Astronomical computers (old ones)

Most people think of astronomy as a very modern science, because of its association with things like space travel, UFOs and science fiction. But it’s really one of the oldest sciences, especially when it comes to quantitative calculations and mathematical modelling. That’s because astronomical objects generally move in a very regular and predictable way. It’s tempting to say “like clockwork”, but that’s putting things back to front. A clock is basically a simple analog computer that models the apparent motion of the Sun across the sky – a fact that was much more explicit in the early “astronomical” clocks that can be found in some old churches (such as this one from Paul Jackson’s blog).

I was reminded of this on a visit to Greenwich last week. It’s the home of the old Royal Observatory (pictured above, with the large red “time ball” clearly visible). By the time it was built, in the 17th century, mechanical clocks were well established. But in earlier times, people had to resort to things like sun-dials. But what if you wanted to know the time in the middle of the night? A short distance from the Royal Observatory, in the National Maritime Museum, I saw a display which explained how you could do just that using an ingenious mediaeval gadget called an astrolabe.

You can see a selection of astrolabes in the picture below. They’re shiny, highly desirable pieces of technology, and I’m sure no fashion-conscious geek would have been seen without one back in the Middle Ages. Like a lot of sexy modern tech, they do something that’s basically very simple in a ridiculously complex (but satisfyingly elegant) way. All you really need to do is measure the elevation angle of a known star. You could do this using a simple sextant made from a protractor, a plumb bob and a drinking straw (Mark Watney makes one in exactly this way in the novel version of The Martian). Then, assuming you know today’s date, you can consult a set of printed look-up tables to interpolate the exact time. All an astrolabe does is remove the need for paper and pencil – you just twiddle its various knobs and dials (it has five moving parts in all), and it automatically tells you the time. But who wouldn’t prefer dial-twiddling to looking things up in a book? If this was 1492 I’d want an astrolabe ... and I bet you would too.

An astrolabe, like a clock, is essentially an astronomical computer. Nowadays we tend to think of computers in terms of information processing, but the word originally comes from the Latin computare, meaning “to calculate”. And in the old days, one of the few things that was amenable to mathematical calculation was astronomy. The famous Antikythera mechanism, which is over 2000 years old, is described by the (never knowingly hyperbolic) Wikipedia as “an ancient analogue computer ... used to predict astronomical positions and eclipses”.

Today it goes without saying that a computer is a machine, and to describe someone as a “human computer” is a pejorative – suggesting they are machine-like and soulless. But the earliest use of the word computer was to refer to a person who carries out calculations. During the 19th and early 20th century, “computer” was actually a job title in some astronomical institutions. That’s a fact I discovered a couple of years ago when I watched Cosmos: A Spacetime Odyssey. One of the episodes, Sisters of the Sun, described the work of the “Harvard Computers” – who as the episode title suggests, were all women. Apparently Greenwich was more egalitarian, employing computers of both sexes – as the following information board in the observatory museum indicates (although the one pictured is female).