Astronomical chronology

Astronomical chronology, or astronomical dating, is a technical method of dating events or artifacts that are associated with astronomical phenomena. Written records of historical events that include descriptions of astronomical phenomena have done much to clarify the chronology of the Ancient Near East; works of art which depict the configuration of the stars and planets and buildings which are oriented to the rising and setting of celestial bodies at a particular time have all been dated through astronomical calculations.

Dating historical events

The use of descriptions of astronomical phenomena to date historical events began in the 16th century, a time of a renewed humanistic interest in history and of increasingly precise astronomical tables.[1] Eclipses in particular are relatively infrequent events and can be dated precisely. When the circumstances are not exact and descriptions leave ambiguities, one can often use other details such as the month of the eclipse or the position of other stars and planets to identify the specific eclipse.[2]

Astronomical dating, like other forms of historical interpretation, requires care in interpreting the surviving written records. John Steele has proposed three questions that must be asked when dating an event: Does the record refer to an actual astronomical event, or is this merely a modern assumption? If it does refer to an actual astronomical event, is the source reliable? Can the record provide an unambiguous date without making unwarranted assumptions about ancient astronomical observational methods?[3]

Babylonian astronomical diaries provide detailed and unambiguous accounts of the positions of all the visible planets, often in relation to specific stars, that have been used to provide precise dates of events like the defeat of Darius III by Alexander the Great at the Battle of Gaugamela on 1 October 331 BCE and of Alexander's subsequent death on 11 June 323.[4]

Since the success of this method depends on the reliability of the written sources and the precision of their accounts of astronomical phenomena, attempts to date literary texts which may describe astronomical events loosely or even as metaphors have led researchers to conclusions that appear precise, but rely on invalid assumptions and are consequently less widely accepted. Thus the attempts to date Vedic texts describing the Pleiades as rising "due East" to about 2300 BCE, which is the time when the Pleiades rose "exactly" due East, is complicated by the fact that poetic descriptions need not be taken as reflecting precise astronomical observations, while precession is a very slow process which makes only small changes in the azimuth of a star rising in the East.[5]

Dating artifacts

Aratea 93v
Ninth century diagram of the positions of the seven planets on 18 March 816.

Among the artifacts that can most readily be dated by astronomical techniques are depictions of the positions of the celestial bodies at a particular time. Since the motions of the celestial bodies are all at different periods, it takes many centuries for all the planets plus the Sun and the Moon, to reach the same positions in the signs of the Zodiac. For a configuration accurate to ±15° (that is, within a single sign) the positions of these seven bodies will only return to the same configuration once in about 3700 years. A particular case involved a medieval illuminated manuscript which portrayed the position of these seven celestial bodies on 18 March 816; corresponding to the period when the manuscript was written. This calculation demonstrated that this illustration was not a copy of an earlier classical depiction of the position of the stars.[6] The rapidly moving Moon is the most sensitive indicator for the exact time; if one can estimate the indicated position of the Moon to within a degree, the time of the diagram can be computed to within an hour.[7]

A striking example of this method was an astrological portrait of Sir Christopher Hatton (1540–1591), which depicted the positions of the seven classical planets in the zodiac and noted the computed positions of the planets to the nearest minute of arc. Here the largest source of error in the date was the uncertainty of 16th-century astronomical calculations. The resulting time was about noon of 12 December 1581.[8]

Dating structures by their orientation

A more controversial archaeoastronomical approach has been used to date structures that are believed to have been oriented on astronomical principles by measuring their orientation and computing the date in the past when a single specified celestial body, whether the Sun or a selected star, rises or sets at the measured azimuth. The astronomer, Norman Lockyer, applied this method to Stonehenge[9] by measuring the orientation of the Stonehenge avenue and comparing it to the position of solstitial sunrise, which changes slowly due to the changing obliquity of the ecliptic. The archaeologist, F. C. Penrose, applied a similar method to ancient Greek Temples, attempting to establish their dates by relating their orientation to the appearance of stars on the horizon, the position of which changes slowly due to the precession of the equinoxes.[10]

