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{{Short description|Cycles used to predict eclipses of the Sun and Moon}}
The '''saros''' ({{IPAc-en|audio=
|url=
|title=A Catalogue of Eclipse Cycles
|date=8 September 2003
|first=Robert Harry
|last=van Gent}}</ref>
A series of eclipses that are separated by one saros is called a ''saros series''. It corresponds to:
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*18.999 eclipse years (38 [[eclipse season]]s)
*238.992 [[anomalistic month]]s
*241.029 [[sidereal month]]s
The 19 eclipse years means that if there is a [[solar eclipse]] (or [[lunar eclipse]]), then after one saros a new moon will take place at the same [[lunar node|node]] of the [[orbit of the Moon]], and under these circumstances another eclipse can occur.
== History ==
The earliest discovered historical record of what is known as the saros is by [[Neo-Babylonian Empire|Chaldean]] [[Babylonian astronomy|(neo-Babylonian) astronomers]] in the last several centuries
The name "saros" ({{lang-el|σάρος}}) was applied to the eclipse cycle by [[Edmond Halley]] in 1686,<ref>{{cite journal |last1=Halley |first1=E. |title=Emendationes & Notae in tria loca vitiose edita in textu vulgato Naturalis Historiae C. Plinii |journal=Philosophical Transactions of the Royal Society of London |date=1686 |volume=17 |issue=194 |pages=535–540 |doi=10.1098/rstl.1686.0101 |s2cid=186208699 |url=https://royalsocietypublishing.org/doi/pdf/10.1098/rstl.1686.0101 |trans-title=Corrections and notes on three badly edited passages in a common edition of C. Pliny's Natural History |language=Latin}} From p. 537: ''"Secundo loco annotare libet hanc Periodum Chaldaeis olim Astronomiae repertoribus ''Saron'' dici, … "'' (In the second passage, it is pleasing to note [that] this period was called "Saron" by Chaldean authors of astronomy, … ) ''" … Sari mensura & numerus apud Chaldaeos, etenim 120 Sari constituunt annos 2222 juxta Chaldaeorum calculum, nempe ''Saros'' constat ex 222 mensibus Lunaribus, qui sunt 18 Anni cum sex mensibus."'' ( … the Sari [was] a measure and number in the writings of the Chaldeans, as a matter of fact 120 Sari constitute 2,222 years according to the Chaldeans' calculation; indeed a ''Saros'' consists of 222 lunar months, which are 18 years and 6 months.)</ref> who took it from the ''[[Suda]]'', a [[Byzantine]] lexicon of the 11th century. The Suda says, "[The saros is] a measure and a number among [[Chaldea]]ns. For 120 saroi make 2220 years (years of 12 lunar months) according to the Chaldeans' reckoning, if indeed the saros makes 222 lunar months, which are 18 years and 6 months (i.e. years of 12 lunar months)."<ref>The Suda entry is online [http://www.stoa.org/sol-entries/sigma/148 here].</ref> The information in the ''Suda'' in turn was derived directly or otherwise from the ''Chronicle'' of [[Eusebius of Caesarea]],{{citation needed|date=December 2014}} which quoted [[Berossus]]. ([[Guillaume Le Gentil]] claimed that Halley's usage was incorrect in 1756,<ref>Le Gentil's criticism of Halley's use of the term "Saros" appeared in two places in the 1756 volume of ''Histoire de l'Académie
* in the ''Histoire'' section: {{cite journal |last1=(Staff) |title=Sur le Saros Chaldaïque |journal=Histoire de l'Académie
* in the ''Mémoires'' section: {{cite journal |last1=le Gentil |title=Remarques sur un mémoire de M Halley, inséré dans les Transactions philosophiques de l'année 1692, No. 194, page 535, dans lequel M. Halley parlé du Saros des Chaldéens |journal=Histoire de l'Académie
[[File:Manual2021-X MOUSSAS SAROS.jpg|thumb|Antikythera Mechanism Saros cycle for the prediction of eclipses ΣΚΓ′, in the red rectangle, and means 223 months. Written between 150 and 100 BCE]]
The Saros period of 223 lunar months (ΣΚΓ') is in the [[Antikythera Mechanism]] user manual of this instrument, made around 150 to 100 BC in Greece, as seen in the picture. This number is one of a few inscriptions of the mechanism that are visible with unaided eye.<ref>Freeth, T., Bitsakis, Y., Moussas, X., Seiradakis, J. H., Tselikas, A., Mangou, H., ... & Edmunds, M. G. (2006). Decoding the ancient Greek astronomical calculator known as the Antikythera Mechanism. Nature, 444(7119), 587-591</ref> Above it the period of [[Metonic cycle]] and [[Callippic cycle]] are visible too. ▼
▲The Saros period of 223 lunar months (
==Description==
[[Image:Lunareclipsediagram.svg|240px|thumb|left|Lunar eclipses occurring near the Moon's descending node are given ''odd'' saros series numbers. The first eclipse in such series passes through the southern edge of the Earth's shadow, and the Moon's path is shifted northward each successive saros, while lunar eclipses occurring near the Moon's ascending node are given ''even'' saros series numbers. The first eclipse in such series passes through the northern edge of the Earth's shadow, and the Moon's path is shifted southward each successive saros.]]
