antimeridian is the meridian opposite of earth's prime meridian.
antimeridian is the meridian opposite of earth's prime meridian.
the meridian opposite of earth's prime meridian?
antimeridian
prime meridian









































a prime meridian is a meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. together, a prime meridian and its anti-meridian (the 180th meridian in a 360°-system) form a great circle. this great circle divides a spheroid, e.g., earth, into two hemispheres. if one uses directions of east and west from a defined prime meridian, then they can be called the eastern hemisphere and the western hemisphere.
prime meridian
when the airy transit circle was built, a mercury basin was used to align the telescope to the perpendicular. thus the circle was aligned with the local vertical or plumb line, which is deflected slightly from the normal, or line perpendicular, to the reference ellipsoid used to define geodetic latitude and longitude in the international terrestrial reference frame (which is nearly the same as the wgs-84 system used by gps). while the local vertical defined at the airy transit circle still points to the modern celestial meridian (the intersection of the prime meridian plane with the celestial sphere), it does not pass through the earth's rotation axis. as a result of this, the itrf zero meridian, defined by a plane passing through the earth's rotation axis, is 102.478 metres to the east of the prime meridian. a 2015 analysis by malys et al. shows the offset between the airy transit circle and the itrf/wgs 84 meridians can be explained by this deflection of the vertical alone; other possible sources of the offset that have been proposed in the past are smaller than the current uncertainty in the deflection of the vertical near the observatory. the astronomical longitude of the greenwich prime meridian was found to be 0.19″ ± 0.47″ east, i.e. the plane defined by the local vertical on the greenwich prime meridian and the plane passing through the earth's rotation axis on the itrf zero meridian are effectively parallel. however, the claim, found, e.g., in a bbc article that this difference between astronomical and geodetic coordinates means that any measurements of transit time across the irtf zero meridian will occur 0.352 seconds (or 0.353 sidereal seconds) before the transit across the "intended meridian" is based on a failure of understanding. the explanation by malys et al. on the other hand is both lucid and correct.
prime meridian (greenwich)