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Precession
A slow conical motion of the axis of rotation of the Earth, largely due to the gravitational torque exerted by the Sun and Moon on the equatorial bulge of the Earth. Lunisolar precession is that component of general precession due to the Sun and Moon only. Planetary precession is a slow change in the orientation of the Earth's orbital plane caused by perturbations from the planets and is the other component of general precession. The period of precession is about 26,000 years. |
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Prima
A watch in the medieval monastery watch system beginning at sunrise, or the first hour. Also called prime. |
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Quad
A term used in SCDTL to indicate a period of 1,461 days, which is the usual number of days encountered in a period of four years, which includes three common years of 365 days and one leap year of 366 days. |
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Saros cycle
A period of 18 years 11 days 8 hours, after which a given pattern of lunar and solar eclipses tends to repeat. |
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Second
The SI second is the unit of time for the TAI scale, defined to be exactly 9,192,631,770 cycles of radiation, corresponding to the transition between two hyperfine levels in the ground state of cesium 133. That is, the international second is defined in terms of atomic time. |
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The term second derives from medieval Latin and is an ellipsis for secunda minuta or second minute, referring to the result of a second sexagesimal division, the first producing the minute or prime minute as a division of the hour. |
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Second Wave
A term referring to a general stage in cultural evolution, which comes between First Wave (developed agriculture) and Third Wave (information and service economies), characterized by industrialization, emphasis on mass production of uniform goods, the factory system, manual processing of information, centralization, and nationalization. |
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Sexta
A watch in the medieval monastery system beginning with the sixth hour, and hence the hour at which the sun is overhead. Therefore, this term denotes the hour called noon today. The word siesta is derived from this term. |
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Sextile
Generally, a portion of something divided into six parts. Historically, the term can refer to leap years in the French Revolutionary calendar, which were called sextile because a sixth epagomenal day was added. |
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Sidereal
Referring to the stars, as opposed to the Sun or Moon or other planetary body, as a reference frame for measuring relative motion. |
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