| 词条 | Culmination |
| 释义 |
In observational astronomy, culmination is the instant of time of the transit of an celestial object (the Sun, the Moon, a planet, a star, constellation or a deep-sky object) across the observer's local meridian. During each day, every celestial object appears to move along a circular path on the celestial sphere due to the Earth's rotation creating two moments when it crosses the meridian.[1][2] Except at the geographic poles, any celestial object passing through the meridian has an upper culmination, when it reaches its highest point above the horizon, and nearly twelve hours later, is followed by a lower culmination, when it reaches its lowest point. The time of culmination (when the object culminates) is often used to mean upper culmination.[1][2][3] An object's altitude (A) in degrees at its upper culmination is equal to 90 subtracted by the observer's latitude (L) and added by the object's declination (δ): {{math|A {{=}} 90° − L + δ}}. CasesThree cases are dependent on the observer's latitude (L) and the declination (δ) of the celestial object:
The third case applies for objects in a part of the full sky equal to the cosine of the latitude (at the equator it applies for all objects, because the sky turns around the horizontal north-south line; at the poles it applies for none, because the sky turns around the vertical line). The first and second case each apply for half of the remaining sky. Period of time{{see also|Sidereal time|Equation of time|Perturbation (astronomy)}}The period between one upper culmination and the next is about 24 hours, while the period between an upper one and a lower one is 12 hours. The orbital motion and proper motion of Earth affect the period between successive upper culminations. Due to the proper and improper motions of the Sun, one solar day (the interval between like culminations of the Sun) is slightly longer than one sidereal day (the interval between like culminations of any reference star).{{Citation needed|date=August 2007}} The mean difference is 1/365.24219, since Earth takes 365.24219 days to complete one orbit around the Sun. The Sun{{main|Sun transit time|Noon#Solar noon}}{{further|Solar zenith angle|Sun path}}From the tropics and middle latitudes, the Sun is visible in the sky at its upper culmination (at solar noon) and invisible (below the horizon) at its lower culmination (at solar midnight). When viewed from the region within either polar circle around the winter solstice of that hemisphere (the December solstice in the Arctic and the June solstice in the Antarctic), the Sun is below the horizon at both of its culminations. Supposing that the declination of the Sun is +20° on a given summer day, the complementary angle of 70° (from the Sun to the pole) is added to and subtracted from the observer's latitude to find the solar altitudes at upper and lower culminations, respectively.
Circumpolar stars{{main|Circumpolar star}}From most of the Northern Hemisphere, Polaris (the North Star) and the other stars of the constellation Ursa Minor appear to circle counterclockwise around the north celestial pole and remain visible at both culminations (as long as the sky is clear and dark enough). Such objects that always remain above the horizon, as viewed from the observer's latitude, are described as circumpolar. See also
References1. ^1 {{cite book |title=The Cambridge Guide to the Constellations |first=Michael E. |last=Bakich |publisher=Cambridge University Press |year=1995 |page=8 |isbn=0521449219}} 2. ^1 {{cite book |entry=Culmination |page=110 |title=The Facts on File Dictionary of Astronomy |first1=John |last1=Daintith |first2=William |last2=Gould |publisher=Infobase Publishing |year=2009 |isbn=1438109326}} 3. ^{{cite book |entry=Meridian |page=993 |title=The National Enyclopaedia |edition=library |volume=8 |location=London, Edinburgh, and Glasgow |first=William |last=Mackenzie |issue=69 |publisher=Ludgate Hill, E.C. |year=1879–81}} 2 : Celestial mechanics|Spherical astronomy |
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