The wide variance of these dates from historically accepted ones led the architect and archaeologist, William Bell Dinsmoor, to mistrust dates established by the slowly changing obliquity of the ecliptic or by stellar alignments, which involve an arbitrary selection of a star that rises on the proper azimuth. Instead he proposed a method employing what was already known from historical records concerning the dates of construction of Greek temples, the festivals associated with specific temples, and the nature of the Greek Lunisolar calendar. Since the date of a festival in the Greek lunisolar calendar only recurs on the same date in the solar calendar every eight or nineteen years, Dinsmoor identified a festival connected with a specific temple and was able to determine the exact year near the historically recorded construction date when the Sun rose in alignment with the temple on the date of the festival.[11]


  1. ^ Anthony Grafton, "Some Uses of Eclipses in Early Modern Chronology," Journal of the History of Ideas, 64 (2003): 213-229.
  2. ^ F. R. Stephenson and J. M. Steele, "Astronomical Dating of Babylonian Texts Describing the Total Solar Eclipse of S.E. 175", Journal for the History of Astronomy, 37 (2006): 55-69.
  3. ^ John M. Steele, "The Use and Abuse of Astronomy in Establishing Ancient Chronologies," Physics in Canada/La Physique au Canada, 59 (2003): 243-8, p. 247.
  4. ^ Jona Lendering, "Astronomical Diaries" Archived 2011-05-14 at the Wayback Machine
  5. ^ Michael Witzel, "The Pleiades and the Bears viewed from inside the Vedic texts," Electronic Journal of Vedic Studies Archived March 20, 2008, at the Wayback Machine, 5 (1999), issue 2
  6. ^ Richard Mostert and Marco Mostert, "Using astronomy as an aid to dating manuscripts, The example of the Leiden Aratea planetarium", Quaerendo, 20 (1999): 248-261; see esp. § 6. Reliability of the Dating Method, p. 258.
  7. ^ William D. Stahlman, "Astronomical Dating Applied to a Type of Astrological Illustration," Isis, 47 (1956): 154-160, p. 158.
  8. ^ Arthur Beer, "Astronomical dating of works of art", Vistas in Astronomy, 9 (1967): 177-223.
  9. ^ Norman Lockyer and F. C. Penrose, "An Attempt to Ascertain the Date of the Original Construction of Stonehenge from Its Orientation", Proceedings of the Royal Society of London, 69. (1901 - 1902): 137-147.
  10. ^ F. C. Penrose, "On the Results of an Examination of the Orientations of a Number of Greek Temples with a View to Connect these Angles with the Amplitudes of Certain Stars at the Time the Temples were Founded, and an Endeavour to Derive therefrom the Dates of their Foundation by Consideration of the Changes Produced upon the Right Ascension and Declination of the Stars by the Precession of the Equinoxes", Philosophical Transactions of the Royal Society of London, A, 184 (1893): 805-834.
  11. ^ William Bell Dinsmoor, "Archæology and Astronomy", Proceedings of the American Philosophical Society, 10 (1939): 95-173.


  • Neugebaer, Otto. A History of Ancient Mathematical Astronomy, (3 vols). New York: Springer, 1975. Vol. 3, pp. 1071–1076 provides a brief introduction to astronomical chronology.


  • Gingerich, Owen and Barbara Welther. Planetary, Lunar, and Solar Positions, A. D. 1650 to 1805, Memoirs of the American Philosophical Society, 59S. Philadelphia, 1983.
  • Neugebauer, Paul V. Astronomische Chronologie (2 vols). Berlin: De Gruyter, 1929.
  • Steele, John M. "The Use and Abuse of Astronomy in Establishing Absolute Chronologies", Physics in Canada/La Physique au Canada, 59 (2003): 243-248.
  • Tuckerman, Bryant. Planetary, Lunar, and Solar Positions, 601 B.C. to A, D. 1, Memoirs of the American Philosophical Society, 56. Philadelphia, 1962.
  • Tuckerman, Bryant. Planetary, Lunar, and Solar Positions, A. D. 2 to 1649, Memoirs of the American Philosophical Society, 59. Philadelphia, 1964.

External links


An astrarium, also called a planetarium, is the mechanical representation of the cyclic nature of astronomical objects in one timepiece. It is an astronomical clock.

