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Ptolemy’s Invention of the Astrolabe Measured Time and the Stars

An exploded view of an astrolabe, an instrument that was invented by the Greek scientist Ptolemy. Credit: Elrond /CC BY-SA 4.0

Nowadays, were are incredibly spoiled by modern technological devices that allow us to know the exact time, the positions to the stars and planets, and our exact location, to the tenth of a degree — but all these things were available to the ancient Greeks as well, in one device, thanks to their invention of the astrolabe.

The scientist Ptolemy, who lived in Alexandria, was the brilliant mind behind this genius machine, which used sets of dials to determine altitude, latitude — as long as the time was known —  the shifting positions of stars and planets, and to survey or triangulate your location on land.

The brilliant machine was nothing less than a handheld model of the universe. Its many functions also made it an elaborate inclinometer and an analogue calculation device that was capable of working out several kinds of problems in astronomy.

One of many ingenious inventions of the ancient Greeks, this piece of technology proved to be incredibly versatile, used for calculating the altitude above the horizon of any celestial body, during the day or nighttime, and was used in classical antiquity into the European Middle Ages and the great Age of Discovery for all these purposes.

The importance of the invention of the astrolabe comes not only from the early discoveries in astronomy, but also in determining latitude on land or on calm water, making it possible to navigate the seas in a limited way.

Much later, the determination of latitude on the open ocean, so crucial in the exploration of the Americas and other continents, was made possible by the invention of the marine chronometer, which made the reliable determination of longitude finally possible.

Celestial globe of Isfahan
The “Celestial Globe of Isfahan,” at the Louvre Museum. Created in Iran in 1144, this globe is the third oldest such object extant in the world. Credit: Captainm /CC BY-SA 3.0

Like the Antikythera Mechanism, which as been called the world’s first computer, the astrolabe was the fruit of the genius of ancient Greek thinkers.

The word astrolabe comes from theGreek word astrolabos, from “astron” (ἄστρον) = star + “lab-” (λαβ-) = to take.

An early astrolabe was invented in the Hellenistic civilization by Apollonius of Perga between 220 and 150 BC, but it was often attributed to Hipparchus. The device, a marriage of the planisphere and the dioptra, was effectively an analog calculator capable of working out several different kinds of problems in astronomy.

Planisphaerium to astrolabe

The Greek scientist Ptolemy of Alexandria, who lived from 85 to 165 AD, created the “Planisphaerium,” which dealt with the problem of mapping figures from the celestial sphere onto a plane, by a method now known as ‘stereographic projection’, that preserves circles.

This stereographic projection later became the mathematical basis of the plane astrolabe, which developed into all later forms of the device.

Theon of Alexandria, who lived from c. 335 – c. 405, wrote a detailed treatise on the astrolabe, and it is believed that Ptolemy used his astrolabe to make the astronomical observations recorded in the “Tetrabiblos.”

The invention of the plane astrolabe is sometimes wrongly attributed to Theon’s daughter Hypatia, who was born c. 350–370 and died in 415 AD; according to researchers, it is known to have already been in use several centuries before Hypatia was born.

The misattribution comes from a misinterpretation of a statement in a letter written by Hypatia’s pupil Synesius (c. 373 – c. 414), which mentions that Hypatia had taught him how to construct a plane astrolabe; it does not state anything about her having invented the astrolabe herself.

Astrolabes continued in use in the Greek-speaking world throughout the Byzantine era. About 550 AD, the Christian philosopher John Philoponus wrote a treatise on the astrolabe in Greek, which is the earliest extant treatise on the instrument.

Αstrolabes made their way to Northern Europe centuries after their invention

The first known metal astrolabe in Western Europe is the “Destombes” astrolabe, made from brass in the eleventh century in Portugal. Metal astrolabes avoided the warping that large wooden ones were prone to, allowing the construction of larger and therefore more accurate instruments.

Metal astrolabes were heavier than wooden instruments of the same size, however, making it difficult to use them in navigation. The constant motion of the waves also made it difficult — although not impossible — to use the instrument.

Herman Contractus of Reichenau Abbey explored the use of the astrolabe in his work “Mensura Astrolai” during the 11th century. Peter of Maricourt wrote a treatise on the construction and use of a universal astrolabe in the last half of the 13th century, entitled “Nova compositio astrolabii particularis.”

Universal astrolabes can be found at the History of Science Museum in Oxford. David A. King, a historian of Islamic instrumentation, describes the universal astrolabe designed by Ibn al-Sarraj of Aleppo, Syria in 1328 as “the most sophisticated astronomical instrument from the entire Medieval and Renaissance periods.”

The first printed book on the astrolabe was “Composition and Use of the Astrolabe” by Christian of Prachatice, in the present-day Czech Republic.

The astrolabe, like so many other Greek inventions, made its way north into Northern Europe centuries after its invention. It was almost certainly first brought north of the Pyrenees by Gerbert of Aurillac (the future Pope Sylvester II), where it was integrated into the curriculum at the school in Reims, France sometime before the turn of the 11th century.

In the 15th century, French instrument maker Jean Fusoris, who lived from 1365–1436, started making and selling astrolabes in his shop in Paris, along with portable sundials and other popular scientific devices of the day. Thirteen of his astrolabes survive to this day.

