The Compass Meridian

To map the world it was necessary to first establish a system of latitude and longitude. Measuring latitude from any given point on earth is fairly straightforward. All that is necessary is to measure the angular altitude of the north star above a flat horizon, or the equivalent point in the southern hemisphere.

Measuring longitude is more difficult. We need to begin with a reference point. Latitude has natural reference points because the earth is divided in half by the equator as it revolves around the polar axis. But with longitude there are no such natural reference points so we have to define one. The reference line that we have defined to measure longitude by is the Prime Meridian, which passes through London.

Measurement of longitude became practical when John Harrison invented an accurate clock that didn't rely on a pendulum. Clocks that were based on pendulums were considered as unreliable as sea because the pitching and rolling of the ship in rough water might affect the timing of the pendulum.

The new clock could be set to Greenwich Mean Time (GMT). Wherever a ship was it could measure the local solar time by means such as a sundial. The difference between the two times reveals the ship's longitude. Since the earth rotates every 24 hours it rotates 15 degrees every hour so that four minutes equals one degree of longitude. If the local time is ahead of Greenwich Mean Time it means the ship is east of the Prime Meridian, since the earth rotates eastward. If the local time is behind Greenwich Mean Time it means west of the Prime Meridian.

There has to be some line of longitude where one day ends and the next begins. By convention it is the International Date Line, at 180 degrees longitude on the opposite side of the world from the Prime Meridian. The International Date Line is fortunately in the middle of the Pacific Ocean. It diverts so that it does not cross any land because it would be very inconvenient to have one's home and work or school in different days.

What I want to point out is another way that longitude might have been measured and I think it could have been done much earlier than waiting for the invention of an accurate clock that didn't rely on a pendulum. It could have been accomplished by using a magnetic compass. The ordinary magnetic compass seems to have faded into history but I think it still has a lot of possibility left. I explained one such use as a measurement tool in the compound posting "Measurement", September 2021, section 7) A VERY USEFUL TOOL.

The earth is a magnet, which is the basis of the compass, but magnetic north is not exactly the same thing as geographic north. The north star is very close to the north celestial pole and determining direction by the stars is more accurate than by a compass. We could have used this difference to our advantage.

The bottom of the yellow line in the following image from Google Earth is magnetic north and the top of the line is geographic north. 

The red dot is on Greenland. The yellow dot is on Quebec. The green dot is on Nunavut. The blue dot is on Alaska. The purple dot is on Siberia, and the white dot is on Scandinavia.

The span of the line, representing the difference between magnetic and geographic north poles may not seem to make a big difference. But if we could build a large magnetic compass, so that the differences in readings could be easily discerned, this would have enabled a mapping of the world long before it was done by means of a clock, as described above. A reading of north would be taken by the stars, and then by the compass. The difference between the two would reveal the longitude.

Instead of the Prime Meridian the line shown above would be extended in both directions as the Compass Meridian. Only when the two measurements were exactly the same would we be on the Compass Meridian. The greater the difference between the two measurements the further we would be from the Compass Meridian, in terms of longitude. The difference would reach a peak at the meridian line 90 degrees from the Compass Meridian and then begin to decrease.

Whether the Compass Error, as we could call it, was to the east or west of the star measurement would tell us whether we were east or west of the Compass Meridian. If magnetic north was showing as east of the stellar measurement it would mean that we were west of the Compass Meridian, and vice versa.

It is true that the magnetic poles are moving towards the geographic poles but that is not taking place very quickly and it wouldn't have interfered in the mapping of the world. The mapping of the world would be essential. It would be of little use to know exactly where we were, without an accurate map. But compasses would have made this possible much earlier than with clocks.