The Astrolabe

Astrolabes on display in the British Maritime Museum. Photo: ©2014 Mary Ames Mitchell(1)

Navigation for 15th century mariners became more difficult after they figured out how to cross the equator. They could no longer see Polaris for reference and there was no equivalent South Star. In the open Atlantic, dead reckoning is not a useful method for figuring out where in the world your ship is.

Fortunately, Medieval astronomers were resurrecting an ancient and extremely useful tool known as an astrolabe. The word astrolabe was derived from two Greek words. Astor meant star. Labe came from the Greek word lambanein, which meant to take or seize. Therefore, astrolabe literally meant to take a star.

Astrolabes had hundreds of uses. With it, a Muslim could tell prayer times and the location of Mecca. An astrologer could read horoscopes. An astronomer could predict eclipses and the position of planets. A navigator could find the Meridian, longitudes, and latitudes, and calculate time. He could predict when and where the sun would rise, its path across the sky during the day, and where and when the moon would appear. Later we will learn how Christopher Columbus used the astrolabe to predict a lunar eclipse while on the island of Jamaica. He needed a magic trick to get himself out of a tight squeeze with the local natives.

The earliest written records we have about astrolabes are Greek from 340 BCE. However the roots of its technology go back to the stone circles built by the Ancient Egyptians and Babylonians, which were already advanced when the stone circle of Nabta Playa was erected in c.9000 BCE.

Layout of Nabta Playa Stone Circle, Egypt, c.9000 BCE.

The basic concept of an astrolabe, when used for navigation, is to match coordinates and angles on earth – i.e. the 360 degrees of north, south, east, west – with coordinates in the heavens – i.e. maps of the celestial bodies, and the path of the moon and sun. If you were standing in the middle of the Sahara, or floating in the middle of the Atlantic where there were no obstructions to your view, both the celestial world and the terrestrial world would seem like circles – one above the horizon and one below the horizon. Translate those circles into two disks. In astrolabe terms, the earth disk would be the tympan and the sky disk would be the rete. According to Roderick and Marjorie Webster, in their book Western Astrolabes(2), the only thing missing from the equation to qualify the device as an astrolabe would be an alidade, the part that helps the user determine altitudes.

Chinese Diviner's Board or Shih.

In 100 CE, the Chinese were using a tool known as a Diviner's Board or Shih that works in a similar way. As you can see on the diagram above, the backboard is marked with the four directions: North, South, East, and West. The center of the board is a circle with the Big Dipper constellation drawn in the middle. To use this board, first make sure you can see the Big Dipper in the night sky. Place the board on a surface that is parallel with the Earth’s surface - such as a table. Then swivel the board around until the position of the Big Dipper in the circle matches the position of the Big Dipper above you. Voila! You have found north.

The trouble is, this board will only work in specific places depending on your latitude and longitude. The angle of the Big Dipper in relation to the North Pole changes when you change your position on Earth. In other words, if the board works in Beijing, it probably won’t work in Florida. And it certainly won’t work below the equator because you can no longer see the Big Dipper below the equator.

The Medieval Astrolabe

By the 1200s, the astrolabe had become a sophisticated computer. We mentioned in the article about John of Gaunt that his house-guest, the author Geoffrey Chaucer, wrote a manual for his son on how to use the astrolabe. Chaucer taught Henry the Navigator’s grandmother Blanche of Lancaster how to use it as well.

Each astrolabe was unique, and each was a fine piece of craftsmanship and art. Ship masters purchased their own astrolabes. It was a big investment. The most sophisticated were made of bronze – sometimes even silver or gold. However, it is possible to create a useful device, though maybe not as precise, out of wood or even paper. The device consists of three disks and a sight vane or rule attached together in the center with a grommet around which the disks and sight vane swivel.

1] The Mater

The disk at the back is called the mater. The word comes from mother since this piece serves as a womb into which the rest of the disks nestle. Attached to the top of the matter is the thumb-ring that allows the navigator to hang the astrolabe above him while lining up a star or the sun with the sight vane. Grooved marks around the outer edge of the mater divide the circumference into 360 degrees and/or twelve hours. Sometimes both equal hours and unequal hours are inscribed around the edge. Sometimes the degrees or hours are inscribed around the back of the mater along with all sorts of other important data.

If you are using an Arabian astrolabe, the data is written in Arabic. If you are using a Portuguese astrolabe from the 1400s, the data is written in the international language, Latin. Most western astrolabes were made in Nuremberg, Germany, the 15th century equivalent of today’s Silicon Valley.