BPL (time service)

BPL is the call sign of the official long-wave time signal service of the People's Republic of China, operated by the Chinese Academy of Sciences, broadcasting on 100 kHz from CAS's National Time Service Center in Pucheng County, Shaanxi at 34°56′54″N 109°32′34″E, roughly 70 km northeast of Lintong, along with NTSC's short-wave time signal BPM on 2.5, 5.0, 10.0, and 15.0 MHz.

BPL broadcasts LORAN-C compatible format signal from 5:30 to 13:30 UTC, using an 800 kW transmitter covering a radius up to 3000 km.


Chronometry (from Greek χρόνος chronos, "time" and μέτρον metron, "measure") is the science of the measurement of time, or timekeeping. Chronometry applies to electronic devices, while horology refers to mechanical devices.

It should not to be confused with chronology, the science of locating events in time, which often relies upon it.

Clock position

A clock position is the relative direction of an object described using the analogy of a 12-hour clock to describe angles and directions. One imagines a clock face lying either upright or flat in front of oneself, and identifies the twelve hour markings with the directions in which they point.

Using this analogy, 12 o'clock means ahead or above, 3 o'clock means to the right, 6 o'clock means behind or below, and 9 o'clock means to the left. The other eight hours refer to directions that are not directly in line with the four cardinal directions.

In aviation, a clock position refers to a horizontal direction; it may be supplemented with the word high or low to describe the vertical direction which is pointed towards your feet. 6 o'clock high means behind and above the horizon, while 12 o'clock low means ahead and below the horizon.

Common year

A common year is a calendar year with 365 days, as distinguished from a leap year, which has 366. More generally, a common year is one without intercalation. The Gregorian calendar, (like the earlier Julian calendar), employs both common years and leap years to keep the calendar aligned with the tropical year, which does not contain an exact number of days.

The common year of 365 days has 52 weeks and one day, hence a common year always begins and ends on the same day of the week (for example, January 1 and December 31 fell on a Sunday in 2017) and the year following a common year will start on the subsequent day of the week. In common years, February has four weeks, so March will begin on the same day of the week. November will also begin on this day.

In the Gregorian calendar, 303 of every 400 years are common years. By comparison, in the Julian calendar, 300 out of every 400 years are common years, and in the Revised Julian calendar (used by Greece) 682 out of every 900 years are common years.


Endurantism or endurance theory is a philosophical theory of persistence and identity. According to the endurantist view, material objects are persisting three-dimensional individuals wholly present at every moment of their existence, which goes with an A-theory of time. This conception of an individual as always present is opposed to perdurantism or four dimensionalism, which maintains that an object is a series of temporal parts or stages, requiring a B-theory of time. The use of "endure" and "perdure" to distinguish two ways in which an object can be thought to persist can be traced to David Lewis.


HD2IOA is the callsign of a time signal radio station operated by the Navy of Ecuador. The station is located at Guayaquil, Ecuador and transmits in the HF band on 3.81 and 7.6 MHz.The transmission is in AM mode with only the lower sideband (part of the time H3E and the rest H2B/H2D) and consists of 780 Hz tone pulses repeated every ten seconds and voice announcements in Spanish.

While sometimes this station is described as defunct, reception reports of this station on 3.81 MHz appear regularly at the Utility DX Forum.

Hexadecimal time

Hexadecimal time is the representation of the time of day as a hexadecimal number in the interval [0,1).

The day is divided into 1016 (1610) hexadecimal hours, each hour into 10016 (25610) hexadecimal minutes, and each minute into 1016 (1610) hexadecimal seconds.

Intercalation (timekeeping)

Intercalation or embolism in timekeeping is the insertion of a leap day, week, or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months.


The minute is a unit of time or angle. As a unit of time, the minute is most of times equal to ​1⁄60 (the first sexagesimal fraction) of an hour, or 60 seconds. In the UTC time standard, a minute on rare occasions has 61 seconds, a consequence of leap seconds (there is a provision to insert a negative leap second, which would result in a 59-second minute, but this has never happened in more than 40 years under this system). As a unit of angle, the minute of arc is equal to ​1⁄60 of a degree, or 60 seconds (of arc). Although not an SI unit for either time or angle, the minute is accepted for use with SI units for both. The SI symbols for minute or minutes are min for time measurement, and the prime symbol after a number, e.g. 5′, for angle measurement. The prime is also sometimes used informally to denote minutes of time.