One more special example of craftsmanship in early 15th-century Europe is the astrolabe designed by Antonius de Pacento and made by Dominicus de Lanzano, dated 1420. Many astronomical clocks use an astrolabe-style display, such as the famous outdoor clock at Prague, adopting a stereographic projection of the ecliptic plane.

In recent times, astrolabe watches have become popular. For example, Swiss watchmaker Dr. Ludwig Oechslin designed and built an astrolabe wristwatch in conjunction with Ulysse Nardin in 1985.

Dutch watchmaker Christaan van der Klauuw also manufactures astrolabe watches today.

Just how are these intricate machines made?

An astrolabe consists of a disk, called the mater (mother), which is deep enough to hold one or more flat plates called tympans, or climates. A tympan is made for one specific latitude, or degree North or South of the equator. Commonly these were made with the latitudes of a large city on each plate.

It is engraved with a stereographic projection of circles denoting azimuth and altitude and representing the portion of the celestial sphere above the local horizon. The rim of the mater is typically graduated into hours of time, degrees of arc, or both.

Above the mater and tympan, the rete, a framework bearing a projection of the ecliptic plane and several pointers indicating the positions of the brightest stars, is left free to rotate.

These pointers are often just simple points, but depending on the skill of the craftsman, they can be very elaborate and artistic. There are examples of astrolabes with artistic pointers in the shape of balls, stars, snakes, hands, dogs’ heads, and leaves, among other decorative objects.

The names of the indicated stars were often engraved on the pointers in Arabic or Latin letters. Some astrolabes have a narrow rule or label which rotates over the rete, and may be marked with a scale of declinations.

The rete, representing the sky, functions as a star chart. When it is rotated, the stars and the ecliptic move over the projection of the coordinates on the tympan. One complete rotation corresponds to the passage of one day. The astrolabe is, therefore, a predecessor of the modern planisphere.

On the back of the mater, there is often engraved a number of scales that are useful in the astrolabe’s various applications. These vary from designer to designer, but might include curves for time conversions, a calendar for converting the day of the month to the sun’s position on the ecliptic, trigonometric scales, and graduation of 360 degrees around the back edge.

The alidade is attached to the back face. An alidade can be seen in the lower right illustration of the Persian astrolabe above. When the astrolabe is held vertically, the alidade can be rotated and the sun or a star sighted along its length, so that its altitude in degrees can be read (“taken”) from the graduated edge of the astrolabe.

This is also what allows time measurements to be taken on the astrolabe, using the rate points on the outer rim.

Devices were usually signed by their maker with an inscription appearing on the back of the astrolabe, and if there was a patron of the object, their name would appear inscribed on the front of the device.

The date of the astrolabe’s construction was often also recorded, which has allowed historians to determine that these devices are the second oldest scientific instrument in the entire world.

Almanacs of the stars important to the ancients

The inscriptions on astrolabes also allowed historians to conclude that astronomers tended to make their own astrolabes, but that many were also made to order and kept in stock to sell, suggesting there was some contemporary market for the devices.

Today, this brilliant scientific device leaves its traces in modern analog inventions like the slide rule and, as noted above, the fanciest Swiss watches made today.

John Huth, a physicist at Harvard University says in an interview with Smithsonian magazine that astronomy and astrology developed hand-in-hand during the time the astrolabe was invented.

“If you look at these almanacs of the stars, in some sense they offered ways of predicting where the planets were going to be, but they were also giving information to astrologers,” says Huth. “This is one of the things that drove the development of astronomy, getting higher precision in astrological predictions.”

Huth points out that the astrolabe would have been among the many essential tools that Christopher Columbus would have used when exploring the New World, along with a quadrant and tables and almanacs with al the navigational information that was down at the time.

Determination of latitude made possible because of astrolabe

Portuguese explorers who had been trained to use the North Star, or Polaris, to find their position on the seas also used the astrolabe when they sailed close enough to the equator that Polaris was no longer visible.

The professor explains “Bartolomeo Dias used the astrolabe to figure out the latitude of the Cape of Good Hope in 1488, because they were so far south that they lost Polaris.”

In fact, many of the astrolabes recovered today are found in shipwrecks from Spanish and Portuguese mariners, often found off the west coast of Ireland, according to Louise Devoy, the curator at the Royal Observatory Greenwich in England, which includes a jaw-dropping collection of astrolabes.

“It was one of many tools used (on board),” Devoy says, of the many journeys undertaken by European and other explorers all around the world.

By the 17th and 18th centuries in Europe, mechanical clocks were becoming more reliable and affordable for many.

The astrolabe also began to fall out of use as sextants for more precise ocean navigation took their place. However, in recent years, we seem to have revived the concept of the astrolabe, perhaps harkening back to another time, when this measurement was taken on an exquisitely beautiful piece of machinery.

Not only can we purchase an exquisitely crafted astrolabe watch from the best watchmakers in the world, we can also turn our ubiquitous smartphones into our very own astrolabes.

Huth explains “I can turn my cellphone into an astrolabe quite easily. It can be used in different locations, it is adaptable and it has a style element, too.” A quick online search during Huth’s interview brought up a range of “Qibla” app options on his smartphone screen.

“Qibla app for iPhone, there you go,” Huth says as he scrolls through his search results. “That’s impressive.”

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