2] The Tympan or Plate

Within the mater fits the middle disk called a tympan or plate. On its face are engraved grids showing the paths that stars, planets, and the moon make as seen from earth. That would include the zenith [highest altitude] and azimuth [direction from the observer, such as the compass bearing] of the celestial body you were observing or measuring. The center of the tympan, where the grommet is located, corresponds to the location of the North Star. In 1440, that would be Polaris. Curved lines indicate the latitudes for the Tropic of Cancer and the Tropic of Capricorn. The line down the middle of the disk indicated the Meridian, which was still in 1440, Cape St. Vincent.

Tympans are location-dependent. Each plate is designed for a specific latitude. A navigator needs a different tympan for the northern sky than for the southern sky. This will be the major problem for Henry the Navigator’s astronomers to figure out. In order to create a tympan for the southern sky, a person first needs to go there and take measurements.

3] The Rete

The third disk is called a rete, from the Latin word for net. You can see through the rete to the tympan. The rete is a star map. The tips of the various pointers represent Sirius, Vega, and other bright stars. The navigator rotates the rete over the tympan to line the stars with their corresponding paths on the tympan. That path is determined by the time of day, latitude, and longitude. To figure out the time of day, latitude, and longitude the astronomer needs the fourth part of the astrolabe called the alidade.

A dismantled astrolabe in the British Maritime Museum. The astrolabe has two different tympans, one on the left and one on the right of the mater. The rete hangs below the mater. Photo: ©2014 Mary Ames Mitchell

4] The Alidade

In order for navigators and astronomers to use the astrolabe to take measurements, they needed an alidade to measure angles. That is where the sight vane and rule come in. The rule or site vane is about a half-inch wide and rotates like an arm on a clock over the three disks, so that the points at each end aim at the various degree marks on the circumference of the mater.

Instructions on How to Use an Astrolabe

Before you can read the information provided by an astrolabe, you must first take a class on the principals of navigation and astronomy. If you are going to use a 14th century astrolabe, you had better learn to read Latin and Roman numerals or Arabic. Once you have those courses under your belt, you are ready for the basic steps for using the device:

  1. Select a star or other celestial body.
  2. Hold the astrolabe out in front of you, hanging it vertically from the thumb ring. With one eye, line the swiveling sight vane with the star. The sight vane has small peep holes sticking out of it perpendicularly that you must look through. This requires clever maneuvering when you are bouncing around on the high seas.
  3. Note the degree mark where the rule lands on the circumference of the mater. That is the measurement of the altitude of the star.
  4. Identify the star on the rete.
  5. Move the rete so that the altitude of the star matches the corresponding points on the grid. [You learned how to do that in your astronomy course.] Additional data for this step is engraved on the back of your astrolabe in either Latin or Arabic.
  6. Now that the mater, tympan, rete, and alidade are lined up properly, the astrolabe, if placed flat on the table, has become a two-dimensional model of the three dimensional sky above you.

Two different astrolabes in the British Maritime Museum, front and back. The astrolabe on the right has the site vane and degree marks on the back. Photo: ©2014 Mary Ames Mitchell

In the upcoming articles, the Portuguese will finally pass the equator where their navigators and astronomers can take measurements of the relationships of the moon and stars. They will find new stars such as the Southern Cross. Then they will take the data home with them. In c.1470 Astronomer Abraham Zacuto and Martin Behaim of Nuremberg will use the information and make new tympans and retes for the astrolabe to allow Vasco da Gama to find his way under Africa to India.

Predecessors to the Astrolabe

The Paleokastro Disk

An ancient artifact known as the Paleokastro Disk may be the oldest physical evidence we have that Bronze Age man used an astrolabe-like tool to study and measure the heavens. [It is actually the cast for such a device, not the device itself.] Archaeologists found it on the island of Crete and dated it to c.1600 BCE, the height of the Minoan Empire [fl. c.2700 – c.1450 BCE]. It is the size of your palm.

The Paleokastro Disk, c.1600 BCE. Photo: Archeology News Network.(3)

According to Minoan scholar and expert Dr. Minas Tsikritsis, the device, like the astrolabe, was an analog computer [a device that compares one thing in proportion to another]. With it a Minoan astronomer or navigator could calculate the position of stars to each other, hence record their movement. He could tell time and determine geographical latitude. He could even predict solar and lunar eclipses. With such a tool, Minoan navigators were able to guide their ships as far north as England, as far south as Yemen and probably India, and maybe even across the Atlantic.