OLB5 was the callsign of a Czech time signal radio station. The station was located at Poděbrady and transmitted time signals which originated from the OMA (time signal) clock at Liblice.

The station transmitted in the HF band, on 3.17 MHz with 1 kW.


The past is the set of all events that occurred before a given point in time. The past is contrasted with and defined by the present and the future. The concept of the past is derived from the linear fashion in which human observers experience time, and is accessed through memory and recollection. In addition, human beings have recorded the past since the advent of written language. The first known use of the word "past" was in the fourteenth century; it developed as the past participle of the middle english verb passen meaning "to pass."

Specious present

The specious present is the time duration wherein one's perceptions are considered to be in the present. Time perception studies the sense of time, which differs from other senses since time cannot be directly perceived but must be reconstructed by the brain.

Tempus fugit

Tempus fugit is a Latin phrase, usually translated into English as "time flies". The expression comes from line 284 of book 3 of Virgil's Georgics, where it appears as fugit inreparabile tempus: "it escapes, irretrievable time". The phrase is used in both its Latin and English forms as a proverb that "time's a-wasting". Tempus fugit, however, is typically employed as an admonition against sloth and procrastination (cf. carpe diem) rather than a motto in favor of licentiousness (cf. "gather ye rosebuds while ye may"); the English form is often merely descriptive: "time flies like the wind", "time flies when you're having fun".

The phrase's full appearance in the Georgics is:

The phrase is a common motto, particularly on sundials and clocks.

Term (time)

A term is a period of duration, time or occurrence, in relation to an event. To differentiate an interval or duration, common phrases are used to distinguish the observance of length are near-term or short-term, medium-term or mid-term and long-term.

It is also used as part of a calendar year, especially one of the three parts of an academic term and working year in the United Kingdom: Michaelmas term, Hilary term / Lent term or Trinity term / Easter term, the equivalent to the American semester. In America there is a midterm election held in the middle of the four-year presidential term, there are also academic midterm exams.

In economics, it is the period required for economic agents to reallocate resources, and generally reestablish equilibrium. The actual length of this period, usually numbered in years or decades, varies widely depending on circumstantial context. During the long term, all factors are variable.

In finance or financial operations of borrowing and investing, what is considered long-term is usually above 3 years, with medium-term usually between 1 and 3 years and short-term usually under 1 year. It is also used in some countries to indicate a fixed term investment such as a term deposit.

In law, the term of a contract is the duration for which it is to remain in effect (not to be confused with the meaning of "term" that denotes any provision of a contract). A fixed-term contract is one concluded for a pre-defined time.

Tomorrow (time)

Tomorrow is a temporal construct of the relative future; literally of the day after the current day (today), or figuratively of future periods or times. Tomorrow is usually considered just beyond the present and counter to yesterday. It is important in time perception because it is the first direction the arrow of time takes humans on Earth.

UTC offset

The UTC offset is the difference in hours and minutes from Coordinated Universal Time (UTC) for a particular place and date. It is generally shown in the format ±[hh]:[mm], ±[hh][mm], or ±[hh]. So if the time being described is one hour ahead of UTC (such as the time in Berlin during the winter), the UTC offset would be "+01:00", "+0100", or simply "+01".

Every inhabited place in the world has a UTC offset that is a multiple of 15 minutes, and the majority of offsets (as well as all nautical time zones) are measured in whole hours.

UTC is the equivalent to GMT.


YVTO is the callsign of the official time signal from the Juan Manuel Cagigal Naval Observatory in Caracas, Venezuela. The content of YVTO's signal, which is a continuous 1 kW amplitude modulated carrier wave at 5.000 MHz, is much simpler than that broadcast by some of the other time signal stations around the world, such as WWV.

The methods of time transmission from YVTO are very limited. The broadcast employs no form of digital time code. The time of day is given in Venezuelan Standard Time (VET), and is only sent using Spanish language voice announcements. YVTO also transmits 100 ms-long beeps of 1000 Hz every second, except for thirty seconds past the minute. The top of the minute is marked by a 0.5 second 800 Hz tone.The station previously broadcast on 6,100 MHz but appears to have changed to the current frequency by 1990.

Yesterday (time)

Yesterday is a temporal construct of the relative past; literally of the day before the current day (today), or figuratively of earlier periods or times, often but not always within living memory.

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