Sunday, December 4, 2011

Stars on oldest heliocentric model were on backwards

One of the world’s oldest mechanical models of the universe has been on display with its constellations in the wrong order for more than 60 years. But - until I found myself scratching my head at its unveiling to the public following a year-long restoration - it seems that no one had noticed.
The Leiden Sphaera, a top attraction in Museum Boerhaave (the Dutch National Museum for the History of Science and Medicine in Leiden), was built around 1670 by the clock maker Steven Tracy for Adriaen Vroesen, a mayor of Rotterdam with a keen interest in science. It is a very early example of an orrery: a dynamic scale model of the solar system.
According to Hans Hooijmaijers, head of collections at Museum Boerhaave, the Sphaera, which measures 1.5 metres across, may well be the first orrery ever to display the Sun-centred universe. “It’s a unique device”, he says.
So what is wrong with the constellations? The twelve patterns of stars in the zodiac are displayed in separate gold-plated reliefs around the “equator” of the Sphaera. Each constellation is pictured in reverse, as it would appear reflected in a mirror - a common practice on celestial globes. As seen from the Earth, in the interior of the globe, the constellations would have the correct orientation, but as seen from the outside, they appear flipped horizontally. For instance, the head of the Lion on the Leiden Sphaera points to the left, whereas looking up to the sky from Earth, it points to the right.
To keep things consistent, the order in which the twelve constellations are displayed should of course also be mirror-reversed. But, as I noticed last week, that’s not the case with the Sphaera: the Lion’s head points towards the Virgin, not towards the Crab, as it does in the sky. Tracy and Vroesen may be turning in their graves.
Indeed, a 1711 engraving of the Sphaera, in a pamphlet published by Leiden Observatory, clearly shows that the original order of the constellations was correct. The same is true for a photo of the instrument made in 1930. But a 1948 photo reveals that the current mistake was made at least 63 years ago.
According to Hooijmaijers, eye-witness reports suggest the Leiden Sphaera was severely damaged during World War II. “It must have been repaired shortly after the war”, he says, “but this has not been documented”. Apparently, the twelve ornamental signs were mounted in the wrong order back then - something that went unnoticed by staff astronomers and historians for decades.
The recent restoration has been the most thorough facelift of the Leiden Sphaera ever. “It’s now a real showpiece again”, says Paul Steenhorst, the museum’s head of restoration.
But before it is truly ready, Steenhorst and colleagues will have to return to the workshop to dismount the twelve zodiac signs and replace them again in the correct order. Rosalijn van IJken, who worked on the team restoring the Sphaera, says “it’s a pity” that no one ever took a close look at the old photographs. “But I wouldn’t call it negligence”, she adds. “I just put the constellations back in the order in which I found them. I don’t know anything about astronomy”.

No comments:

Post a Comment