Prime meridian and continental drift

[dylan_73]

Hiya.

Just finished reading “Longitude” by Dava Sobel and I wondered how do people keep the prime meridian exactly in place when the Earth’s plates are moving (even if pretty slowly)?

Do they just assume that Greenwich is stationary and the rest of the world moves slightly, or is the difference so small that no-one really cares for that much exactness?

Stupid question I know, but these things pop into my head from time to time…

[pravnik]

Here’s a couple of phd types dicussing this very question:

Higher Education Support

[dylan_73]

heh…from your link:

Everybody I’ve contacted says, “hmm, what an interesting question”. Yours is surely a candidate for question of
the year.

I guess it wasn’t such a stupid question after all!

Thanks pravnik. I wonder if anyone since has actually checked to see if there is an ofset from Greenwich, since that was posted in 1999.

[sunfish]

I would be pretty surprised to find that anyone has measured the drift of Greenwich away from the prime meridian. It’s not technically difficult - one just has to place sensitive GPS (global positioning system) receivers in the appropriate locations in order to track plate motions at the level of fractions of centimeters per year. However, that sort of attention is usually reserved for actively deforming plate boundaries (like the San Andreas Fault, which IS monitored this way) where seismic hazards are a concern, and not relatively peaceful intraplate areas like merrye olde England. So if you’re really dying to know, I’d suggest renting some receivers and hiring a few starving seismology grad students to place and maintain the GPS stations for you.

[Squink]

From History of the Prime Meridian:

The link between longitude and time in an earth-fixed is defined by the IERS_Terrestrial Reference Frame (ITRF). ITRF is based on observations to satellites_and celestial compact radio sources (quasars) from various coordinated stations around the globe.
_In Europe ITRF is realised as ETRF, the European Terrestrial Reference Frame. In 1989 they were identical but are slowly moving apart due to tectonic plate movements and inconsistencies of the movement of the earth in its orbit and rotation about its axis. Because of small, but observable movement of a centimetre or so reference frames are suffixed by a date. In 1989 ITRF89 and ETRF89 were identical. Since then ETRF has moved with the stable part of Europe. This is a small movement of a few centimetres relative to ITRF.

Apparently, the prime meridian is part of a fixed, earth based coordinate system, so it’s not moved to account for continental drift. However, it’s been supplanted by other coordinate systems which do include corrections for plate movement.

[bonzer]

I’d typed up a post based on Carl Calvert’s appendix note in the 1997 reprint of Derek Howse’s classic Greenwich Time and the Discovery of the Longitude (Oxford, 1980), put it to one side while the hamsters risked strokes and then find five minutes later that Squink has found a website with a variation on Calvert’s note. The odd think is that this has made me notice that in 1997 he had the International Reference Meridian 102.5 metres west of the Airy transit, while the website puts it the same distance east. Someone’s clearly had to correct a minus sign at some point.
For what it’s worth, the post concluded:

You can get into the semantics here and argue that the Reference Meridian is only an approximation to that defined by the telescope. But in practical applications to this sort of accuracy, it’ll be the Reference Meridian that will be used. Needless to say, while they reprinted Howse for the occasion of the Millennium, the folks at the National Maritime Museum (who run the Old Observatory) don’t go out of their way to explain this to the tourists.
From memory, the Reference Meridian still falls within the Park. One would, however, like it to be west of that annoying shop on Nelson Road that claims to be the first in the Western Hemisphere.

No such luck.