Meridian, simply means line of longtitude.
Long definition:
A (geographic) meridian (or line of longitude) is the half of an imaginary great circle on the Earth's surface, a coordinate line terminated by the North Pole and the South Pole, connecting points of equal longitude, as measured in angular degrees east or west of the Prime Meridian.
The term meridian comes from the Latin meridies, meaning "midday"; the subsolar point passes through a given meridian at solar noon, midway between the times of sunrise and sunset on that meridian.[
Methods of determining longitude and Earth rotation
Traditional optical instruments for measuring Earth rotation included visual and photographic zenith tubes, circumzenithals, Danjon astrolabes, and transit telescopes, such as the Airy Transit Circle. These instruments measured local sidereal time by the passage of specially designated “clock stars” across a plane of reference established in part by the local direction of gravity. These measurements were realized with respect to the local vertical using a basin of mercury, and the astronomical latitudes and longitudes of the observatories were thus affected by local gravity, because their coordinates were determined from the same observations.
The sidereal time observed from these instruments was converted to mean solar time according to a conventional relationship, and time signals were adjusted to match the astronomical observations. The difference between astronomically determined local mean times t (in hours) is proportional to the difference in local longitudes Λ
(in degrees):
t−t0=(Λ−Λ0)/15,
(1)
where Λ0
and t0 refer to the astronomical longitude and time of the reference, or “prime”, meridian. The 1884 International Meridian Conference recommended that the prime meridian, Λ0
= 0, pass through the Airy Transit Circle at Greenwich (Explanatory Supplement 1961), establishing the mean solar time determined at the Airy Transit Circle—Greenwich Mean Time (GMT)—as an internationally recognized basis for global timekeeping and navigation. Any astronomical time service could thereby convert its own determination of local mean time to GMT by removing its longitude difference from Greenwich.
Because of significant discrepancies between the earliest radio time-signal broadcasts, the Bureau International de l’Heure (BIH) was established at the Paris Observatory in 1912 to help coordinate the national time bureaus and improve longitudes globally (Guinot 2000). For purposes of global timekeeping by the BIH, the Greenwich Observatory was superseded in 1929 by a statistical construct known as the “mean observatory”, of which Greenwich became one of many contributors. The formation of GMT, now called Universal Time, from this “mean observatory” involved adopted corrections to the contributed data and weights that could change annually (Feissel 1980). To maintain a continuous and consistent series of Universal Time, the BIH also assumed the responsibility for establishing and refining the terrestrial reference systems used for the analyses (Guinot 2000). The early BIH terrestrial reference frames were entirely based on astronomical coordinates (Mueller 1969).
Delays in the analysis of polar motion from the International Latitude Service (ILS) made it impractical for the BIH to correct Universal Time for the motion of the rotational pole until the middle of the twentieth century (Guinot 2000). Universal Time observed from a given observatory was designated UT0, whereas the distributed astronomical time scale corrected for polar motion became known as UT1. After the BIH adopted various reference poles of different epochs during the 1950s and 1960s (Robbins 1967), the BIH standardized on the average ILS pole from 1900–1905—known as the Conventional International Origin—as part of an update to celestial and terrestrial reference systems. To preserve the continuity of UT1–UT0 for contributing stations within this new “1968 BIH System”, the adopted longitudes of observatories were systematically adjusted for the change of pole. Although the Airy Transit Circle was no longer operational as a BIH station, its astronomical longitude would have changed from zero to 0.2927′′
W (about 6 m) within this new system (MacDonald 1985; Schmid 1974).
meridian passage
The meridian passage is the moment when a celestial object passes the meridian of longitude of the observer. At this point, the celestial object is at its highest point. When the sun passes two times an altitude while rising and setting can be averaged to give the time of meridian passage. Navigators utilized the sun's declination and the sun's altitude at local meridian passage, in order to calculate their latitude with the formula.[18]
Latitude = (90° – noon altitude + declination)
The declination of major stars are their angles north and south from the celestial equator.[19] It is important to note that the meridian passage will not occur exactly at 12 hours because of the inclination of the earth. The meridian passage can occur within a few minutes of variation.[20]
astronomical basis of time keeping Greenwich
Greenwich astronomical observations 1843
apparent RA of sun observation vs nautical almanac 1831
Nowadays, planets transit are observed using scanning-slit micrometer and photoelectric detector system, Carlsberg Meridian Telescope (CMT), La Palma
HKO open data sun and moon transit time
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