A page from the "Calendars" exhibit...


(see also books, and Museum store)

Claus Tøndering

Key contributor

The primary contributor is Claus Tøndering, based in Denmark, who first published his Calendar FAQ in 1997. This novel document used the then-new "Frequently Asked Questions" style of writing to explain the otherwise complex working of calendars in an inviting and understandable way. Since then, he has published several editions online, adding his own research and improvements from other contributors.

Other text credits

The Maya calendar was compiled by Claus Tøndering based on information from Chris Carrier.

Various texts, including the background on our year, the introduction to the Calendars section, the Indian calendar, and the history of the Chinese calendar are adapted from L. E. Dogget. Explanatory Supplement to the Astronomical Almanac, P. Kenneth Seidelmann, editor, with permission from University Science Books, Sausalito, CA 94965.

Detailed meanings of the days of the week, and portions of the explanation of the origin of seven day week are adapted from Mario Hilgemeier, with permission.

The introduction to "Calendars" and the timeline are adapted from Calendar, by David Ewing Duncan, Avon Books, 1998; with corrections provided by Simon Cassidy. Comments about the politics of the Julian and Gregorian calendars are adapted from a review by by Mike Musgrove of Duncan’s book in Salon Magazine, December, 1998.

Text scans of the legislative texts which established le Calendrier Républicain are courtesy of Philippe Chapelin, Recherches généalogiques sur toute la France.

Select details about the Chinese calendar, and the holidays of the Singapore calendar are courtesy of Helmer Aslaksen, Department of Mathematics, National University of Singapore.

Additional information about the early Chinese calendar, and oracle bones; and newer reforms from Encyclopædia Britannica.

Information about the Himba people is from an article by Rachel L. Swarns, in the New York Times, January 1, 2000, A5.

Information about the Church and astronomy is adapted from a review of John L. Heilbron’s The Sun in the Church in "How the Church Aided ‘Heretical’ Astronomy" by William J. Broad, in the New York Times, October 19, 1999.

Information about the Icelandic calendar is adapted from an article by Thorsteinn Vilhjálmsson. Published in Clive L.N. Ruggles (ed.), 1991. Archaeoastronomy in the 1990s. Loughborough, UK: Group D Publications. Pp. 69-76.

Ethiopian (Coptic) calendar adapted from EthiO Systems and Holiday Festival.

Rationale for the Martian Calendar’s Structure adapted from an essay by Bill Hollon, Leonard Bromberg, edited by Thomas Gangale.

Naming of months 9-12, and 28 days of February adapted from Bill Hollon.

Early Roman calendar from Plutarch. Numa Pompilius. Sanctum Library: 8th-7th Century B.C. Translated by John Dryden. Info on Plutarch and Numa Pompilius from Encyclopædia Britannica.

Ancient calendars: Overviews from World Book, details and references from Encyclopædia Britannica. Some information about Roman and Greek calendars from Bill Hollon.

Greek Astronomy adapted from an article by J J O'Connor and E F Robertson.

Information about Cro-magnon man (Lascaux caves in France) from article by Dr David Whitehouse in France for BBC News Online. Monday, 16 October, 2000.

Info about cutting trees during a full moon from "Now the Ancient Ways Are Less Mysterious", Associated Press, February 2, 2000.

Information about Star Wars from the Completely Unofficial Star Wars Encyclopedia by Bob Vitas.

Information about planets from Jet Propulsion Laboratory of the California Institute of Technology.

Specialized text sources

Some of the technical details in the "timeline" have the following sources: Jean Meeus’ book "Astronomical Algorithms", 1991; his article co-authored with Denis Savoie, "The history of the tropical year" J.Br.Astron.Assoc. v102 #1, 1992 pp40-42; Duncan Steel’s newspaper article "How Britain got the calendar wrong" in The Guardian (formerly Manchester, now London), published on 24th June 1999.


Claus Tøndering would like to thank the following for their help with this document: Dr. Monzur Ahmed of the University of Birmingham, UK, Michael J Appel, Jay Ball, Tom Box, Chris Carrier, Simon Cassidy, Claus Dobesch, Leofranc Holford-Strevens, David B. Kelley of the Hamamatsu University School of Medicine in Japan, H. Koenig, Graham Lewis, Duncan MacGregor, Marcos Montes, James E. Morrison, Waleed A. Muhanna of the Fisher College of Business, Columbus, Ohio, USA, Stefan Potthast, Yves Sagnier of the Centre d'Etudes de la Navigation Aerienne, Paul Schlyter of the Swedish Amateur Astronomer’s Society, Dr John Stockton.


