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| Saros (astronomy) | |
|---|---|
| Name | Saros |
| Caption | Path of a solar eclipse in a Saros series |
| Discipline | Astronomy |
| First | Antiquity |
| Period | 18 years, 11 days, 8 hours (approx.) |
Saros (astronomy) is a period used to predict solar eclipses and lunar eclipses based on the alignment of the Moon, Earth, and Sun. The cycle links eclipse recurrence with the motions described by Hipparchus, refined by Ptolemy and applied by medieval astronomers such as Ibn al-Shatir and Abu Ma'shar. Modern use of the period appears in catalogs maintained by institutions like NASA, Royal Astronomical Society, and observatories including Greenwich Observatory.
The Saros ties together eclipse events through the interaction of three orbital periods: the synodic month, the draconic month, and the anomalistic month, producing near-repeat geometry after about 18 years; this concept connects historical records from Babylon, Ancient Greece, Antioch, and Alexandria to modern datasets curated by Harvard Observatory, Smithsonian Astrophysical Observatory, and Jet Propulsion Laboratory. Observers from China, Maya civilization, and Islamic Golden Age astronomers recorded eclipse periodicities that later informed European scholars such as Tycho Brahe and Johannes Kepler.
A Saros arises because 223 synodic months ≈ 242 draconic months ≈ 239 anomalistic months, aligning lunar phase, nodal crossing, and lunar distance; this near-integer relation yields a repeat after about 6585.32 days (18 years, 11 days, 8 hours) described by Simon Newcomb and used by computational programs from Besselian elements to modern numerical ephemerides like DE430. The extra ~8-hour fraction shifts visibility westward by ~120° of longitude, linking successive eclipses observed in regions such as Europe, North America, Africa, and Asia and requiring cross-referencing with timekeeping standards like Greenwich Mean Time and Coordinated Universal Time.
Saros cycles produce families of eclipses called Saros series, each starting with a partial eclipse near a lunar pole and evolving through partial, annular, total, or hybrid eclipse phases before ending as partial; individual series span over a millennium and comprise about 70–80 events, cataloged by agencies such as United States Naval Observatory and researchers at International Astronomical Union. Lunar Saros series follow analogous progression for penumbral, partial, and total phases, documented in catalogs produced by Royal Observatory Greenwich and universities like Cambridge University and Harvard University.
The term "Saros" was introduced into modern astronomy by Edmond Halley and popularized by Gustave de Pontécoulant after studies of Babylonian astronomy records such as the Enûma Anu Enlil tablets; earlier scholars including Pliny the Elder and Claudius Ptolemy referenced eclipse cycles without the modern nomenclature. Medieval contributions from Al-Battani and Al-Biruni improved nodal calculations, while Renaissance figures like Christopher Clavius and Giovanni Cassini used Saros intervals for prediction, informing eclipse expeditions undertaken by scientists such as Edmond Halley and explorers linked with institutions like Royal Society.
Predictive application of the Saros involves translating the 18-year interval through corrections for the ~8-hour longitudinal shift and secular orbital variations cataloged in modern ephemerides like VSOP87 and processed by software from Jet Propulsion Laboratory and projects at European Space Agency. Accurate predictions require incorporating perturbations by Jupiter, Saturn, and other bodies accounted for in tables by Simon Newcomb and later improvements by E. W. Brown and Jean Meeus. Contemporary eclipse prediction combines Saros indexing with high-precision astrometry from observatories such as Mauna Kea Observatory and space missions like Gaia.
Well-known Saros series include those producing landmark eclipses observed during events involving figures like Christopher Columbus (New World reports), Gustav Kirchhoff-era observations, and the widely studied total solar eclipse of 1919 associated with tests of Albert Einstein's General relativity; these events are traced to specific Saros series listed in databases at NASA and the United States Naval Observatory. The Saros that generated the total solar eclipse of 2017 crossed the United States and was extensively documented by institutions including NASA, Smithsonian Institution, and universities such as Cornell University and University of California, Berkeley.
Saros cycles influenced ritual, historical chronicles, and scientific development across cultures—from Babylonian astronomy and Mesoamerican codices to observations by Chinese imperial court astronomers and Islamic scholars linked to institutions like House of Wisdom. Scientifically, Saros-based records enabled chronology work by historians such as F. E. Robbins and informed orbital theory developments used by astronomers at Royal Astronomical Society and space agencies including NASA and European Space Agency. The cycle continues to bridge heritage records from Mesopotamia, Greece, India, and China with modern astrophysical research at facilities like Harvard-Smithsonian Center for Astrophysics and missions such as SOHO.