The saros, a period of 6585.3211 days (15 common years + 3 leap years + 12.321 days, 14 common years + 4 leap years + 11.321 days, or 13 common years + 5 leap years + 10.321 days), is useful for predicting the times at which nearly identical eclipses will occur. Three periodicities related to lunar orbit, the [[Month#Types of months in astronomy|synodic month]], the [[Month#Types of months in astronomy|draconic month]], and the [[Month#Types of months in astronomy|anomalistic month]] coincide almost perfectly each saros cycle. For an eclipse to occur, either the Moon must be located between the Earth and Sun (for a [[solar eclipse]]) or the Earth must be located between the Sun and Moon (for a [[lunar eclipse]]). This can happen only when the Moon is [[new moon|new]] or [[full moon|full]], respectively, and repeat occurrences of these [[lunar phase]]s result from solar and lunar orbits producing the Moon's ''synodic period'' of 29.53059 days. During most full and new moons, however, the shadow of the Earth or Moon falls to the north or south of the other body. Eclipses occur when the three bodies form a nearly straight line. Because the plane of the lunar orbit is inclined to that of the Earth, this condition occurs only when a full or new Moon is near or in the [[plane of the ecliptic|ecliptic plane]], that is when the Moon is at one of the two [[lunar node|nodes]] (the ascending or descending node). The period of time for two successive lunar passes through the ecliptic plane (returning to the same node) is termed the ''draconic month'', a 27.21222 day period. The three-dimensional geometry of an eclipse, when the new or full moon is near one of the nodes, occurs every five or six months when the Sun is in conjunction or opposition to the Moon and coincidentally also near a node of the Moon's orbit at that time, or twice per [[Year#Draconic year|eclipse year]]. Two eclipses separated by one saros have very similar appearance and duration due to the distance between the Earth and Moon being nearly the same for each event: this is because the saros is also an integer multiple of the ''anomalistic month'' of 27.5545 days, the period of the
[[File:Saros15122015.gif|thumb|Visualization of a period of one saros cycle in 3D.]]
After one saros, the Moon will have completed roughly an integer number of synodic, draconic, and anomalistic periods (223, 242, and 239) and the Earth-Sun-Moon geometry will be nearly identical: the Moon will have the same phase and be at the same node and the same distance from the Earth. In addition, because the saros is close to 18 years in length (about 11 days longer), the Earth will be nearly the same distance from the Sun, and tilted to it in nearly the same orientation (same season).<ref name="totality">{{Cite book| last = Littmann| first = Mark |author2=Fred Espenak |author3=Ken Willcox | title = Totality: Eclipses of the Sun | publisher = Oxford University Press | date = 2008 | isbn = 978-0-19-953209-4}}</ref> Given the date of an eclipse, one saros later a nearly identical eclipse can be predicted. During this 18-year period, about 40 other solar and lunar eclipses take place, but with a somewhat different geometry. One saros equaling 18.03 years is not equal to a perfect integer number of lunar orbits (Earth revolutions with respect to the fixed stars of 27.32166 days [[sidereal month]]), therefore, even though the relative geometry of the Earth–Sun–Moon system will be nearly identical after a saros, the Moon will be in a slightly different position with respect to the stars for each eclipse in a saros series. The axis of rotation of the Earth–Moon system exhibits a [[Lunar precession|precession]] period of 18.59992 years.