Many of the sources for the FAQ’s are from Claus Tøndering, and they are written in Danish. Following are a sampling of sources (these are not neccessarily the best sources.)

General works

An important book on both the development of the calendar and its calculation and possible reform is Alexander Philips, The Calendar: Its History, Structure and Improvement (1921). A shorter and more up-to-date reference is the section on the calendar in the Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac (1961, reprinted with amendments, 1977). Also useful are Frank Parise (ed.), The Book of Calendars (1982), a general reference source with a number of conversion tables; and William Matthew O'Neil, Time and the Calendars (1975). Ludwig Rohner, Kalendergeschichte und Kalender (1978), discusses the history of Western calendars. Vladimir V. Tsybulsky, Calendars of Middle East Countries (1979, originally published in Russian, 1976), examines modern calendars.

A F Aveni, Empires of time: Calendars, clocks and cultures (New York, 1989).


See references to special studies in E.J. Bickerman, Chronology of the Ancient World, 2nd ed. (1980). On astronomy and calendar, see Otto Neugebauer, The Exact Sciences in Antiquity, 2nd ed. (1957, reprinted 1969); and his chapter on "Ancient Mathematics and Astronomy," in the History of Technology, ed. by Charles Singer et al.,vol. 1 (1954). On the later Babylonian calendar cycle, see Richard A. Parkerand Waldo H. Dubberstein, Babylonian Chronology 626 B.C.E.-C.E. 75 (1956). Current bibliography is published in the quarterly review Orientalia.

Early Greek and Roman

For the octateris, see D.R. Dicks, "Solstices, Equinoxes, and the Presocratics," J. Hellenic Stud., 86:26-40 (1966); see also his Early Greek Astronomy to Aristotle (1970). Sterling Dowand Robert F. Healey, A Sacred Calendar of Eleusis (1966), describes a calendar other than that of Athens. Benjamin D. Meritt, The Athenian Year (1961), contains a reconstruction of the Athenian civil years. Jon D. Mikalson, The Sacred and Civil Calendar of the Athenian Year (1975), includes useful bibliographical references. For water clocks, see Otto Neugebauerand H.B. Van Hoesen, Greek Horoscopes (1959, reprinted 1978). William Kendrick Pritchett, Ancient Athenian Calendars on Stone (1963), is good for the Athenian calendar. See also his "Gaming Tables and I.G., I2, 324," Hesperia, 34:131-147 (1965); and, with Otto Neugebauer, The Calendars of Athens (1947). Also useful are Bickerman(op. cit.); and Alan E. Samuel, Greek and Roman Chronology (1972). For Roman calendars, see Agnes Kirsopp Michels, The Calendar of the Roman Republic (1967, reprinted 1978); and Pierre Brind'amour, Le Calendrier romain (1983).


The oldest systematic and complete book on the present fixed Jewish calendar is the work of Abraham bar Hiyya(born c. 1065), known as Savasorda of Barcelona, that bears the title Sefer ha-'Ibur. A pr*cis of this is contained in a section (ch. 6-10) in Moses Maimonides, Sanctification of the New Moon, trans. from the Hebrew by Solomon Gandz, with an "Astronomical Commentary" by Otto Neugebauer(1956), and supplemented in the "Addenda and Corrigenda" by Ernest Wiesenbergto Moses Maimonides, The Book of Seasons (1961). These treatises from the Code of Maimonides are published as vols. 11 and 14 of the "Yale Judaica Series." Additional details of the Jewish calendar of both the rabbinic and sectarian varieties have been outlined by Ernest Wiesenberg in "Calendar," and Jacob Lichtin "Sectarian Calendars," both in Encyclopaedia Judaica, vol. 5, pp 43-53 (1971).

Other Middle Eastern

Assyria: Hildegard Lewy, The Cambridge Ancient History, 3rd ed., vol. 1, pt. 2, ch. 25 (1971); and, on the "week," see also The Assyrian Dictionary, vol. 5 (1956). Hittites: Albrecht Gstze, Kleinasien, 2nd ed. (1957). Ugarit: Cyrus H. Gordon, Ugaritic Textbook (1965). Phoenicians: J. Brian Peckham, The Development of the Late Phoenician Scripts (1968). Mari: Archives royales de Mari XII, vol. 2 (1964). Iran: E.J. Bickerman in The Cambridge History of Iran, vol. 3, pt. 2, ch. 21 (1983).

