BrightKidz Hub
Subjects
Two clocks reading different times, and the place that picks out Two round dials side by side, each with two straight hands. The left dial's hands point straight up. The right dial's hands are turned round to a later hour. Below them runs a long horizontal scale with evenly spaced upright ticks, and a solid block stands on the scale between the two dials. The gap between the two readings is what puts the block where it is.

Longitude Is a Time, Not a Distance

About 15 minutes

Part 1 ended on a puzzle. The Earth is a ball, and a ball has no favourite direction — so why does the sky give away north and south for nothing, while east and west are written nowhere at all?

The answer is that the Earth does have a favourite direction, and it is the one it spins about.

The spin picks a direction

The Earth turns once a day, and it turns about a line through the North and South Poles. That line is the axle, and everything about the sky follows from it.

Because the axle points north, the sky's whole turning is arranged around a point in the north. Polaris happens to sit almost exactly on that point, so it stays put while the rest wheels round it. Your distance from the axle — how far north or south you are — changes how high that point sits in your sky. That is why latitude falls out of a single glance upward.

East and west are the direction of the spin itself. Move east or west and you are simply somewhere else along the same circle, going round at the same rate, with your axle pointing at the same star. Nothing about your view of the sky's framework changes at all.

Why moving north changes the sky and moving east does not A circle for the Earth with a dashed line running straight through it from top to bottom: that is the axle it spins about, and it points north. A flat ring round the middle shows the direction of the spin, with a small arrow on it. A second ring higher up carries two filled dots at the same height as each other but on opposite sides. Both dots are the same distance from the axle, so both see the sky arranged the same way. the axle the spin fixed, pointing north so the sky wheels round it east and west lie along it both dots see the same sky
The axle is fixed and the spin is not. Move north and your angle to the axle changes, so the sky changes with it. Move east and you have gone round the same circle to a different seat, with the same view.

The sky is a clock with no numbers

Here is a better way to think about it, and it is the idea the whole rest of this book hangs on.

The turning sky is a clock face. It goes round once a day, taking the stars with it, exactly like a hand sweeping round a dial. And you can read that hand from anywhere on Earth — a sailor with a clear night can tell you precisely how far the sky has turned.

But a hand is only useful if there is a dial behind it. And here there is not.

Suppose you look up and the sky is turned to some particular angle. There are two completely different reasons that could be so:

From where you stand, at the moment you look, those two are impossible to tell apart. Going a quarter of the way round the world and waiting six hours produce exactly the same sky. That is not a limit of old instruments — no instrument can separate them, because in the sky they genuinely are the same thing.

So longitude is a time difference

Turn that around and it stops being a problem and becomes a method.

If moving east is the same as waiting, then how far east you are can be measured in hours. The Earth turns 360 degrees in 24 hours, which is a famously tidy sum:

360 ÷ 24 = 15 degrees every hour

That is the same fifteen degrees an hour that gives the world its time zones, being used here for a completely different job. There it explains why your friend abroad is eating breakfast while you eat lunch. Here it is a ruler.

So: find the time where you are standing. Find the time at some agreed place — call it home. Take the difference and multiply by fifteen. That is your longitude, exactly, with no fudging and no table.

A scale of degrees, and the ship the clock difference puts on it A long horizontal scale carries nine upright ticks, labelled underneath from zero degrees up to a hundred and twenty. A pale post at the far left marks home. A solid block with a short mast slides along the scale, and it always comes to rest on a tick rather than between two, because every hour of clock difference is exactly fifteen degrees. 0° 15° 30° 45° 60° 75° 90° 105° 120° home

Drag the difference between your clock and home's clock, and watch the ship slide to the only place on Earth it can be.

Nothing here is being estimated. Each hour of difference is exactly fifteen degrees, so the ship does not drift along the scale — it lands on a mark. A clock reading and a position are the same measurement, said in two different units.

Finding your own local time is easy and always was. Watch for the moment the Sun stands highest and your own shadow is shortest: that is noon where you are, and everything else follows from it. On a clear day a ship's officer could pin local noon to within a minute or so with a hand instrument.

So one of the two clocks was never a problem.

Which leaves one question, and only one

The whole of longitude has now shrunk down to a single sentence:

What time is it at home, right now?

Answer that on the deck of a ship in the middle of an ocean and you have your position. Fail to answer it and you have nothing, no matter how good your eyes, your charts or your arithmetic.

And it is worth being clear about how sharp the demand is, because this is what made the problem so hard for so long.

Your local noon happens when the clock you brought from home says three in the afternoon. How far from home are you, and in which direction?

Two readings, one subtraction, and the answer counted out in cells At the top left a filled disc for the Sun stands directly over a small post: that is local noon. Below it a clock dial with its hands at three o'clock: that is the clock still keeping home's time. To the right, three equal cells are filled in a row, each marked fifteen degrees, and the line ends with the total, forty-five degrees. One cell for each hour of difference. noon where you stand the clock you brought the Sun is at its highest and your shadow is shortest still keeping home's time three hours ahead of the Sun 15° 15° 15° = 45°
Two readings and one subtraction. The Sun overhead gives local noon. The travelling clock still keeps home's time. The gap between them, multiplied by fifteen, is the whole answer.

Now, "what time is it at home?" sounds like the easiest question in this book. Everybody knows how to answer it. You take a clock with you.

Except that for four hundred years nobody could. Not because clocks had not been invented — Europe had had good clocks since the 1300s and superb ones by the 1600s, accurate to seconds a week, standing in cathedrals and observatories all over the continent.

Put one of those on a ship and it became worthless within a week. Every single thing about a ship is hostile to a clock, and it took the best part of a lifetime to find out why.