The Technical World, March, 1905, pages 18-26:
Distribution of Time Signals
Many-Sided Activities of the U. S. Naval Observatory at Washington, D. C.--The Determination of Time, and Other Important Astronomical Work
By WALDON FAWCETT
SEVERAL notable achievements and a progressive advance in aims and administration, have of late brought into special prominence the work of the United States Naval Observatory at Washington, D. C. Though the youngest among the great astronomical institutions of the New World, this observatory unquestionably ranks with that at Greenwich, near London, and that at Pulkowa near St. Petersburg. It is but little known, however, to the general public in the United States. Though its unusual attainments have won the enthusiastic admiration of scientific circles throughout the world, it is less heard of than are many private institutions in this country. This is doubtless due in great measure to the fact that the object of the governmental institution is not the further discovery of the unknown, but the development and application of the known--the one purpose which, of the two, is likely to be by far the more beneficial to mankind in general.
Growth of the Institution
The United States Naval Observatory was established in 1842, prior to which time this Government was almost wholly dependent upon Greenwich, Pulkowa, and Paris, and on college observatories on this side of the Atlantic. As early as 1833, however, there appeared a forerunner of the present magnificent institution, in the form of an unpretentious frame structure sixteen feet square, erected under the shadow of the National Capitol at the expense of Lieutenant Wilkes, an American naval officer. This small building was equipped with a 5-foot Troughton transit which had been manufactured in England early in the century for the use of the United States Coast Survey, but which was not needed for that service when Congress refused to authorize the establishment of a national observatory.
The object of Lieutenant Wilkes in establishing his naval observatory in embryo, was solely to provide facilities for obtaining the clock errors needed for the determination of true time and the rating of naval chronometers then under his charge. However, the testing of all chronometers and other naval instruments used by United States vessels was found to be so wise and beneficial that the Secretary of the Navy speedily took it upon himself to give the little observatory official recognition under the title "A Depot for Charts and Instruments," and an officer was regularly detailed to look after the work of the institution.
In 1838, there were made upon the young institution unexpected demands which in their fulfilment so demonstrated its usefulness that its future was in a measure assured. An exploring expedition in the results of whose researches the United States was very deeply interested, sailed for the South Seas and, in perfecting the plans for its work, the discovery was made that it was well-nigh essential to the accurate determination of the longitude of the places visited by the expedition, that corresponding observations should be made at home, in order to afford comparisons with the data obtained, by the explorers. The Secretary of the Navy turned this task over to Lieutenant Gilliss, who had succeeded Lieutenant Wilkes, the founder of the Observatory; and, under stress of the new exactions, an achromatic telescope was added to the equipment of the miniature institution. For four years, though handicapped by inadequate equipment, Lieutenant Gilliss discharged faithfully and accurately the duties thus suddenly thrust upon him, and won by his success the applause of many of the foremost astronomers of Europe.
The most clearly defined development of this interesting institution dated from the time when, as a result of the object-lesson afforded by the work of Gilliss, an appropriation of $25,000 was secured. Even after this, however, the establishment continued to be known officially as the Depot of Charts and Instruments, partisan reasons preventing Congress from allowing it to be designated as an "observatory."
The first site chosen for the permanent home of the institution was selected by President Tyler. It consisted of a tract of nineteen acres which had originally been set aside for a national university--one of the pet schemes of George Washington. This location was soon found to be unsatisfactory, owing to the fact that its proximity to the heart of the city involved almost constant vibration; but it was not until 1884 that Congress could be induced to make appropriation for the purchase of the present admirable site on Georgetown Heights, overlooking the national capital.
Once the new institution was formally established, the Observatories at Berlin, Paris, Greenwich, and Vienna generously made a donation of some two hundred rare volumes of the highest standard, as a nucleus for an astronomical library. With this modest start, the library of the Naval Observatory has grown until the collection now numbers more than 22,000 volumes and pamphlets, and ranks second only to that at Pulkowa. In 1874 the institution installed the largest refracting telescope then in existence--the famous 26-inch equatorial. This telescope was set in place just in time to observe the transit of Venus, which occurs but about once in a lifetime and which affords a valuable method of determining the sun's parallax--the baseline measurement for determining celestial distances.
The present home of the Naval Observatory was completed in 1893. As has been explained, the site has a considerable elevation over much of the surrounding country; and the original site has been so extended by supplementary purchases of land, that the proper and essential isolation of the institution would seem to be insured for all time.
The equipment of the Naval Observatory is all that could be desired. The dome that houses the great equatorial is a particularly wonderful piece of mechanism. Although exceeding six tons in weight, it is so perfectly balanced that its huge bulk can be swung round, raised, or lowered by one man without excessive effort. The dome rolls around on a circular wall so that an opening for the telescope can be presented toward any part of the heavens. The entire floor of the apartment containing the large telescope, rises and falls by means of hydraulic power, after the fashion of an elevator, thus giving easy access to the instrument in all positions.
