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a highly accurate clock used in astronomical observations. The accurate determination of time is essential to the solution of most problems of astrometry, as well as problems in other areas of astronomy.
From ancient times to the 15th century astronomical time was measured with sundials, sandglasses, or water clocks. Clocks with gear-wheel mechanisms were first used in astronomical observations in 1484. Owing to the primitive nature of the regulating devices, however, these clocks did not provide accurate readings. The pendulum clock, which was invented in 1657 by C. Huygens, came to be widely used by time services. The impracticability of using pendulum clocks on ships at sea provided the stimulus for the development of the chronometer, which, although not as accurate as the pendulum clock, was accurate enough for use on oceangoing expeditions.
The principal requirement made of an astronomical clock is that its regulating device—in a pendulum clock, the pendulum—maintain a period of oscillation that is extremely stable. If the acceleration of gravity is constant, the period of a pendulum is a function of the length of an equivalent simple pendulum, the amplitude of its motion, and the density of the medium in which it swings.
Variations in these quantities have a considerable effect on clock rate. A 1-micrometer change in the length of an equivalent simple pendulum—usually caused by temperature variations—will result in a change in clock rate of 0.04 sec over a 24-hour period. To minimize the influence of temperature, pendulum rods are made of materials with low expansion coefficients, various compensating devices are used, and the clocks are placed in isothermal chambers.
The amplitude of the pendulum of an astronomical clock does not usually exceed 120′. A change of 0′.1 alters the clock rate by 0.011 sec per day. To eliminate the influence of changes in ambient density, either the pendulum or the entire clock is placed in a partially evacuated vessel.
The Riefler clock, the first to employ a detached-lever escapement, was used in the late 19th and early 20th centuries; its variation in clock rate did not exceed ±0.01 sec/day. A Shortt pendulum clock with a variation in clock rate not exceeding ±0.01 sec/day was designed in 1910. The basic feature of this clock is its use of two pendulums. The master, or free, pendulum, which performs no mechanical work, is located in a glass cylinder in which the pressure is maintained at 20 mm Hg. The cylinder is installed in a room in which a constant temperature is maintained year round. All the mechanical work needed to trigger the clock’s mechanism is supplied by the secondary, or slave, pendulum, the swing of which is synchronized with that of the master pendulum by a special electrical system. The secondary pendulum produces pulses that maintain the oscillation of both pendulums.
The most accurate pendulum clock is the Fedchenko clock, which has an isochronous pendulum suspension that assures a stable amplitude of swing. The accuracy of this clock is comparable to that of the best quartz clocks. Quartz clocks, which appeared in the 1940’s and 1950’s, are considerably more accurate than astronomical observations, but only for relatively brief periods of time. Because of the aging of the quartz plate, quartz clocks cannot determine a uniform time scale independently. Quartz clocks have revolutionized accurate time-measurement and timekeeping, which are accomplished by comparing the readings of several quartz clocks and those of astronomical observations.
As a result of advances in science and technology, the accuracy attained in astronomical clocks is no longer unique. The transmission of time signals by radio and television has enabled regular checks of the reference clocks at astronomical observatories against the best clocks of the official unified time service and thus has made possible a considerable increase in the reliability of the reference clocks.
E. A. IUROV