN NAVIGATION

Introduction.

Time-keeping and navigation are two important practical applications

of astronomy. Navigation will be discussed in detail in Unit V,

Section 9.

Measurement of Time.

The measurement of time is based on the laws of motion. Two

intervals of time are equal if, during each, the earth rotates through

equal angles.

Foucault's Pendulum Proved That the Earth Rotates.

In 1851 Leon Foucault hung a long pendulum with a heavy bob from

the top of the dome of the Pantheon in Paris and set the pendulum

swinging. The direction of the swinging was carefully marked. In a

short time it was observed to have changed its direction with respect

to the building at the rate of about eleven degrees per hour. The

pendulum could not have changed its direction without a force acting

on it. But no force had been applied, so the only conclusion was that

the earth, as a result of its rotation, changed its position relative to the

pendulum and thus caused the apparent change in the direction of the

swing of the pendulum.

The Rotation of the Earth Is Our Master Clock.

Our watches and clocks are regulated to agree with a reference clock

in an observatory, and this clock is regulated in turn by the earth's

turning round and round on its axis. Is this master clock perfectly

reliable? There is evidence that the speed of rotation of the earth is

diminishing and that the length of the day is increasing at the rate of

one or two thousandths of a second per century, but this change in our

master time clock is of no practical importance to the everyday prob-

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