BrightKidz Hub
Subjects
The same pole star seen from two different places Two scenes side by side on one ground line, divided by an upright rule. In the left scene a star stands high above the ground and a dashed sight line rises steeply to it from a small block, with a wide angle marked at the block. In the right scene the same star sits low, close to the ground, the sight line is nearly flat, and only a narrow angle is marked. The angle is different because the place is different.

Half of Your Address Is Free

About 16 minutes

Stand outside on a clear night, far enough from streetlights, and you can work out how far north you are to within about the width of a county. You need no instrument, no map and no help. You need your own arm and about a minute.

That is a strange thing to be true. Knowing where you are sounds like the hardest question there is — people spent centuries and enormous sums of money on it, and the story of that effort runs through the next five parts of this book. Yet half of the answer has been lying in plain sight the whole time, free, for anyone who looked up.

The other half is the hard half. This part is about why the two halves are so different.

Your address is two numbers

Any place on Earth can be named by two numbers, and only two.

The first says how far north or south you are. It is called latitude, and it runs from 0 at the equator to 90 at each pole. The second says how far east or west you are. It is called longitude, and it runs from 0 to 180 in each direction, starting from a line we will come back to in part 6.

Together they are an address that works everywhere and belongs to nobody. Give me those two numbers and I can find you on any map on Earth.

A globe ruled with latitude rungs and longitude slices A circle stands for the Earth. Three straight lines cross it from side to side: those are lines of latitude, and they are rungs stacked from bottom to top. Curved lines run from top to bottom like the segments of an orange: those are lines of longitude, and they are slices going round. A filled dot marks the single point where one rung crosses one slice, and a line runs from it to a label reading your place. your place latitude longitude one rung crosses one slice, once rungs, north to south slices, round the middle
Two numbers doing two different jobs. Latitude counts rungs from the equator up to a pole. Longitude counts slices around the middle. A place is where one rung crosses one slice.

Here is what this book is about. One of those numbers is easy and the other is brutally hard, and for most of history that was simply a fact of life everybody had to sail around.

The sky hands you the first number

Look north. If the night is clear you can find the seven bright stars people call the Plough or the Big Dipper, and the two stars at the outer edge of its scoop point almost exactly at another star a little way off. That is Polaris, the pole star.

Polaris sits nearly straight above the North Pole. Not exactly — it is about half a degree off, so it traces a tiny circle over the course of a night, far too small for anyone to notice by eye. Every other star wheels slowly around it all night long. Polaris barely moves at all.

Now measure how high it stands above the horizon, in degrees. That measurement is your latitude. Not a hint about your latitude. Not something to look up in a table afterwards. The angle is the number.

At the North Pole, Polaris is directly overhead: 90 degrees up, latitude 90. At the equator it sits right on the horizon: 0 degrees up, latitude 0. In London it stands about 51 degrees high, and London's latitude is about 51.5. In Cairo, 30 degrees, and Cairo is at 30. It works everywhere in the northern half of the world, and it always has.

Counting fist-widths from the horizon up to the pole star Seven joined dots at the upper left make the shape people call the Plough. A dashed line runs from its two end dots across to a star on the right. A long straight line across the bottom is the horizon. Under the star, square blocks are stacked one on top of another from the horizon upwards, each block standing for one fist-width of about ten degrees. Five blocks reach the star.

Measure the sky with nothing but your own arm. Step through it here, then take it outside on the next clear night.

  1. Find the Plough — seven bright stars, four making a scoop and three making a handle. The two stars at the outer edge of the scoop point the way.
  2. Follow that line onwards, about five times the gap between those two stars. It lands on a fairly ordinary star that sits low and hardly moves. That is Polaris.
  3. Find the horizon — the flat line where ground meets sky. If a hill or a building is in the way, guess where the true flat line would fall, and expect your answer to be a little out.
  4. Hold your arm straight out and make a fist. A fist at arm's length covers about ten degrees of sky, and that holds for almost everybody: bigger hands come with longer arms.
  5. Stack fists from the horizon up to Polaris, counting as you go. Multiply the count by ten. That is roughly how far north you are.

Do it properly and you will land within a few degrees, which is a few hundred kilometres. That sounds sloppy. It is not — it is a person on a dark hillside with no equipment at all getting the same answer a ship's officer would get with a brass instrument, only less precisely. The method is identical. What the instrument buys is decimal places.

Nothing up there tells you the other number

Now try the same trick for east and west.

Go outside tonight and look. Then imagine sailing a thousand kilometres due west and looking again at the same hour. The Plough is in the same place. Polaris is at the same height. Every constellation sits where it sat. The sky has not changed at all.

That is not a failure of your eyes or of the night. There is genuinely nothing in the sky that is fixed to east and west. The stars are pinned to the north–south framework, which is why they give up latitude so cheaply. They are not pinned to the east–west one at all.

So a sailor a thousand kilometres out in the Atlantic could tell you, confidently and correctly, exactly how far north they were. Ask how far west and the honest answer was a shrug and a sum.

What people did instead

If you can find your latitude perfectly and your longitude not at all, there is one obvious way to sail, and for centuries it was what nearly everybody did.

Go north or south first, until your latitude matches the latitude of the place you are heading for. Then turn and sail straight along that line, holding the pole star at the same height night after night, until land appears.

It is called running down the latitude, and it works. Ships crossed oceans that way for hundreds of years.

It is also slow, because the straight line to your destination is almost never the line you are allowed to take. And it carried a danger nobody could design away: everyone bound for the same port ran down the same latitude, so shipping piled onto a handful of invisible lanes across an entire ocean — which pirates worked out very quickly indeed.

You measure the pole star and find it standing 40 degrees above the horizon. What have you just learned?

So one half of the problem was solved before anybody thought to write it down, and the other half stayed unsolved for four hundred years after ships began crossing oceans in earnest.

That lopsidedness is the odd thing, and it wants explaining. The Earth is a ball. A ball has no favourite direction. So why should north and south be written all over the sky, while east and west are not written anywhere at all?