The Nebra Sky Disk

Nebra Sky Disk. Housed in the State Museum of Prehistory in Halle, Germany, c1600 BCE.(4)

The bronze Nebra Sky Disk is also dated to c.1600 BCE. Like the Paleokastro Disk, scientists have not figured out how it works yet.(5) It was found by a young German a few miles from a Bronze Age astronomical stone circle near Leipzig in the center of today’s Germany. He was illegally scavenging in the forest with a metal detector. As we discussed in the article about trade during the Bronze Age, the Minoans and Egyptians traded with the northern German countries around the Baltic for amber. It is interesting to compare this disk with the Neolithic dimpled stones that we think were star maps.

‘Cup-holders’ on a stone from Göbleki Tepe c9000 BCE.(6) Dimpled stone from Monolith 8 of the Almendres Cromlech c6000 BCE.(7) Nebra Sky Disk c.1600 BCE.(8)

The bronze disk is about a foot in diameter [30 cm]. The surface is inlaid with gold. Both the gold and bronze have been traced to Cornwall, England, which was a primary source of tin during the Bronze age. Note the notches around the perimeter of the disk. Possibly a rete-like device fit over the disk at one time. One excavator asserts that the two arcs on the face of the disk indicate the degrees that the sun travels during the winter and summer solstices, like the ovals on the rete of an astrolabe. The rest of the markings appear to be the sun, the moon, and the constellations. The defining feature is the seven-star cluster of the Pleiades.

As noted before, the ancients could only see seven stars in the Pleiades constellation with their naked eyes. [Today’s astronomers have found eleven major stars and more than five hundred smaller stars in the cluster.] According to Greek mythology, the seven stars were the seven daughters of the Titan Atlas and the Oceanid Pleione. One day, when Orion the Hunter was chasing after the Pleiades sisters, Zeus changed them into a cluster of stars.

The Antikythera Mechanism

The Antikythera Mechanism [aka Item No. 15,087] in the National Archaeological Museum in Athens, c.205 BCE. Side A and B.(9) The portion of the mechanism that shows the degree scale.(10)

Recreations of the Antikythera Mechanism. [L] Michael Wright’s version. [R] Replica by Massimo Mogi Vicentini, 2007.(11)

A two-thousand-year-old device known as the Antikythera Mechanism tells us how much the Ancient Greeks knew about astronomy and how sophisticated they were when it came to building machines. The device was not used to gather the data. It demonstrated data that was already gathered by an extremely sophisticated version of the astrolabe that no longer exists. We strongly recommend you watch the BBC video about this extraordinary ancient computer on YouTube. You will be amazed. In the time being, here is our humble summary.

Sponge divers discovered the mechanism back in 1900 along with a hold-full of ‘some of Ancient Greece’s most beautiful artifacts’ that had been dumped on the bottom of the sea two thousand years earlier. A huge Roman trade galley, overloaded with the treasure, wrecked during a storm offshore from the Greek island of Antikythera. The ‘largest hoard of Greek artifacts every found,” is now on display at the National Archaeological Museum in Athens.

The shipwreck occurred during the period when Rome was in the process of taking over Greece’s colonies around the Mediterranean. Several dozen Roman silver and bronze coins – ‘an archaeologists dream for dating artifacts’ – and amphorae [tall clay jugs] place the shipwreck to between 65 and 50 BCE. The port of origin was probably Pergamon or Ephesus. From there the ship sailed to the island of Kos, then to Rhodes. In Rhodes the huge trade galley was loaded to the gunwales with treasure that, it is believed, was meant to pay for “a triumphal parade being staged by Julius Caesar” in Rome. Following the busy trade route to Italy, the galley ran into a storm off the coast of the tiny, rocky Greek island of Antikythera.

Scientists believe the mechanism was created earlier by “someone with truly staggering genius” – someone like astronomer, inventor, and mathematician Archimedes (c.287–c.212 BCE). Archimedes lived in Syracuse in Sicily, Ancient Greece’s third largest city state. Legend states he was killed when the Roman’s conquered the colony in 212 BCE. The Roman Emperor Marcellus had ordered Archimedes’ life spared. But an ignorant legionnaire did not recognize ‘the old man drawing circles in the sand’ and slew him. Marcellus received two instruments that ‘could read the movements of the celestial bodies’ that Archimedes had created.