In addition to the 26-inch equatorial, the equipment of the Naval Observatory includes a 9-inch transit, a 6-inch transit circle, a 12-inch equatorial, a prime vertical transit instrument, a 6-inch azimuth, and a 40-foot photo-heliograph. The latter is used in the daily photographing of the sun.
A feature of the equipment which is worthy of special mention, is the Clock Room, or, rather, the vault connected therewith, which was designed and constructed with a view to removing from all disturbing influences the delicately adjusted time-pieces upon which dependence must be placed in much of the work of the Observatory. The clock vault is 8 feet square and 7 feet high, and its whole construction has been with a view to keeping the temperature very nearly constant throughout the year. In the vault is a frame structure of the dimensions above given; and, inclosing this, is a 9-inch brick wall. Between the frame and brick construction there is an air space of 12 inches which contains hot-water pipes for heating. The entire structure is closely roofed with boards, inclosing a 6-inch layer of asbestos wool. The vault contains three brick piers for clocks, and one smaller pier for mounting a pendulum apparatus designed for testing the minor irregularities of clock rates. The vault is provided with triple doors and with means for slow ventilation; and, inasmuch as it is located on the summit of a hill, the drainage conditions are such that the basement containing the vault is remarkably dry, so that there has thus far been little damage from rust.
Time Signals Flashed
One of the most important functions of the Naval Observatory is found in the daily distribution of the correct time to every portion of the United States. This is effected by means of telegraphic signals, which are sent out from Washington at noon daily, except Sundays. The original object of this time service was to furnish mariners in the seaboard cities with the means of regulating their chronometers; but, like many another governmental activity, its scope has gradually broadened until it has become of general usefulness. The electrical impulse which goes forth from the Observatory at noon each day, now sets or regulates automatically more than 70,000 clocks located in all parts of the United States, and also serves, in each of the larger cities of the country, to release a time-ball located on some lofty building of central location. The dropping of the time-ball--accompanied, at some points, with the simultaneous firing of a cannon--is the signal for the regulation by hand of hundreds of other clocks and watches in the vicinity.
The large telescope--which, by the way, cost $46,000, and has few rivals save the 36-inch Lick telescope in California and the 40-inch Yerkes telescope of the University of Chicago--is not used in the determination of time. In this work, only the transit instrument is used. By means of this latter piece of apparatus, an observer watches the movements of the stars on each clear night, and, by their aid, corrects the large Frodsham clock which stands in the Signal Room of the Observatory.
In this phase of the observations, spiders' webs play an important part, being used to form cross-lines extending at right angles across the field of view so as to divide it into mathematical spaces. The web of the spider, it may be explained, is not only exceedingly fine, but of great strength considering its delicacy of texture. These nets of fragile strands also possess remarkable stability in another respect--namely, that they are not affected by moisture, and do not expand or contract as a result of changes in temperature. Formerly the directors of the Naval Observatory sent to China for all the spiders' webs required, it being supposed that the large spiders of the Orient produced the most desirable webs for scientific purposes; but of late years it has been discovered that the webs of spiders found in certain portions of the United States are equally suitable.
How Time is Determined
The time determination at the Naval Observatory is manifestly one of its most important activities, since the sun is irregular and cannot safely be depended upon as a measurer of time. The actual elapsed time required for one revolution of the earth on its axis can be accurately determined only by measuring the interval between two passages of a given star across a designated meridian of the earth--intervals which do not vary from century to century. This, then, becomes the basis of the work of time determination. It is, however, a foundation not secured without considerable effort, for the number of revolutions which the earth actually makes on its axis is one greater than the number of so-called solar days in the year as prescribed by the calendar in common use. Accordingly, the day, hour, minute, and second as determined by the stars are shorter than those of the sun as told off by our clocks; and consequently the time of the "star clock"--which is corrected directly from the stars by means of the transit--must needs be translated into solar time ere it can be of use to Uncle Sam's citizens in the readjustment of their timepieces.
Time correction by the stars is in itself an interesting process. As an aid, the observer has the Nautical Almanac, a practically infallible time-table of the stars. This great work is compiled year by year as the combined result of that careful timing of the days of the stars which is ever in progress at all the principal observatories scattered over the globe. By consulting the Almanac, an observer learns at exactly what hour, minute, and second the star under observation should cross the meridian. Taking his place under the telescope, he awaits the scheduled passage of the star. Precisely as the latter crosses the imaginary line, the observer presses a telegraph key, and the exact time of passage is accurately registered by the chronograph. This instrument, in popular parlance, might be compared to a clock with a revolving cylinder of paper for a dial. Moving over this paper, instead of a second hand, is a fountain pen which answers the electrical impulse by making a mark on the paper cylinder at the exact instant that the circuit is closed. The "star clock" whose inaccuracies are thus determined at such cost of time and labor, is never literally set right. Allowance is merely made in subsequent calculations for any errors which may have been discovered, these errors amounting only to an infinitesimal part of a second in many days.
