Celestial Navigation

"So you are a sailor huh?" The guy was clearly tipsy as he started chatting with Rose.
"Is it true that the sailors of the olden sail ship days could navigate by the stars?" The man waved his beer around as if he was pointing to the sky. Splashing half of it out of the glass, luckily on the other side and not on Rose.
-"They could indeed."
"But you can't anymore, right? I mean, it's a mystery how they did that."
-"Well, if you want to call mathematics a mystery, then it is I guess." Rose countered.
"Huh?" The man was clearly confused by that statement.

North South

Finding one's latitude, the position relative to the poles or the equator, is relatively "We" have agreed that the poles are 90° and the equator is at 0°. On the Northern hemisphere we have the luck of the star Polaris, the northern star or the pole star. This star is straight above the North pole. So with the use of a sextant, we can measure the angle between the horizon and the star. This number is equal to our latitude.

So if you are on the North Pole, Polaris will be straight above you. If you are on the equator, Polaris will touch the horizon.

On the Southern hemisphere, we have the Southern Cross, you may know it as the constellation of stars featured on Australia's flag. If you extend the long leg of this constellation 4½ times, it is straight above the south pole. So here we can do the same measurement.

East West

This is a bit more difficult, as you need to know the exact time. Not where you are, but in Greenwich. Your clock should match Greenwich solar noon. At Greenwich lays the 0° meridian, from here we measure East and West around the globe, meeting up at the International Date Line

This measurement we can take off the sun. We can again take our sextant, and we know how fast and in which direction we sailed, so we roughly know when noon should be, so we take a good time before that and we start measuring the height of the sun above the horizon. At some point the sun has reached its top and appears to stop moving up, before it goes down again. This is noon. At this exact same moment, we check which time it is over in Greenwich on our fancy clock. If it's 11 in the morning. That means we are one hour ahead of Greenwich, one hour west. One hour equals 15°.


The above method is working, but quite unprecise. You can't always see Polaris, it may be cloudy, also you can't actually see the stars close to the horizon.

Also this way you will only have a latitude in the evening/morning. During nautical twilight. And a longitude at noon, these numbers will be hours apart. Good enough when you are slowly going and in the middle of the ocean.

Other stars

Astronomists have gathered information on stars for thousands of years now. For shipping they gather a bunch of the brightest and easily recognisable in books. With information about their position in the sky.

By measuring the height of three or more stars, one can do some mathematics and create three intersecting lines on a map. At the intersection you are. An excellent navigator can do this very accurate, to 0,01 Nautical mile precise is not uncommon.

This way you can have a precise position during the evening and morning twilight. Why at twilight? Because then you can see both the horizon and the stars.

Another check at noon on the sun, and you can safely sail across the oceans.


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20 Aug, 2022 09:24

I mentioned it in our DMs, but I seriously love how much technical detail & effort you put in these.

20 Aug, 2022 18:36