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==Origin==
==Origin==
[[Latin]] ''trepidātiōn-em'', n. of [[action]]; ''trepidāre''
[[Latin]] ''trepidātiōn-em'', n. of [[action]]; ''trepidāre''
*[http://en.wikipedia.org/wiki/17th_century 1605]
*[https://en.wikipedia.org/wiki/17th_century 1605]
==Definitions==
==Definitions==
* 1. Tremulous [[agitation]]; confused hurry or [[alarm]]; [[confusion]]; flurry; perturbation.
* 1. Tremulous [[agitation]]; confused hurry or [[alarm]]; [[confusion]]; flurry; perturbation.
*2. Tremulous, [[vibratory]], or reciprocating movement; vibration; [http://en.wikipedia.org/wiki/Oscillation oscillation], rocking; an instance of this; also, involuntary trembling of the limbs, as in paralytic affections; tremor.
*2. Tremulous, [[vibratory]], or reciprocating movement; vibration; [https://en.wikipedia.org/wiki/Oscillation oscillation], rocking; an instance of this; also, involuntary trembling of the limbs, as in paralytic affections; tremor.
*3. Astronomy. A libration of the eighth (or ninth) [[sphere]], added to the system of [http://en.wikipedia.org/wiki/Ptolemy Ptolemy] by the Arab astronomer [http://www.worldcat.org/title/notice-sur-une-theorie-ajoutee-par-abul-hasan-thabit-ben-korrah-al-harrani-a-larithmetique-speculative-des-grecs/oclc/493040850 Thabet ben Korrah], c950, in order to account for certain [[phenomena]], esp. precession, really due to [[motion]] of the [[earth]]'s [[axis]]
*3. Astronomy. A libration of the eighth (or ninth) [[sphere]], added to the system of [https://en.wikipedia.org/wiki/Ptolemy Ptolemy] by the Arab astronomer [https://www.worldcat.org/title/notice-sur-une-theorie-ajoutee-par-abul-hasan-thabit-ben-korrah-al-harrani-a-larithmetique-speculative-des-grecs/oclc/493040850 Thabet ben Korrah], c950, in order to account for certain [[phenomena]], esp. precession, really due to [[motion]] of the [[earth]]'s [[axis]]
==Description==
==Description==
According to a [http://en.wikipedia.org/wiki/Middle_Ages medieval theory] of [[astronomy]], '''trepidation''' is [http://en.wikipedia.org/wiki/Oscillation oscillation] in the [http://en.wikipedia.org/wiki/Precession_of_the_equinoxes precession of the equinoxes]. The [[theory]] was popular from the 9th to the 16th centuries.
According to a [https://en.wikipedia.org/wiki/Middle_Ages medieval theory] of [[astronomy]], '''trepidation''' is [https://en.wikipedia.org/wiki/Oscillation oscillation] in the [https://en.wikipedia.org/wiki/Precession_of_the_equinoxes precession of the equinoxes]. The [[theory]] was popular from the 9th to the 16th centuries.
The origin of the [[theory]] of trepidation comes from the Small Commentary to the Handy Tables written by [http://en.wikipedia.org/wiki/Theon_of_Alexandria Theon of Alexandria] in the 4th century CE. In [http://en.wikipedia.org/wiki/Precession precession], the [http://en.wikipedia.org/wiki/Equinoxes equinoxes] appear to move slowly through the [http://en.wikipedia.org/wiki/Ecliptic ecliptic], completing a revolution in approximately 25,800 years (according to [[modern]] astronomers). Theon states that certain (unnamed) ancient astrologers believed that the precession, rather than being a steady unending [[motion]], instead reverses direction every 640 years. The equinoxes, in this [[theory]], move through the ecliptic at the rate of 1 degree in 80 years over a span of 8 degrees, after which they suddenly reverse direction and travel back over the same 8 degrees. Theon describes but did not endorse this [[theory]].
The origin of the [[theory]] of trepidation comes from the Small Commentary to the Handy Tables written by [https://en.wikipedia.org/wiki/Theon_of_Alexandria Theon of Alexandria] in the 4th century CE. In [https://en.wikipedia.org/wiki/Precession precession], the [https://en.wikipedia.org/wiki/Equinoxes equinoxes] appear to move slowly through the [https://en.wikipedia.org/wiki/Ecliptic ecliptic], completing a revolution in approximately 25,800 years (according to [[modern]] astronomers). Theon states that certain (unnamed) ancient astrologers believed that the precession, rather than being a steady unending [[motion]], instead reverses direction every 640 years. The equinoxes, in this [[theory]], move through the ecliptic at the rate of 1 degree in 80 years over a span of 8 degrees, after which they suddenly reverse direction and travel back over the same 8 degrees. Theon describes but did not endorse this [[theory]].
