First on latitude, then the Greeks studying the results of the measurements by the explorer Pytheas who voyaged to Britain and beyond, in 325 BC, used several methods to measure latitude, including
- the height of the sun above the horizon at midday, measured using a gnōmōn,
- the length of the day at summer solstice
- and the elevation of the sun at winter solstice.
A gonom is just any vertical stick such as the triangular blade here, which you use to cast a shadow. You can estimate your latitude from the length of the shadow at summer solstice Sundial Taganrog
He went as far north as the Arctic circle (observing the Midnight sun), ending up at a place he called Thule, which he thought was an island to the north of Britain, but from its latitude was probably Trondheim in Norway.
The Greek Marinus of Tyre (AD 70–130) was the first to assign a latitude and longitude to every place on his maps.
From the late ninth century, the Arabian Kamal was used in equatorial regions, to measure the height of Polaris above the horizon. This instrument could only be used in latitudes close to the horizon.
This is how you use it:
The idea is you hold the string between your teeth and then position the card so that the lower edge is on the horizon and you can just see Polaris above it. Then by counting the number of knots, equally spaced along the string, between your teeth and the card, you can figure out your latitude.
The Mariner's astrolabe which gives the angle of the sun from the horizon at noon, or the angle of a known star at night, was used from around the fifteenth century to the seventeenth century.
The Backstaff, which measures the length of a shadow was used from the sixteenth century
A Jacob's staff, from John Sellers’ Practical Navigation(1672)
It was replaced by more accurate methods such as the Davis quadrant and then the Elton's quadrant in the sixteenth century
Elton’s quadrant from Philosophical Transactions of the Royal Society, No. 423, Vol 37, 1731-1732. Adds an index arm with spirit levels for an artificial horizon to Davis quadrant,
The Sextant, which is still used to this day, was mentioned by Isaac Newton (1643–1727) in his unpublished writings, and first implemented about 1730 by John Hadley (1682–1744) and Thomas Godfrey (1704–1749)
It has a system of mirrors which lets you superimpose the sun on your view of the horizon, and by adjusting it up and down you can adjust it until the sun is just touching the horizon, then by reading the angle from the dial you can figure out your latitude. It can also be used with stars such as Polaris at night, and also to measure the angles between any other objects. Using sextant swing
See also: History_of_navigation and Ocean_exploration. Also History_of_latitude_measurements (section which I just added to wikipedia after discovering they don’t seem to have an article or section about it, marked it as “needing expansion”)
FINDING LONGITUDE
Longitude is much harder, indeed up to the seventeenth century it was really hard to determine it, leading to the famous Scilly naval disaster of 1707 when many ships were wrecked on the isles of Scilly, due to an error of longitude, leading Britain to set up the Board of Longitude to solve the problem.
If you can tell the exact time, then you can find your longitude for instance by measuring the elevation above the horizon of a star just before it sets or soon after it rises. The problem is though, that this needs you to know the time exactly. That’s easy for us nowadays but not so easy when you have to rely on pendulum clocks, which don’t keep time that accurately and anyway don’t work properly on a boat or indeed on any of the vehicles they had for land travel back then either, it wouldn’t work very well in horse and carriages either.
Once you have the time, you can find the longitude by comparing the time it reaches a particular elevation with the time it reaches that same elevation, say, at Greenwich (usual zero line for longitude).
Early methods of finding longitude approximately include the Lunar distance method. The idea there is to look at the position of the Moon in the sky. Since it moves against the background stars quite quickly, then if you know where it is against the stars, you get a good estimate of the time. Typically they used the angle from the bright star Regulus which they could measure, and then compare with tables to find the current time in Greenwich approximately.
Galileo had another idea, to use the moons of Jupiter as a kind of clock in the sky. He saw them disappear behind or in front of Jupiter - but couldn't tell whether they were behind or in front, and he had a lot of trouble telling the moons apart at first, but eventually got a breakthrough when they all vanished, and using that as his starting point for all the orbital periods he was able to figure it out. This is about Galileo's observations. Galileo Galilei
But that wasn’t nearly accurate enough, as Nicholas Claude Fabri de Peiresc in Aixea found out in 1612
Luckily the moons of Jupiter pass into its shadow from time to time. Galileo observed the first such eclipse in the same year 1612, and those eclipses can be measured very accurately.
Jupiter and its satellites, during the Juno satellite approach to Jupiter - notice how the satellites disappear from time to time - that’s because they get eclipsed by the shadow of Jupiter.
(added a bit more about Jupiter’s satellites in a comment)
So, anyway, he proposed to use a table of eclipses of Jupiter’s moons to solve the problem of finding your longitude at sea. He even invented a special instrument, the Celatone, to attach a telescope to your face in front of one of your eyes to help observing the eclipses at sea.
Reconstructed Celatone for observing the moons of Jupiter at sea to use to find the longitude by timing the eclipses Celatone (ricostruzione)
But he never published his table of eclipses, they remained in his unpublished work, maybe he felt they weren’t accurate enough. Later, Cassini did, and they tried all sorts of things including gimbaled observing seats to stay steady while observing Jupiter. But they couldn't get it to work at sea with enough accuracy and were still working on this when the problem was solved in a different way by John Harrison
It was easier to do this on land. In France they did use it on the land and found that all the existing maps had the west coast of France 1 degree to far West, causing King Louis XIV reportedly to say that he was losing more land to the astronomers than to his enemies :).
Eventually the problem was solved by John Harrison
Harrison’s first chronometer H1 marine chronometer It uses rocking bars and coiled springs in place of a pendulum, so would work better on a moving ship.
His later “sea watch” - File:H4 low 250.jpg
The clockwork movement inside his sea watch - File:Harrison H4 clockwork 1.jpg
If you have the exact time, you can also figure out your longitude in the daytime by measuring the elevation of the sun, so long as you know your latitude. But small errors in latitude can lead to large errors in calculation of the longitude.
In 1837, Thomas Hubbard Sumner was returning to Britain and on a cloudy day he only managed one brief observation of the sun. So he didn’t know his latitude that accurately, but he was a brilliant mathematician, and in that situation, his life depending on the result he invented the idea of the Sumner line. He made three different assumptions of latitude, and then based on his sun elevation measurement, he then went through the calculations of longitude, and found that the three poionts he plotted lay on a straight line.
Image from https://timeandnavigation.si.edu...
Luckily that line intersected with a lighthouse and was a safe direction to travel. So he just navigated along the direction of that line until he reached the lighthouse. See Original Sumner lines introduced to the LOP
On longitude, then History of longitude
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