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- 118
lems of time. TIME AND THE CALENDAR 119 The tides act as friction brakes to slow down the rate of the earth's rotation. There are also sudden and unpredictable changes in the rate of rotation of the earth, perhaps because of expansion or contraction of the earth due to variations in solar radiations ; but these changes never throw the earth off schedule more than a few seconds. Primitive Man Told Time by the Sun and Moon. Primitive man told time day by day in terms of the relative positions of the sun and earth. Later he felt the need of a timepiece that could Fig. 33. This time exposure of the stars at the north pole shows the rotation of the earth. (From Moulton, F. R., Astronomy, The Macmillan Company.) indicate more than a single day. Again man looked to the sky and found the desired timepiece. He perceived the changes in the shape of the moon and its failure to appear from time to time. He also observed that these changes occurred over and over again — always in the same order — and the same number of days apart. The moon thus made it possible to fix a future date. Its name means "The Measurer of Time," and from it comes our word "month." Sidereal Time Is the Hour Angle of the Vernal Equinox. The sidereal day equals the interval of time required for the earth to make one complete rotation.