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Episode 3 - The First Computer

David Grider • Apr 05, 2021
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“Give us a reading on the 1202 program alarm.” - Armstrong

“Roger. We got…. We’re go on that alarm.” - Duke

In 1969, a pair of digital computers, each the size of a suitcase, guided a pair of spacecraft to the moon and back. One of those spacecraft landed on the surface and returned to lunar orbit. Those two computers, each called an Apollo Guidance Computer, or AGC, performed their tasks admirably, even when it looked like everything would go horribly wrong. During the descent of the lunar module, the computer repeatedly warned that it was doing too much and had to stop low-priority tasks. Without the quick thinking of some engineers at Mission Control and the forethought of Margarette Hamilton, the inventor of software engineering, it’s possible that Armstrong and Aldrin would have had to abort before landing.


Had that happened, another computer, the Abort Guidance System, or AGS, could have taken over and brought the LM back into lunar orbit.


Despite what you might think, the AGC and the AGS were not the only computers used aboard the spacecraft. Two other computers, the Launch Vehicle Digital Computer (LVDC), and the analog Flight Control Computer resided in the Instrument Unit of the Saturn V rocket. And these are just the computers aboard the actual spacecraft. This doesn’t include the five IBM System/360 computers bought for NASA’s Manned Spacecraft Center in Houston, or the dozens of other computers spread across facilities all around the world.


The computers that aided manned flight to the moon could be considered rudimentary, even primitive, by today’s standards. However, during a time when most computers took up an entire room, the Apollo Guidance Computer was exceedingly small: less than one cubic foot of volume, not counting peripheral devices, and had to perform tasks that were traditionally done by hand. One of the most important tasks it had to perform was keeping track of where the Earth, sun, moon, and 37 stars were located at any given time. Today, a task like this can be performed by a computer very easily. In fact, several free apps for mobile devices exist that do this with reasonable accuracy.


One would think that keeping accurate positioning of stars and planets would have been something that we’d have been really good at by the mid-20th century. After all, as a species, we’ve been fascinated by the sky from time immemorial. Despite this, accurately prediciting the future positions of celestial bodies is not an easy task without precision instruments that were not invented until fairly recently, and it’s even more difficult and time consuming without computers. Highly accurate navigation, which traditionally relies on determining the positions of celestial objects, navigation tables, an accurate timebase, and quite a bit of hand-calculated trigonometry, was only possible after the invention of the marine chronometer in 1761.


However, some of these problems were solved once before. Today’s story takes us back to the year 100 BC.


Hi, I’m David Grider, and welcome to How We Went to the Moon. In this episode, we’re going to explore the only surviving example of the ancient Greek analog computer, and first computer known to exist: the Antikytheria mechanism.


The year 654 Ab urbe condita, corresponding to 100 BC, turned out to be one that would change world events forever. On July 12, Gaius Julius Caesar was born into a patrician family in Rome. At this time, the Roman Republic had taken over most regions of the Mediterranean sea, from the Iberian peninsula to western parts of Anatolia, modern day Turkey.


Around the same time, during the late Hellenistic period, a device about the size of a shoebox was constructed by the Corinthians or the Rhodians that was designed specifically to track the movement of celestial bodies. It wasn’t the first device of its type, as several examples have been mentioned going back several hundred years prior, including in the workshop of Archimedes of Syracuse. The device was a collection of precision gears and pointers with highly sophisticated mechanical motion made of brass and placed into a case, probably of wood. At first glance, one might think it was a clock. The whole machine was driven by a hand-turned crank.


At some point after its construction, the device was placed aboard a Greek merchant ship traveling from Anatolia to Rome, alongside bronze and marble statues, coins, pottery, jewelry, glassware, and many works of art. As the ship passed the tiny island of Antikythera, between the islands of Kythira and Crete, it ran into a storm and sank.


Two millennia later, a group of sponge divers en route to Tunisia were also caught in a storm near the island and, to pass the time, they chose to look for sponges on the ocean floor. Instead, they found what would be known as the Antikythera shipwreck. An expedition was launched by the Greek navy, during which a number of antiquities were discovered, including a few hunks of corroded bronze with splinters of a wood box. The pieces of metal were a mystery, and so were largely ignored by historians. It wasn’t until the 1970s that the artifact was scanned by x-ray, but the images seemed impossible to interpret, and historians once again ignored the item. Nature abhors a vacuum, so the curiosity was labeled by some as an alien artifact. It wasn’t until the 21st century that breakthroughs started occurring.


In 2006, a team at Cardiff University in Wales published the results of a CT scan that showed the internal mechanics of the device as well as inscriptions that were not previously visible. Using this information, and the information gleaned over the last 15 years, we now have a good idea of what the device was, how it worked, and for what purposes it was made.


The device could display the positions of each of the classical planets: the sun, moon, Mercury, Venus, Mars, Jupiter, and Saturn. Each planet had a dial with a ball corresponding to their apparent color. The phase of the moon was displayed with a rotating silver ball painted black on one side. The rise dates of major stars were also displayed. On the back of the device was a dial showing the date, and another for predicting lunar and solar eclipses. This specific version of the device was also designed to calculate the timing of the original Olympic games as well as the festivals of Naa and Halieia. Given the inclusion of the latter, it’s hypothesized that the device may have been built by Posidonius of Rhodes, a Greek philosopher of the school of Stoicism, who is known to have built a complex mechanical model of the solar system.


As of the recording of this podcast, there are several theories regarding the exact mechanics of the device and how it calculated the positions of the planets. When the device was constructed, the accepted model of the universe was the Greek model of deferent-and-epicycle geocentrism, which allowed for the loops that planets trace during parts of their orbits. As such, one theory for the functionality of the device is that it duplicated this by using concentric tubes that are not known to have been possible to make fit in the required space during the second century BC without modern technology. Only 82 pieces of the Antikythera mechanism have been found, and more may still lie on the ocean floor. More research is needed.


It would not for nearly 2000 years that mechanical devices of such complexity would be seen again, and they, like the Antikythera mechanism, would be one more piece to the puzzle of How We Went to the Moon.


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