<P> 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 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 . </P> <P> As of 2017 the Greenwich meridian passes through: </P> <Ul> <Li> United Kingdom, </Li> <Li> France, </Li> <Li> Spain, </Li> <Li> Algeria, </Li> <Li> Mali, </Li> <Li> Burkina Faso, </Li> <Li> Togo, </Li> <Li> Ghana, </Li> <Li> Queen Maud Land in Antarctica . </Li> </Ul> <Li> United Kingdom, </Li>

Three countries in africa that the greenwich meridian passes through