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Each saros series starts with a partial eclipse (Sun first enters the end of the node), and each successive saros the path of the Moon is shifted either northward (when near the descending node) or southward (when near the ascending node) due to the fact that the saros is not an exact integer of draconic months (about one hour short). At some point, eclipses are no longer possible and the series terminates (Sun leaves the beginning of the node). An arbitrary solar saros series was designated as solar saros series 1 by compilers of eclipse statistics. This series has finished, but the eclipse of November 16, 1990 BC ([[Julian calendar]]) for example is in solar saros series 1. There are different saros series for solar and lunar eclipses. For lunar saros series, the lunar eclipse occurring 58.5 synodic months earlier (February 23, 1994 BC) was assigned the number 1. If there is an eclipse one [[inex]] (29 years minus about 20 days) after an eclipse of a particular saros series then it is a member of the next series. For example, the eclipse of October 26, 1961 BC is in solar saros series 2. Saros series, of course, went on before these dates, and it is necessary to extend the saros series numbers backwards to negative numbers even just to accommodate eclipses occurring in the years following 2000 BC (up till the last eclipse with a negative saros number in 1367 BC). For solar eclipses the statistics for the complete saros series within the era between 2000 BC and AD 3000 are given in this article's references.<ref name="Meeus-Mathematical Astronomy Morsels III">{{cite book|last=Meeus|first=Jean|date=2004|title=Ch. 18 "About Saros and Inex series" in: Mathematical Astronomy Morsels III|publisher=Willmann-Bell, Richmond VA, USA}}</ref><ref name="Espenak-Five Millennium Canon of Solar Eclipses">{{cite web|last=Espenak|first=Fred|author2=Jean Meeus|title=Five Millennium Canon of Solar Eclipses, Section 4 (NASA TP-2006-214141)|publisher=NASA STI Program Office|date=October 2006|url=http://sunearth.gsfc.nasa.gov/eclipse/5MCSE/5MCSE-Text.pdf|access-date=2007-01-24|url-status=dead|archive-url=https://web.archive.org/web/20070620150046/http://sunearth.gsfc.nasa.gov/eclipse/5MCSE/5MCSE-Text.pdf|archive-date=2007-06-20}}</ref> It takes between 1226 and 1550 years for the members of a saros series to traverse the Earth's surface from north to south (or vice versa). These extremes allow from 69 to 87 eclipses in each series (most series have 71 or 72 eclipses). From 39 to 59 (mostly about 43) eclipses in a given series will be central (that is, total, annular, or hybrid annular-total). At any given time, approximately 40 different saros series will be in progress.
Saros series, as mentioned, are numbered according to the type of eclipse (lunar or solar).<ref>{{cite book|author=G. van den Bergh|date=1955|title=Periodicity and Variation of Solar (and Lunar) Eclipses (2 vols.)|publisher=H. D. Tjeenk Willink & Zoon N. V., Haarlem}}</ref><ref>{{cite book|author=Bao-Lin Liu|author2=Alan D. Fiala|date=1992|title=Canon of Lunar Eclipses, 1500 B.C. to A.D. 3000|publisher=Willmann-Bell, Richmond VA}}</ref> In odd numbered series (for solar eclipses) the Sun is near the ascending node, whereas in even numbered series it is near the descending node (this is reversed for lunar eclipse saros series). Generally, the ordering of these series determines the time at which each series peaks, which corresponds to when an eclipse is closest to one of the lunar nodes. For solar eclipses, the 40 series numbered between [[Solar Saros 117|117]] and [[Solar Saros 156|156]] are active (series 117 will end in 2054), whereas for lunar eclipses, there are now 41 active saros series (these numbers can be derived by counting the number of eclipses listed over an 18-year (saros) period from the eclipse catalog sites).<ref name="NASA Solar eclipses">{{
===Example===
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== {{anchor|sar}} Relationship between lunar and solar saros (sar) ==
After a given lunar or solar eclipse, after 9 years and {{frac|5
For example, if the Moon's penumbra partially covers the southern limb of the Earth during a solar eclipse, 9 years and {{frac|5
==See also==
*[[List of saros series for solar eclipses]]
*[[List of saros series for lunar eclipses]]
*[[Eclipse cycle]]
*[[Exeligmos]]
*[[Solar eclipse]]
*[[Lunar eclipse]]
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==Bibliography==
*[[Jean Meeus]] and [[Hermann Mucke (astronomer)|Hermann Mucke]] (1983) ''Canon of Lunar Eclipses''. Astronomisches Büro, Vienna
*[[Theodor von Oppolzer]] (1887). [https://archive.org/details/canonderfinstern00oppo ''Canon der Finsternisse'']. Vienna
* Jean Meeus, ''Mathematical Astronomy Morsels''
==External links==
{{wikiquote}}
*[https://www.solar-eclipse.info/en/saros/ List of all active saros cycles]
*[http://eclipse.gsfc.nasa.gov/SEsaros/SEsaros.html NASA – Eclipses and the Saros]
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{{DEFAULTSORT:Saros cycle}}
[[Category:1st-millennium BC introductions]]
[[Category:Eclipses]]
[[Category:Time in astronomy]]
[[Category:Technical factors of astrology]]
[[Category:Neo-Babylonian Empire]]
[[Category:Chaldea]]
[[Category:Units of time]]
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