1 Mar 2016

The Aerial Telescope of Robert Hooke

Robert Hook (28 July 1635 – 3 March 1703)
Robert Hooke was born at Freshwater, Isle of Wight, son of John Hooke, curate at All Saints' Church. The church stands at the end of what is now Hooke Road, which also has the Hooke Museum. Robert Hooke was one of the most brilliant and versatile of seventeenth-century English scientists, but he is also one of the lesser known; his persona and his contributions are far outweighed in public perception by those of Newton and of Wren. This is unfair.

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He was a scientist who made valuable contributions with his splendid illustrations of insects, and made astronomical observations. He was also a British architect who assisted in redesigning London after the great fire of 1666.

He constructed his own aerial telescope which he used to observe the star Gamma Draconis, which is of unusual design.

In 1669 he lodged at Gresham College, London, where he carried out astronomical observations with an aerial telescope of his own design. On 15 July 1669 the then Astronomer Royal John Flamsteed informed the Royal Society he was observing in Gresham college the parallax of the earth’s orb, and hoped to give a good account of it. The Astronomer Royal later described to Isaac Newton how Hook ‘by ingenious means fixed the objective lens of his telescope tube, 36 feet in length, in the roof of his chamber, in order to observe the distances of the stars from the vertex. The measurements Hook passed to Flamsteed covered precisely the period of the summer recess of 1669.

After a few months. Hooke observed a displacement in the expected position of Gamma Draconis, but he tells us that unfortunately the objective lens of his telescope accidentally broke and he therefore discontinued his observations.

In 1725, four years after his Oxford appointment, James Bradley began a research programme with Samuel Molyneux (1689-1728), a brilliant amateur telescope maker. Molyneux had been impressed by the efforts of Robert Hooke, who, unwilling to let any new idea pass him by, had in 1669 made a series of measurements of the star Gamma Draconis that appeared straight overhead from London. By observing a star at so high an altitude, Hooke hoped he would be able to reduce the effects of the Earth's atmosphere on the accuracy of his measurements. In this his reasoning was sound, for when a star is observed overhead, the light from it passes through a thinner layer of air than when it lies close to the horizon, and so is less disturbed by air currents and by distortions due to the air itself. With an overhead star the measurements he made of its position with reference to other stars would have every likelihood of being far more accurate than those made at any other angle, and since he noticed a shift in position he thought that what he measured might well be parallax.

James Bradley became the third Astronomer Royal at the Royal Greenwich Observatory, and popularised the theory of the aberration of starlight.

The English astronomers James Bradley (1693-1762) and Samuel Molyneux (1689-1728) repeated Hooke's observations in 1725 with a better telescope and discovered that the observed displacements could not be due to parallax, but they were unable to interpret them. By 1727, however, after erecting another telescope and continuing his observations, Bradley found the explanation as due to the finite velocity of light, which the Danish astronomer Ole Romer (1644-1710) had determined in 1676.

Bradley thus was able to establish the motion of the Earth, finally accomplishing Hooke's Attempt to Prove the Motion of the Earth from Observations.
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