In 1845, at the request of the Secretary of the Navy, the Observatory installed a time ball atop the 9.6-inch telescope dome. The time ball was dropped every day precisely at Noon, enabling the inhabitants of Washington to set their timepieces. Ships in the Potomac River could also set their clocks before putting to sea. The Observatory's Time Service was initiated in 1865. A time signal was transmitted via telegraph lines to the Navy Department, and also activated the Washington fire bells at 0700, 1200, and 1800. This service was later extended via Western Union telegraph lines to provide accurate time to railroads across the nation. The Observatory participated in a program of determining longitude by comparing local time with that telegraphed from a clock at another fixed observatory, and thus exchanged time signals with other observatories and with the Coast Survey field parties.

Beginning in 1934, the Observatory determined time with a photographic zenith tube (PZT), a specialized instrument that points straight upward toward the zenith and automatically photographs selected stars crossing the zenith. This gave a measure of the Greenwich Mean Time (now called Universal Time) , the "time of day" based on the rotation of the Earth. Improvements in clock technology, including the Shortt free-pendulum clock and quartz crystal clocks, soon proved conclusively that the Earth's rotation was not uniform, and a new uniform time scale known as Ephemeris time came into use in 1956.

Defined by the orbital motion of the Earth about the Sun, in practice Ephemeris time was determined by observations of the Moon, first undertaken with the dual rate moon camera, invented by William Markowitz at the Naval Observatory in 1951. In 1984 the family of time scales known as dynamical time replaced Ephemeris time as the time based on the motion of celestial bodies according to the theory of gravitation, now taking relativistic effects into account. In the meantime, the development of atomic clocks brought about the introduction of a much more accessible time - the Atomic time scale based on the vibration (an energy level transition) of the cesium atom.

In 1958 the Naval Observatory and Britain's National Physical Laboratory published the results of joint experiments that defined the relation between Atomic time and Ephemeris time. (An interesting scientific and philosophical question is whether the relationship between Atomic time and gravitational time remains constant.) Since 1967 the international definition of the second has been based on these joint experiments. Atomic time is kept synchronized with universal time by the addition or subtraction of a leap second whenever necessary.

Time dissemination has also been continuously improved. In 1904 a naval radio station transmitted the first radio time signals ever; they were derived from a U.S. Naval Observatory clock. This was the beginning of a system of radio time, constantly improved and increasingly automated through the century, that now spans the globe. The function of rating, repairing and disseminating chronometers and other nautical instruments, a major and especially critical effort during World War II , was transferred from the Observatory to the Optical Section of the Norfolk Naval Shipyard in Portsmouth, Virginia in 1950.

The U.S. Naval Observatory continues to be the leading authority in the United States for astronomical and timing data required for such purposes as navigation at sea, on land, and in space, as well as for civil affairs and legal matters. Its current Mission Statement, promulgated in 1984 by the Chief of Naval Operations, reads:

"To determine the positions and motions of celestial bodies, the motions of the Earth, and precise time. To provide the astronomical and timing data required by the Navy and other components of the Department of Defense for navigation, precise positioning, and command, control, and communications. To make these data available to other government agencies and to the general public. To conduct relevant research; and to perform such other functions or tasks as may be directed by higher authority."

The U.S. Naval Observatory carries out its primary functions by making regular observations of the Sun, Moon, planets, selected stars, and other celestial bodies to determine their positions and motions; by deriving precise time interval (frequency), both atomic and astronomical, and managing the distribution of precise time by means of timed navigation and communication transmissions and portable clocks; and by deriving, publishing, and distributing the astronomical data required for accurate navigation, operational support, and fundamental positional astronomy. In addition, the U.S. Naval Observatory conducts the research necessary to improve both the accuracy and the methods of determining and providing astronomical and timing data.

By a Department of Defense directive, the U.S. Naval Observatory is charged with maintaining the DoD reference standard for Precise Time and Time Interval (PTTI). The Superintendent is designated as the DoD PTTI Manager. The U.S. Naval Observatory has developed the world's most accurate atomic clock system. Increasingly accurate and reliable time information is required in many aspects of military operations. Modern navigation systems depend on the availability and synchronization of highly accurate clocks. This holds for such ground-based systems as LORAN-C as well as for the Department of Defense satellite-based NAVSTAR Global Positioning System (GPS). In the communications and the intelligence fields, time synchronized activities are essential. The U.S. Naval Observatory Master Clock is the time and frequency standard for all of these systems. Thus, that clock system must be at least one step ahead of the demands made on its accuracy, and developments planned for the years ahead must be anticipated and supported.

The Master Clock system now incorporates hydrogen masers, which in the short term are more stable than cesium beam atomic clocks, and mercury ion frequency standards, which are more stable in the long run. These represent the most advanced technologies available to date. Highly accurate portable atomic clocks have been transported aboard aircraft in order to synchronize the time at Naval Bases and other Department of Defense facilities around the world with the Master Clock. Accurate time synchronization with the Master Clock is now beginning to be carried out through the use of atomic clocks in satellites, such as the GPS satellites, which will provide the primary means of time synchronization and worldwide time distribution in the future.


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