Or maybe Hipparchus had something to do with the mechanism. Hipparchus (fl.162-127 BCE) lived in Rhodes, the Roman galley’s last port of call. He is credited with discovering the precession of the equinoxes even though we know the Babylonians knew about that long ago. He constructed models showing the motion of the sun and moon that still survive. Historians think he could have been the inventor of the western astrolabe. For sure he had an armillary, and maybe he invented that, too.

As shown in the photo above, the remains of the mechanism were barely recognizable. The sponge divers thought the rock chunks were of little value. In fact, they are among the most important scientific and historical artifacts ever found. It has caused a rewriting of the history of western technology.

Layers of the tiny, thin gears were fused together and crusted over with rust and barnacles. The corroded chunk of hardened sediment and bronze was no larger than a modern laptop.(12) The original wooden box in which it was housed – about the size of a boot box(13) – succumbed to the underwater elements long ago. Archaeologists rescued eighty-two fragments. Only seven contained gears or had significant inscriptions on them. Each would help solve the puzzle when scientists tried to recreate the mechanism.

In 1902, archaeologist Valerios Stais examined the artifact and noticed a gear wheel embedded in it. He thought it was an astronomical clock, but the scholars around him thought that to be impossible.

English physicist and ‘father of scientromentrics’ Derek John de Solla Price (1922 – 1983) examined the remains in the 1950’s. He also noticed the precision metal engineered gears, as well as arcs with degree marks, and patches of Greek text. X-Rays revealed some of the twenty-seven surviving gears. When Price counted 127 teeth on one gear and 235 on another, he recognized the numbers were astrologically key to numbers having to do with our moon’s cycles. We explain the 235 below.

X-Rays taken by John de Solla Price. Price’s sketch of the gears.(14)

In 1976, famous diver Jacques Cousteau hauled up more of the treasure. About that time, Michael Wright (b.1948), formerly the curator of mechanical engineering at the Science Museum and later at Imperial College in London, took his own close look at the remains. At the suggestion of radiologist, Alan Partridge, he and Allan George Bromley obtained more images using linear X-ray tomography. They were able to see the rest of the gears. Wright found flaws in Derek Price’s model. Using the same tools and methods the Greeks would have used, he started to recreate his own version of the device.

Michael Wright working on his model of the Antikythera Mechanism. The image of the artifact is on the screen of his computer to the right.(15)

Meanwhile, starting in the year 2000, a team of international historians, mathematicians, and astronomers(16) who called themselves the Antikythera Mechanism Research Project Group, worked closely with the National Archaeological Museum to perform their own detailed investigation. Communicating with Michael Wright all the while, they wanted to know 1) where the mechanism came from, 2) who made it, and 3) what function it performed. The BBC video shows you how they went about finding the answers.

We have already told you the answers to the first two questions. Regarding the mechanism’s function, it displayed the mathematics behind the orbits of the planets and the moon in relation to the earth and sun.

On the front of the box were overlapping dials that represented the five planets. [The Greeks knew of only five planets because that is all you can see with the naked eye.] These dials work like a mini-planetarium or orrery (17). With the aid of a hand-crank, the dials rotate the planets in relation to each other just as they do in real life.

Dials on the front side of the Antikythera Mechanism that display the orbits of the planets.(18)

On the back side of the box were two dials in the form of spirals. [This may be a clue to the spiral shape that was so common in Neolithic petro-glyphs.] The top dial had a five-turn scale and displayed the Metonic cycle of the moon [named after Athenian scholar Meton]. As we are learning, the Babylonians and the Greeks loved figuring out mathematical functions for what seemed like irregular celestial patterns. The Greek word for the Metonic cycle was Enneadecaeteris, which translates to Nineteen Years.

Upper dial on the back side of the Antikythera Mechanism exhibiting the Metonic calendar.(19)

Here is a description of the Metonic Calendar in a nutshell.

However, all things come out more evenly if your calendar covers a nineteen-year period rather than a one-year period. A Metonic Calendar contains 235 months of equal length.

[From this cycle we inherited the complicated function for figuring out the Easter New Moon.]

The bottom dial on the back of the mechanism mimics the sorus, which we described in our article on Mesopotamia, and which the ancient Babylonians described in their Enuma Anu Enlil tablets that are now housed in the British Museum. With this device, a person could predict both lunar and solar eclipses – a trick that gave him a lot of power. Not only could he predict the month, but the exact hour of the eclipse, what color the eclipse would be [red or black], and from which direction the shadow was going to cross.

