Royal Greenwich Observatory - Information Leaflet No. 52: 'The Year AD 2000'

Wanneer begint de 21ste eeuw?

U.S. Naval Observatory

8 July 1998

U.S. Naval Observatory to Celebrate Millennium with Washington Time Ball

The U.S. Naval Observatory, which maintains the Master Clock of the United States, will usher in the years 2000 and 2001 by dropping a time ball from the telescope dome atop its main building in Washington, D.C.

By order of the Secretary of the Navy, the Naval Observatory dropped the first time ball in the United States in 1845, as the primary means for disseminating time to the city of Washington, and for ships on the Potomac to set their chronometers for navigation. The ball was dropped every day at noon from the Observatory's Foggy Bottom site until 1885, when it was moved to the State, War and Navy Building (now the old Executive Office Building) next to the White House. It was last dropped there in 1936. The dropping of the time ball to usher in the year 2000 will therefore commemorate an old tradition for Washington and the U.S. Navy. It will, however, be dropped at midnight EST on New Year's Eve rather than noon. The event is also part of the White House Millennium Program.

Time balls were used in many cities around the country and around the world during the 19th century. Those in the U.S. were sometimes dropped by a signal from Washington. Beginning in September, 1877 a time ball atop the Western Union Building in New York City was dropped by telegraphic signal from the Naval Observatory. In the era before time zones, the signal for New York was issued 12 minutes before that for Washington to take into account the longitude difference. At the turn of the 20th century dozens of time balls were being dropped around the world. A few are still ceremonially dropped, ranging from New Zealand to the Old Royal Observatory, Greenwich.

The dropping of the time ball to mark the millennium raises the contentious question of when the millennium actually begins. The official answer is clear, if not widely understood: January 1, 2001. This is because there is no year zero. Years of the Gregorian calendar, which is currently in use today, are counted from AD 1. Thus, the 1st century comprised the years AD 1 through AD 100. The second century began with AD 101 and continued through AD 200. By extrapolation we find that the 20th century comprises the years AD 1901-2000. Therefore, the 21st century will begin with 1 January 2001 and continue through 31 December 2100. Similarly, the 1st millennium comprised the years AD 1-1000. The 2nd millennium comprises the years AD 1001-2000. The 3rd millennium will begin with AD 2001 and continue through AD 3000.

But admittedly, all those zeros turning over for the year 2000 is cause for celebration! For this reason, the USNO will drop time balls for both the years 2000 and 2001. There is also a countdown clock on the USNO Millennium Web page. Among its other millennium activities, the Naval Observatory will also hold a symposium in March, 1999 to commemorate the 150th anniversary of the founding of the Nautical Almanac Office, a part of the Naval Observatory since the late 19th century.

Another frequently asked question is whether the year 2000 is a leap year. The answer is yes. The rule, according to the Gregorian reform of 1582, is that years evenly divisible by four are leap years, except for centurial years, which must be evenly divisible by 400. Thus 1700, 1800 and 1900 were not leap years, but 1600 was, and 2000 will be.

The Naval Observatory began as the Depot of Charts and Instruments for the Navy in 1830. From 1844 to 1893 it was located at Foggy Bottom. Since 1893, the Naval Observatory has been located at 34th and Massachusetts Avenue, about 2 miles from the Foggy Bottom site and the center of Washington, D.C.

Determination and dissemination of time have been an essential part of the Naval Observatory mission since its beginning. During that time clock technology has evolved from precision pendulum clocks to quartz crystal clocks to the present hydrogen maser and cesium-beam atomic clocks. Similarly, time dissemination has evolved from the visual signal represented by the time ball, to the telegraph beginning in the 1860s, to radio signals and finally the Global Positioning System (GPS), for which USNO still supplies the time. Clock accuracy has advanced from one-thousandth of a second with the most elaborate pendulum clocks at the beginning of the century, to one billionth of a second per day with the present atomic clocks. Time dissemination, accurate to a few tenths of a second with the time ball, is now accurate to within a few billionths of a second with GPS.

Aside from maintaining the Master Clock of the United States, the Observatory produces star catalogs, astronomical and navigational almanacs, and conducts forefront astronomical research. It is building a "cesium fountain" atomic clock to improve time accuracy ten-fold. It has constructed the Navy Prototype Optical Interferometer, which will help improve the celestial reference system and may discover Earth-size planets. The Observatory accurately measures the rotation of the Earth using radio telescopes around the world. And it continues a variety of efforts to improve navigation by land, sea, air, and space.

For more details on the millennium and USNO millennium activities, visit the USNO Millennium Web site, accessible via the home page or the Public Affairs page. Time ball-related illustrations are available at this site. The USNO Web site also has much information on the Master Clock and other USNO projects. More detailed information on time, calendars and related matters is also found in the Explanatory Supplement to the Astronomical Almanac, ed. P. Kenneth Seidelmann (University Science Books, 1992).

For more details on the Naval Observatory time ball see Ian R. Bartky and Steven J. Dick, "The First North American Time Ball," Journal for the History of Astronomy, 13 (1982), 50-54. On the first time balls in the world, erected in Portsmouth and Greenwich, see Ian R. Bartky and Steven J. Dick, "The First Time Balls," Journal for the History of Astronomy, 12 (1981), 155-74. These articles are available at the web site.

For the spread of time balls in the United States see Ian R. Bartky, "Naval Observatory Time Dissemination Before the Wireless," in Sky with Ocean Joined, Steven J. Dick and LeRoy Doggett, eds. (Washington, 1983), 1-28. The latter contains numerous illustrations of time balls, as does Bartky's article "The Bygone Era of Time Balls," Sky and Telescope (January, 1987), 32-35.

For a comprehensive bibliography on the problem of when a new century begins, see Ruth S. Freitag, The Battle of the Centuries (Library of Congress, Washington, 1995).

Illustrations supporting this release are available at


(Illustration 1)
Earliest evidence of a time ball at the Naval Observatory. From Astronomical Observations (1845) of the U. S. Naval Observatory. Washington: J. & S. Gideon, 1846.

(Illustration 2)
Washington time ball. From Executive Office Building; General Services Administration Historical Study No. 3, Washington: Government Printing Office, 1970

(Illustration 3)
From Harper's New Monthly Magazine, 56 (1878), 655-71

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