Logitude

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Graham_Wright

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I've just started reading Tom Cunliffe's book on astro. Very readable.

It set me wondering about the Prime Meridian. Naively, I expected the sun to be directly overhead at noon at some time. As the earth isn't round and the orbit is irregular, I expected some variation. It seems to be closest in April this, year.

What was the history of the creation? Was the line set by the sun?
 
The primary meridian was set up outside the place where the work was being done.

Paris and Moscow also have a primary meridian, but as the UK was such a major seafaring power everybody used the documents that came out of Greenwich.
 
Longitude is set arbitrarily; basically, they painted a N-S line on the ground (actually a brass strip) at Greenwich and said: "This is the zero longitude". These days it is set with respect to models of the Earth's shape; these are known as a datums, of which WGS84 is the best known. The zero of longitudes is still close to Greenwhich, but no longer coincides with the original brass line through the centre of the transit instrument (which was the official designation in the days when astronomical observations were the only means of determining position). The difference would have been regarded as insignificant in the days when astronomical observations were king; it wasn't until the advent of Long Baseline Interferometry, which is capable of measuring the separation between radio telecopes with an accuracy of millimetres, that it became necessary to consider global datums. GPS gives positions according to the WGS84 datum, but does NOT define it.

Incidentally, the plural IS "datums" - this isn't the same word as data!
 
AntarcticPilot said:
Longitude is set arbitrarily; basically, they painted a N-S line on the ground (actually a brass strip) at Greenwich and said: "This is the zero longitude". These days it is set with respect to models of the Earth's shape; these are known as a datums, of which WGS84 is the best known. The zero of longitudes is still close to Greenwhich, but no longer coincides with the original brass line through the centre of the transit instrument (which was the official designation in the days when astronomical observations were the only means of determining position). The difference would have been regarded as insignificant in the days when astronomical observations were king; it wasn't until the advent of Long Baseline Interferometry, which is capable of measuring the separation between radio telecopes with an accuracy of millimetres, that it became necessary to consider global datums. GPS gives positions according to the WGS84 datum, but does NOT define it.

Incidentally, the plural IS "datums" - this isn't the same word as data!
Click to expand...

So time was set to the line?

GHA and all that.
 
Graham_Wright said:
I think time is involved!
Click to expand...
But in this case time is derived from longitude. In the sense that you have local time everywhere (noon when the sun is overhead, subject to variations due to the equation of time). This is enough to find relative longitude; set one watch with local time, travel east or west, find the new local time and the difference gives you the difference in longitude. This is all very well but for charts (and maths) it is nice to have an absolute 0 point (the literal meaning of datum) to set your 1st watch to. It would be great if this were based on some fundamental property (as per LAT for heights) but as others have said there is no privileged location on the Earth, so an arbitrary point was chosen (at the observatory where various navigation techniques were developed).

Except that this isn't quite true, as there is a privileged position, the point where the sun crosses the equator at the equinox. This point moves very slowly, but is used in certain astronomical cases:

https://en.wikipedia.org/wiki/Ecliptic_coordinate_system

Edit: sorry, didn't realise we had moved on. Signed, MudbankPilot
 
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