Mariner’s Astrolabe, Portuguese, 1645, by Nicholao Ruffo, The Mariners’ Museum, (2000.52.1)
One of the oldest of all the altitude measuring devices, the Astrolabe is an angle-measuring tool that’s name comes from the Greek, "to take a star." It was possibly invented by the Greek astronomer and mathematician, Hipparchus (190-120 BCE). However, in its earliest uses, it was for astronomy and astrology. Only when the need to measure angular heights of Polaris became important did we see these instruments adopted for sea-going use. As an astronomer's tool, the Astrolabe was introduced to the Europeans by Arab astronomers in the 10th century, CE. But the first documented use of it used at sea is in 1481 on a voyage down the African coast by Portuguese explorers. It is likely, though, that it was in use by sailors for many years before that.
Using an astrolabe, Le Novveau Phalot de la Mer, 1635, From The Library at The Mariners’ Museum, VK801.J35.1635rare.
So how does it work? To correctly measure the angle of the sun or a star, the Astrolabe
must hang down so that it is perpendicular to the ocean. If it's tilted right or left, or front to back, the angle will not be accurate. To keep it straight, the user holds it with a finger through a ring and lets the Astrolabe
dangle. Next, take a look at the diagram and see that there are two plates on a rotating arm. They each have a pinhole perfectly lined up so that when the sun shines through the top one, and hits the second pinhole, the angle is accurate. You then read from the scale along the circumference.
For Polaris, you can sight over the edge of the two plates. One advantage of the Astrolabe is that you do not need a clear horizon to use the instrument; you do need a clear horizon when measuring the height of the sun or Polaris with other navigational instruments.
Astrolabe, A New Collection of Voyages, Discoveries and Travels: Containing Whatever is Worthy of Notice in Europe, Asia, Africa, and America, 1767, From The Library at The Mariners’ Museum, G160.K75 rare.
It is not a particularly accurate tool at sea because of the difficulty in keeping it steady in a rolling ship and high winds. Usually, however, the Portuguese explorers would take their Astrolabe
ashore and set it up to avoid this problem; this is what they did when they were mapping the coast of Africa in their early exploration. Using it at sea could result in errors as much as five degrees, or 300 miles. However, ashore, it would be much more accurate, certainly less than one-half degree, or 30 miles. The sea-going version might be 6" in diameter, whereas the one they took ashore (and which would be awkward to use at sea) might be two feet in diameter, making it more accurate and easier to read.