A more sophisticated version of this theory was adopted in the 9th century to explain a variation which Islamic astronomers incorrectly believed was affecting the rate of precession.[2] This version of trepidation is described in ''De motu octavae sphaerae'' (''On the Motion of the Eighth Sphere''), a [[Latin]] translation of a lost Arabic original. The book is attributed to the Arab astronomer by [http://en.wikipedia.org/wiki/Ptolemy Ptolemy] by the Arab astronomer [http://www.worldcat.org/title/notice-sur-une-theorie-ajoutee-par-abul-hasan-thabit-ben-korrah-al-harrani-a-larithmetique-speculative-des-grecs/oclc/493040850 Thābit ibn Qurra], but the attribution has been contested in modern times. In this trepidation model, the oscillation is added to the equinoxes as they precess. The [http://en.wikipedia.org/wiki/Oscillation oscillation] occurred over a period of 7000 years, added to the eighth (or ninth) sphere of the [http://en.wikipedia.org/wiki/Ptolemaic_system Ptolemaic system]. "Thabit's" trepidation model was used in the [http://en.wikipedia.org/wiki/Alfonsine_Tables Alfonsine Tables], which assigned a period of 49,000 years to precession. This version of trepidation dominated Latin astronomy in the later Middle Ages.
A more sophisticated version of this theory was adopted in the 9th century to explain a variation which Islamic astronomers incorrectly believed was affecting the rate of precession.[2] This version of trepidation is described in ''De motu octavae sphaerae'' (''On the Motion of the Eighth Sphere''), a [[Latin]] translation of a lost Arabic original. The book is attributed to the Arab astronomer by [https://en.wikipedia.org/wiki/Ptolemy Ptolemy] by the Arab astronomer [https://www.worldcat.org/title/notice-sur-une-theorie-ajoutee-par-abul-hasan-thabit-ben-korrah-al-harrani-a-larithmetique-speculative-des-grecs/oclc/493040850 Thābit ibn Qurra], but the attribution has been contested in modern times. In this trepidation model, the oscillation is added to the equinoxes as they precess. The [https://en.wikipedia.org/wiki/Oscillation oscillation] occurred over a period of 7000 years, added to the eighth (or ninth) sphere of the [https://en.wikipedia.org/wiki/Ptolemaic_system Ptolemaic system]. "Thabit's" trepidation model was used in the [https://en.wikipedia.org/wiki/Alfonsine_Tables Alfonsine Tables], which assigned a period of 49,000 years to precession. This version of trepidation dominated Latin astronomy in the later Middle Ages.
Islamic astronomers described other models of trepidation. In the West, an alternative to ''De motu octavae sphaerae'' was part of the [[theory]] of the [[motion]] of the [[Earth]] published by [http://en.wikipedia.org/wiki/Nicolaus_Copernicus Nicolaus Copernicus] in [http://en.wikipedia.org/wiki/De_revolutionibus_orbium_coelestium ''De revolutionibus orbium coelestium''] (1543). Copernicus' version of trepidation combined the oscillation of the equinoxes (now known to be a spurious [[motion]]) with a change in the obliquity of the ecliptic ([http://en.wikipedia.org/wiki/Axial_tilt axial tilt]), acknowledged today as an authentic motion of the [[Earth]]'s [[axis]].
Islamic astronomers described other models of trepidation. In the West, an alternative to ''De motu octavae sphaerae'' was part of the [[theory]] of the [[motion]] of the [[Earth]] published by [https://en.wikipedia.org/wiki/Nicolaus_Copernicus Nicolaus Copernicus] in [https://en.wikipedia.org/wiki/De_revolutionibus_orbium_coelestium ''De revolutionibus orbium coelestium''] (1543). Copernicus' version of trepidation combined the oscillation of the equinoxes (now known to be a spurious [[motion]]) with a change in the obliquity of the ecliptic ([https://en.wikipedia.org/wiki/Axial_tilt axial tilt]), acknowledged today as an authentic motion of the [[Earth]]'s [[axis]].
==Notes==
==Notes==
#a fully quoted translation is found in Jones A., Ancient Rejection and Adoption of Ptolemy’s Frame of Reference for Longitudes in Ptolemy in Perspective, (ed) A. Jones, Spinger, 2010, p.11
#a fully quoted translation is found in Jones A., Ancient Rejection and Adoption of Ptolemy’s Frame of Reference for Longitudes in Ptolemy in Perspective, (ed) A. Jones, Spinger, 2010, p.11