The Moon Is the Only Hand on the Sky

Part 3 left a ship without a chronometer, which for most of the 1700s meant almost every ship. The question it ended on: if you cannot carry the time, how do you find out what it is at home?

You read it off the sky. Which sounds impossible, because part 2 spent a whole section proving the sky is a clock face with no numbers on it.

That was true of the stars. It is not true of the Moon.

One thing does not keep formation

Watch the sky for an hour and everything in it moves together. The stars rise, sweep across, and set, all in step, holding their patterns exactly. That is the whole sky wheeling as one, and it is what makes it useless for telling east from west.

Watch the Moon for a few nights and something else is happening. The Moon rises later each night — about fifty minutes later — and it is not where it was against the stars. It has slid eastward, past them.

It has to. The stars only look as though they move; that is the Earth turning underneath them. The Moon really is moving, going round us once in about four weeks, and that real motion shows up as a slow drift against the fixed background.

The drift is about thirteen degrees a day, which works out at a little over half a degree an hour. The Moon itself is about half a degree wide. So:

In roughly one hour, the Moon moves its own width against the stars.

That is a hand on a dial. Not a fast one — a clock hand that takes a month to go round is a slow hand — but it is the only object in the sky that moves against everything else, and it moves for real.

The dial comes in a book

A hand needs a dial. The Moon supplies the hand; the dial had to be calculated on land and printed.

In 1766 the Astronomer Royal, Nevil Maskelyne, published the first Nautical Almanac, for the year 1767. It gave, for every day of the coming year, the angle between the Moon and each of several bright stars, worked out in advance for every three hours of Greenwich time.

Now the method is complete, and it needs no clock at all:

1. Measure the angle between the Moon and a named star.
2. Open the almanac and find which Greenwich time that angle belongs to.
3. Work out your own local time from the Sun or the stars.
4. Take the difference and multiply by fifteen.

The instrument for step 1 is a sextant, and it is cleverer than it looks. It does not measure anything's height above the horizon. Using two mirrors, it lets you see the Moon and the star at the same moment, in the same eyepiece, and you turn a screw until they touch. Because you are lining up two things against each other rather than against the deck, it does not matter that the ship is rolling. Both images pitch together, and the angle between them is unchanged.

What it cost

This was not an easy method, and pretending otherwise would be unfair to the people who used it.

The raw angle you measure is not the angle in the book. The air bends light, and the Moon is close enough to us that it appears in a slightly different place depending on where on Earth you stand. Both have to be taken out by calculation, and the process even had a name — clearing the distance. In its early form it took about four hours of paper arithmetic for one fix. Later tables and shortcuts brought that down a great deal, but it was never quick.

And the sky does not always cooperate. You need a clear night, and you need the Moon in it. Around new moon the Moon is too close to the Sun to use at all, so for roughly a week of every month the method simply is not available.

Two answers, and a story that got tidied up

So there were two ways to find longitude, and they were in competition for the same prize.

Ships did not choose. Once chronometers became affordable — which took decades, as watchmakers learned to build them in numbers rather than one at a time — the sensible ship carried both. The chronometer gave a fast answer every day. The lunar sight, taken when the sky allowed, checked the chronometer for drift, which was exactly the thing part 3 said no clock at sea could check for itself.

The Royal Navy kept lunar distance tables in print into the twentieth century. The Nautical Almanac is still published today.

There is one more thing to say, and it belongs in a book that keeps asking how we know.

The version of this story you will most often meet has a hero and a villain in it: the outsider craftsman with the answer, and the Astronomer Royal blocking him to favour his own method. That version was shaped a long time afterwards, mostly in the twentieth century, and historians who have gone back to the Board's own records find it far too tidy. Maskelyne tested Harrison's watch and reported its results. His almanac was genuinely useful and saved ships. The Board was slow, suspicious and sometimes unfair — and it was also being asked to bet an enormous public sum on one unrepeatable object made by one elderly man.

A good story is not evidence. It is worth being careful with this one, because it is so much more fun than what the papers actually say.

From a dot to a map

Either method, in the end, hands you the same thing: two numbers, for one ship, at one moment. You are here.

But knowing where you are is not the same as having a map. A chart shows a coastline, a river mouth, a reef, a mountain — thousands of places, and you cannot sail to every rock in the world and take a sight on it. Some of them you cannot stand on at all.

So how does anybody measure a whole country?