Make your own Astrolabe

by Dominic Ford

10. Epilogue: The Decline and Fall of the Astrolabe

By the turn of the seventeenth century, the astrolabe was starting to be superseded.

In 1576, Tycho Brahe laid the foundation stone of Uraniborg, a research institute on the small Danish island of Hven. Over the following 21 years until its abandonment in 1597, this institute brought about a revolution in pre-telescopic astronomical instrumentation.

Among the instruments pioneered under Tycho's direction were the sextant – which allowed the angular distances between stars to be precisely measured – and the mural quadrant – which allowed the altitudes of transiting stars to be measured with respect to a plumb line indicating the local vertical. At Uraniborg's observatory, Stjerneborg, these instruments achieved arcminute accuracy, close to the theoretical resolving power of the human eye.

Though Tycho guarded his intellectual property closely in his lifetime, knowledge of such instruments spread rapidly after his death in 1601, as his former observing assistants received appointments at observatories across Europe and Asia.

As this was happening, Tycho's observations of planetary positions were being analysed by one of his theoretically-minded former assistants, Johannes Kepler. He found that the path followed by Mars could not be reproduced by either Ptolemaic planetary theory or Tycho's proposed replacement for it.

Motivated by this, Kepler went on to develop his own planetary theory, showing that Tycho's data could be explained if the planets followed not circular, but elliptical orbits about the Sun. This conclusion vindicated Tycho's campaign of precise observation by demonstrating, as Tycho had hoped to do, that precise measurements of planetary positions could challenge ancient planetary theory. Ironically, though, Kepler had at the same time disproved Tycho's own cosmological ideas.

Once the case for precision observation had been made, the astrolabe, a small hand-held instrument, was no longer sufficient for astronomers' needs. By the mid-eighteenth century, the staple instrument for positional astronomy would be the transit instrument – an evolved form of Tycho's mural quadrant with a telescopic sight.

Even outside observatories, the astrolabe was by now becoming largely redundant: as a chronometer, it could no longer compete with the clockwork rivals which were increasingly widely available and reliable.


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