Variations in the color of the moon during an eclipse.(20)

The Antikythera Mechanism was so precise, that for one function, it measured to a fraction as small as 0.112571655 days. The brilliant person who made it, figured out the timing of our universe and translated it into twenty-seven little gears with just the right amount of teeth to interact with each other. He even figured out a way to accommodate the moon’s irregular orbit of the earth. He created a tiny mechanism on one of the gears that made the movement of the ‘moon dial’ speed up when it was closest to Earth and slow down when it was farthest from Earth. [Again, you have to watch the video to see how this works.]

The ‘moon gear,’ about the size of a quarter, has a tiny pin and groove feature that allows it to change speed as as it orbits the earth. Screen-shot from BBC movie.(21)

Working together, the museum team and Michael Wright figured out the functions of all twenty-seven surviving gears. Cardiff University professor Michael Edmunds, who led another study of the device in 2006, regarded the Antikythera Mechanism as “more valuable than the Mona Lisa.” The latest news is that there may be another ship near where the first was found. Michael Wright found a gear that he thought did not fit the first device. He suspects there is another mechanism to be discovered.


  1. Photo ©2014 Mary Ames Mitchell. All rights reserved.
  2. Webster, Roderick and Marjorie. Western Astrolabes: Historic Scientific Instruments of the Adler Planetarium & Astronomy Museum, Vol. I, ©1998 Adler Planetarium & Astronomy Museum, 1300 South Lake Shore Drive, Chicago, Illinois, 60605. Printed in Japan
  3. The Paleokastro Disk. Image source: Richard’s blog. “Is the Minoan Disc Discovered at Paleokastro in 1898 the World’s First Analog Computer?” Posted January 10, 2014. URL: Also found on the Archeology News Network web site. URL:
  4. Nebra Sky Disk. Image from wikipedia. Image source url:
  5. There are many theories on Internet about how the Nebra Sky disk worked.
  6. ‘Cup-holders’ on a stone from Göbleki Tepe c9000 BCE. Image source:
  7. Dimpled stone from Monolith 8 of the Almendres Cromlech c6000 BCE.
  8. Nebra Sky Disk c.1600 BCE. Image source url:
  9. The Antikythera Mechanism, dated to 205 BCE. Housed in the National Archaeological Museum, Athens, Greece. Attributions. Photos of Side A and B: No attribution provided. Marsyas, own work assumed (based on copyright claims). CC BY 2.5, Both provided by Wikimedia Commons.
  10. Portion of Antikythera Mechanism artifact showing grid. Image screen-shots from the BBC|Images First Ltd. Video, ‘The 2000 Year-Old Computer - Decoding the Antikythera Mechanism,’ Published March 20, 2014.
  11. Model of the Antikythera Mechanism Massimo Mogi Vicentini, 2007 on display next to the models by Derek Price and the Michael Wright in the National Archeology Museum in Athens: Photo attributed to I, Mogi [GFDL (, CC-BY-SA-3.0. [] or CC BY 2.5 [], via Wikimedia Commons
    Image source: Image source url:
  12. The largest gear, which was about 5.5 inches [140 mm] in diameter.
  13. The wooden box measured 13.4 inches [340 mm] by 7 inches [180 mm] by 3.5 inches [90 mm].
  14. Image screen-shots from the BBC|Images First Ltd. Video, ‘The 2000 Year-Old Computer - Decoding the Antikythera Mechanism,’ Published March 20, 2014.
  15. Ibid.
  16. Members of the team included : Tony Freeth, Mathematician; Dimitris Kourkoumelis, archaeologist; Professor Michael Edmunds; Panagiotsis Tselekas, ancient coin expert; Yanis Bitsakis, physicist; Professor Xenophon Moussas, Astronomer; Mary Zafeiropoulou, Senior Archaeologist; and Professor John Steele, Babylonian Expert.
  17. An orrery is a mechanical model of the solar system, or of just the sun, earth, and moon, used to represent their relative positions and motions. Orreries were named after the fourth Earl of Orrery, for whom one was made in the early 18th century.
  18. BBC movie, ibid.
  19. Ibid.
  20. Ibid.
  21. Ibid.
  22. Michael Wright’s video can be viewed at:

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