That's not junk: this is only the 8th astrolabe quadrant ever found.Canterbury Archaeological Trust Ltd

When the owners of a restaurant in the historic British city of Canterbury dug the foundations for an extension to their period property, they may have hoped to find an old coin or two. Instead, they unearthed a rare scientific instrument.

Nestled among shards of pottery in a fourteenth-century rubbish pit, archaeologists watching over the dig spotted an astrolabe quadrant - a complex astronomical calculator for telling the time and calculating latitude.

Such devices are extremely rare — it is only the eighth such instrument ever to be discovered. And finding it in what was probably a rubbish dump is even more unusual.

The building that houses the restaurant dates back to the sixteenth century, so when work on the extension began in 2005, Andrew Linklater of the Canterbury Archaeological Trust was assigned to watch over it — just in case.

As the foundations were dug, the builders uncovered the remains of a medieval building. By the back wall was a pit containing shards of pottery dating to 1375-1425, which Linklater speculates might have been a rubbish pit. Among the fragments, he picked out a dirty brass plate in the shape of a quarter circle.

Unable to identify the plate, he took it to the British Museum. "They held their hands up and went 'wow'," he recalls. Experts there identified it as an incredibly rare astrolabe quadrant, and suggested sending it to Elly Dekker, a Dutch independent scholar with expertise in historical astronomical instruments, for full analysis.

Eye on the sky

Dekker's study is now complete, and she believes that the find dates to around 1388.

The quadrant has a series of curves and markings engraved on it. Along one edge of the instrument are two sighting holes, which the user would have lined up with the Sun. An attached plumb bob (now missing) would have hung down vertically, aligning with the engraved calibrations and allowing the user to tell the time of day. It could also be used to determine the time of sunrise and sunset, or to work out the latitude.

Dekker says that the quadrant is quite recent compared with the few others that are known. But the Canterbury version can only be used during the day, unlike the other instruments that were designed to line up with the stars as well as the Sun. It may be that its designers realized it was too difficult to fit so much information onto a small instrument, and so created a simplified, stripped down version.

Brass in pocket

Dekker says the object would have been very expensive in its time, and that it is extremely rare to find such a valuable object buried in the earth - most pieces such as this get passed down through the generations and are eventually rediscovered in someone's attic or private collection. "We don't find instruments of this type in archaeological sites," she says.

Linklater adds that he is more used to digging up coins or buckles, and is mystified by how the quadrant ended up in the pit. "It would have been exceptionally highly prized; it was the peak of the technology of the day," he says. "It was the sort of thing you had to have if you wanted to be ahead of everything in science. But it must have been discarded."

He speculates that the owner might have been a member of the Church, which was interested in science. And this person may have been travelling, perhaps even on a pilgrimage. Astrolabe quadrants were the 'pocket' version of a more common, circular instrument called an astrolabe. Back in the fourteenth century, the street where the restaurant stands was lined with inns for pilgrims coming to Canterbury.

The quadrant is being sold by London auctioneers Bonhams on 21 March, and is predicted to net the owners between £60,000 (US$118,000) and £100,000. The extension to the restaurant is now complete, and has been named the Quadrant Bar.

Linklater says archaeologists generally frown on the selling of artefacts, as they often then don't end up in a museum. But he's happy just to have discovered it. "To find something as rare as this is incredibly exciting," he says. "It's certainly the most valuable thing I've ever found."

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