Randal L. Schwartz wrote:
And you should also note that the location where they charge you to take that photograph is about 20 meters to the west of where it should be.
It's more complicated than that. The line on the ground (and laser beam at night) is genuinely lined up with the historical transit circle instrument which defines the meridian which in 1884 was selected as the international prime meridian. There is a discrepancy between that and where a GPS unit will read zero longitude, of about 5.3 arcseconds (about 350 ms of rotational time, and about 100 m on the ground at Greenwich). This discrepancy mainly arises from differing definitions of longitude, interacting with non-uniform mass concentration in the Earth. One would normally expect that if one erected a plumb line, and extended the vertical of that line to the north and south, then the resulting meridian plane would pass through the centre (and entire rotational axis) of the Earth. In fact it's not quite so. In the case of Greenwich, there is a relative mass concentration to the west, which gravitationally attracts any plumb bob, making a plumb line lean slightly, by some 5.3 arcseconds. A meridian plane based on this local vertical misses the centre of the Earth, passing about 100 m to the west of it. When a transit circle is installed, it is aligned very precisely to match the local vertical. In the case of the Greenwich instrument, installed in 1851, this process involved a dish of mercury to provide a horizontal plane. The result of the mass concentration is that, where we think of the transit circle being directed precisely upwards, this does not mean that it's looking precisely away from the Earth's axis. The direction in which the Greenwich instrument looks is 5.3 arcseconds to the east of that. All of its observations, which defined GMT and our prime meridian, are based on this local vertical direction. It is the direction of that meridian plane, not the location of the observatory relative to the Earth's axis, that defined the prime meridian. GPS takes a different view of longitude. It doesn't measure local vertical, but instead works in a strictly geometrical manner, producing coordinates in 3D space. When those coordinates are geometrically reduced to latitude and longitude, one's implied local meridian is strictly the plane passing through one's location and the Earth's axis. One's longitude, then, is the angle between this geometrically-defined meridian and the prime meridian, the latter necessarily still being based on the observations made using the local gravitational vertical at Greenwich. Thus it is that the GPS-based (geometric) longitude at the location of the Greenwich instrument is 5.3 arcseconds west. So neither the line on the ground nor GPS is wrong; they're just the zeroes of different ways of reckoning longitude. There is also presumably some discrepancy between the astronomical longitude of the transit circle and the modern prime meridian due to inaccuracy in historical switches between ways of realising the prime meridian, but this is probably two orders of magnitude smaller than the difference between astronomical and geometric longitude. -zefram