The translation and transmission of the correct time from the star clock to the clocks which are directly responsible for the sending of the time signal, is accomplished by mechanical methods very similar to those already referred to. The paper-covered cylinder of the recording device is made to revolve at a fixed speed, passing under a stationary pen loaded with red ink. Behind the dial in each clock, marking the seconds, is a cogged wheel each cog of which in turn touches a brass spring, thereby closing the circuit of a battery, and, by a mechanical arrangement, causing the pen above mentioned to make a horizontal mark on the paper enveloping the cylinder. This affords a permanent record of each second. The star clock is also in circuit with the recording pen. A cup of mercury resting in the clock is connected with one pole of the battery. The pendulum is connected with the other. As the latter swings, it touches the mercury in the cup, closing the circuit and sending an electric impulse which causes the pen to do its work. It is comparatively easy to set one of the ordinary clocks within a second of the star clock; but an adjustment of a fraction of a second requires measurements of great nicety. Corrections such as above described are made a comparatively short time before noon, so that there will be little opportunity for the clocks to gain or lose before the time at which the all-important signal is dispatched.
The clocks which contain the mechanism for sending the time signal, cost upwards of $800 each. As a precaution against accident, two of these clocks are provided; but only one is used in sending the signal. At three and a-quarter minutes before noon, the signal clock is switched into the telegraph circuit which covers the entire country; and from that moment until the sending of the signal, all business is suspended throughout the 350,000 miles of telegraph lines over which it is to be flashed. Warnings of the approach of the time signal precede by short intervals the actual announcement of the noon hour. These warnings are in part sent automatically. The signal clock is fitted with a toothed wheel which is located directly behind the wheel that marks the seconds, and which is divided into sixty spaces corresponding to the seconds in a minute. However, the tooth representing the twenty-ninth second is missing; and so, likewise, are those representing the fifty-fifth to the fifty-ninth second, respectively. As the wheel revolves, the teeth come in contact with a spring which is in connection with the electric current, closing the circuit and causing the sounder to respond. The absence of the twenty-ninth tooth causes the twenty-ninth signal to be omitted, and indicates the approach of a half-minute; and then the skipping of the five beats announces the approaching conclusion of the minute. All this takes place in the next to the last minute of the final hour. There is a third warning interval of twenty seconds before the supreme signal; but this is produced not automatically but by the telegraph operator at the Observatory, and occurs when he moves the switch key which throws out of the circuit the wheel marking the seconds, and throws into the circuit the wheel marking the minutes.
In the final one-hundredth of the last second of the fifty-ninth minute of the last hour at Washington, the tooth of the minute wheel touches the spring which closes the circuit. Simultaneously, the announcement is flashed to every part of the country, the flow of the current serving of itself to release the time-balls which have shortly before been hoisted to the tops of the staffs in various cities. How rapidly the signal travels may be appreciated from the fact that it is flashed from Washington to San Francisco in one-fifth of a second. Since the time signal is sent out from Washington at noon, or at 12 o'clock standard Eastern time, and there are four different standard times in the United States, determined by geographical locations, it will be appreciated that the signal from Washington will reach the Central, Mountain, and Pacific Time Belts at 11 o'clock, 10 o'clock, and 9 o'clock A. M., respectively. On the last night of the year, the time signal--which in this instance marks the advent of the New Year--is sent entirely around the world, traveling over 1,180,000 miles of wire and cables, and making the circuit of the globe in ten seconds.
Yet another phase of the activities of the Naval Observatory is found in the regulation of all the chronometers of the Navy. How important is this duty, may be appreciated from the fact that a second of error in a ship's chronometer at the equator means a variation of more than sixteen miles east or west of the mariner's calculation of his position.
Photographing the Sun
In the photographing of the sun to which reference has previously been made, extreme care and skill are required, it being necessary to use extremely delicate plates and to develop them by special methods. Considerably more than one hundred photographs of the sun are made each year. The camera with which the sun is photographed has a triangular box 35 feet in length. The lens is five inches in diameter, and has a focal length of forty feet. The shutter travels across a six-inch opening. The negatives, which are taken on every clear day in the year, are carefully filed for reference, and are measured with the greatest accuracy in order to show the changing positions of the sun spots, their sizes, shapes, etc.