Early Egyptian

See Neugebauer (op. cit.); see also his Commentary on the Astronomical Treatise (1969); "The Origin of the Egyptian Calendar," J. Near Eastern Stud., 1:396-403 (1942); and Otto Neugebauerand Richard A. Parker(eds. and trans.), Egyptian Astronomical Texts, 3 vol. (1960-69). Richard A. Parker, The Calendars of Ancient Egypt (1950), is a good source on the subject–all older material is out of date; his "Lunar Dates of Thutmose III and Ramesses II," J. Near Eastern Stud., 16:39-43 (1957), is important for later lunar dates. H.E. Winlock, "The Origin of the Ancient Egyptian Calendar," Proc. Am. Phil. Soc., 83:447-64 (1940), is also an important discussion.


The most complete account of the lunar-solar calendar of India may be found in "Indian Calendar," ch. 5 of the Calendar Reform Committee Report of the Government of India (1955). A good summary of the materials was published by Jean Filliozatin "Notions de chronologie," an appendix of the encyclopaedic work on Indian history and culture, L'Inde classique, by Louis Renouand Jean Filliozat,vol. 2 (1953).


The Chinese calendar is discussed in Joseph Needhamand Wang Ling, "Mathematics and the Sciences of the Heavens and the Earth," Science and Civilisation in China, vol. 3 (1959).


The following are useful and authoritative references for the Mayan calendar: Sylvanus G. Morley, An Introduction to the Study of the Maya Hieroglyphs (1915, reprinted 1975); and J. Eric S. Thompson, Maya Hieroglyphic Writing: An Introduction, 3rd ed. (1971), the most complete and authoritative account. See also Floyd G. Lounsbury,"Maya Numeration, Computation, and Calendrical Astronomy," Dictionary of Scientific Biography, ed. by Charles Coulston Gillispie et al., vol. 15 (1978); and Miguel Len-Portilla, Time and Reality in the Thought of the Maya (1973). For the Mexican calendar: Alfonso Caso,"El Calendario Mexicano," Memorias de la Academia Mexicana de la Historia, vol. 17, no. 1 (1958); Thirteen Masterpieces of Mexican Archaeology (1938, reprinted 1976); and Los Calendarios prehispanicos (1967). See also Fray Diego Durn, Book of the Gods, and The Ancient Calendar(1971; originally published in Spanish, 1867), containing illustrated explanations of the Aztec calendar. For the Inca and related calendars: Alexander von Humboldt, Vues des Cordill*res, et monuments des peuples indig*nes de l'Am*rique, 2 vol. (1816); Alfred L. Kroeber,"The Chibcha," in The Handbook of South American Indians, ed. by Julian H. Steward,vol. 2 (1946, reissued 1963); and John Howland Rowe,"Inca Culture at the Time of the Spanish Conquest. Astronomy and the Calendar," also in The Handbook of South American Indians. See also Reiner Tom Zuidema,"The Sidereal Lunar Calendar of the Incas," in Archaeoastronomy in the New World, ed. by A.F. Aveni (1982).

For North American Indian chronologies, see the chapter by Cyrus Thomas,"Calendar," in The Handbook of American Indians North of Mexico, ed. by Frederick W. Hodge,vol. 1 (1907, reprinted 1979).

Photo credits

Variuos photos from Corbis, Eyewire, Photodisc, Michael Douma, ILM.

Shang oracle bones from Wan-go Weng. Collection Academia Sinica, Taipei. In Philip F. Riley et al., The Global Experience 2nd ed., vol. 1 (Prentice Hall, 1992), p. 34. David N. Keightley, Sources of Shang History: The Oracle-Bone Inscriptions of Bronze Age China (Berkeley: University of California Press, 1978), fig. 14.

Roman calendar Fasti from Penn State Architectural Engineering Graduate Students Association

Phases of the moon photo collage by F. Espenak.

The image on the home page is from a french textbook on astronomy: Camille Flammarion (1842-1925): L'Atmosphere Meteorologie populair, Paris 1888.

Legal disclaimer

While we have used our best efforts to verify that the information contained herein is accurate, we make no warranties to that effect, and shall not be liable for any damage that may result from errors or omissions in this exhibit.