The mission of the U. S. Naval Observatory, Washington, D.C., is to make such astronomical observations, to derive and publish such data, and to arrange for the supply of such nautical instruments, as will afford to United States naval vessels and aircraft, as well as to all availing themselves thereof, means for safe navigation, including the provision of accurate time, and, while pursuing this primary function, to contribute material, within the capabilities of the available astronomical equipment, to the general advancement of navigation and astronomy.
The idea of establishing a national observatory was brought to the attention of Congress in 1809 by an amateur astronomer, William Lambert, who suggested the establishment of a first meridian in the United States at the seat of government. The appeals of Lambert, and those of John Quincy Adams, as President and as a member of Congress, and of the Secretary of the Navy, for an observatory to secure accurate positions of the heavenly bodies and to increase knowledge by discovery, were unheeded by Congress until 1842. In the meantime the Secretary of the Navy, in 1830, ordered the establishment of a depot of charts and instruments, to be in charge of Lieutenant L. M. Goldsborough, U. S. Navy, for the purpose of assembling and caring for the nautical instruments, books, charts, and chronometers not in use but to be issued to naval vessels. The rating of chronometers and the correction of their errors required the use of a transit instrument, which was secured and mounted. In 1833, Lieutenant Goldsborough was succeeded at the depot by Lieutenant Charles Wilkes, U. S. Navy, who increased the facilities for time derivation. In 1837, Lieutenant Wilkes was relieved at the depot by Lieutenant J. M. Gilliss, U. S. Navy, and was ordered upon survey and exploration work. Lieutenant Gilliss, by authority of the Secretary of the Navy, secured sufficient equipment for, and commenced, in 1838, continuous observations in astronomy, magnetism, and meteorology.
In 1842, Congress authorized a “suitable house for a depot of charts and instruments,” thus permitting the expansion of the services of the depot, particularly in astronomy. The depot was located at the site of the present naval hospital on Twenty-fourth Street, N.W., its functions being hydrographical as well as astronomical.
Lieutenant M. F. Maury, U. S. Navy, relieved Lieutenant Gilliss in 1842, and commenced his great work in hydrography which resulted in the Wind and Current Charts, the beginning of these useful charts which finally covered every navigable sea. The benefit to navigation and commerce of Lieutenant Maury’s investigations have been of lasting and universal value.
In 1846, the Secretary of the Navy approved the course being pursued by the observatory in making the series of astronomical observations necessary for the preparation of a nautical almanac and the employment of the spare time of observers and instruments for the general advancement of astronomy.
The first American Ephemeris and Nautical Almanac was issued in 1852, for the year 1855. The hydrographic work of the observatory was transferred in 1866, and assigned to the then newly established Hydrographic Office, Navy Department. The observatory, however, retained all of its astronomical functions and those functions concerned with the procurement and repair of nautical instruments for the use of the Navy, and with the supplying of the correct time.
By the time of the removal of the observatory to its present site in 1893, its work in astronomy had placed it in the first rank of then existing observatories.
The present site is circumscribed by a circle of a 1,000-foot radius about a clock house. This distance from street traffic and external disturbances was decided upon as a safeguard against vibrations which might affect the standard clocks and astronomical instruments of precision, and is assured by a provision of the federal law of the United States.
Through the time service rendered by the observatory, longitude determinations are readily obtained not only by navigators, but also by scientific investigators and by surveyors, many of whom are actively at work for various governmental departments.
The Navy has a primary interest in the maintenance of a thoroughly up-to-date national observatory concerned with the astronomy of position, because it is by means of this and the data of nautical almanacs obtained from it, that deep-sea navigation is rendered possible, time is derived, and maritime independence along navigational lines is secured. It follows, therefore, that, in addition to the preparation and publication of the American Ephemeris and Nautical Almanac, the derivation and dissemination of correct time, and the procurement and repair of nautical instruments, the field of the observatory comprises the astronomy of position, continuous observations of the sun, moon, planets, and fundamental stars for the determination of absolute positions, including the position of the equator and the equinox among the stars, and essential contributory observations. Other astronomical investigations of general or special scientific value, either independently or in cooperation with other observatories, are undertaken, so far as time and opportunity permit.
About three thousand visitors annually view the heavens through the Naval Observatory telescopes. The increasing interest in astronomy is apparent in the increasing requests for cards of admission to the observatory.
Accompanying this article are photo graphs and descriptions of some of the most important instruments in use at the observatory, including the 26-inch Equatorial, the 6-inch Transit Circle, the Altitude-Azimuth Instrument, and the Time- Service Transmitting Clock.
Others, not shown, are:
Nine-inch Transit Circle.—This instrument made by Pistor and Martins, was mounted in 1865 and has been in almost continuous use since that date. It does work similar to the 6-inch Transit Circle, observing the heavenly bodies during their transit across the meridian for accurately determining their positions. Aperture, 9 inches; focal length, 108 inches.
Zenith Tube.-—-This photographic reflex instrument, devised by Dr. Ross, was made in 1911 by Gaertner. The objective, which has an aperture of 8 inches and a focal length of 17 feet, was made by Petitdidier. The telescope is mounted vertically and is immovable. It is used to photograph stars as they pass near the zenith, in order to determine the variation of latitude. This variation of latitude is used in correction of all observations for declinations.
Photoheliograph.—This instrument is for taking daily photographs of the sun. The diameter of the solar image is about 4.2 inches. The objective has an aperture of 5 inches and a focal length of about 40 feet. Negatives are measured for positions and areas of sun spots and groups, which data are used in the study of magnetic disturbances supposed to be related to sun spots.
Photographic Equatorial.—The guiding telescope has an object glass of 9.6 inches and a micrometer. The photographic lenses are 10-inch triple twin lenses of 113 inches focal length. Its work is principally the observation of asteroids (minor planets) for position. Several new asteroids have been discovered with this instrument.
Prin Portable Transit Instrument.—This is the type of instrument used by expeditions requiring accurate time. Two of these little transits of 3-inch aperture and 33-inch focal length, made by Prin of Paris, were used in longitude operations at Paris and Washington in 1913-14, and at San Diego and Washington in 1926. One of them is now being used regularly for time determinations for the daily time signals. It is provided with a motor-driven self-registering micrometer by which the personal equation of the observer is practically eliminated.
Sidereal Clocks.—In an underground vault beneath the clock house three sidereal Riefler clocks are securely mounted on piers sunk well into the ground. The vault is kept at a uniform temperature of 84° F. The clocks are encased in sealed glass cases from which the air is partially exhausted. The clocks are wound electrically. The tick from the clock in use as the standard is transmitted to the chronographs located for the convenience of the time service and of the observers at the instruments. The rates of the clocks are carefully determined. Sidereal time is sometimes called “star time” as distinguished from solar time, and indicates the hour angle of the vernal equinox.