Posted by on Nov 26, 2016 in Main |


Mr David Sellers from Leeds AS is welcomed at the beginning of his presentation by society secretary Dominic Curran.


Mr David Sellers from Leeds astronomical society was the guest speaker at the November society meeting. Mr Sellers has spent much time and dedication researching the life and works on one Willian Gascoigue who lived from 1612 to 1644. Mr Sellers explained that in his previous occupation he worked in a building located in Middleton south Leeds, that stands on the site of William Gascoigne’s home. Mr Sellers investigations revealed a scientific genious who’s life was cut short in 1644 after his probable death in the battle of Marston Moor.

It is probably because of his early death and the later loss of his papers and letters that Gascoigne’s work became quickly forgotten.

In the late 1630s, Gascoigne, was working on a Keplerian optical telescope when a thread from a spider’s web happened to become caught at exactly the combined optical focal points of the two lenses. When he looked through the arrangement Gascoigne saw the web bright and sharp within the field of view. He realised that he could more accurately point the telescope using the line as a guide, and went on to invent the telescopic sight by placing crossed wires at the focal point to define the centre of the field of view. He then added this arrangement to a sextant modelled on the instrument used by Tycho Brahe, although Tycho’s sextant was only a naked-eye instrument. Gascoigne’s sextant was five feet in radius, and measured the distance between astronomical bodies to an unprecedented degree of accuracy. Gascoigne then realised that by introducing two points, whose separation could be adjusted using a screw, he could measure the size of the image enclosed by them. Using the known pitch of the screw, and knowing the focal length of the lens producing the image, he could work out the size of the object, such as the Moon or the planets, to a hitherto unattainable degree of accuracy. This invention was later taken up and improved by the scientist and astronomer Richard Towneley who was the nephew of Gascoigne’s friend Christopher Towneley. Towneley later brought the instrument to the attention of Robert Hooke, who used it to calculate the size of comets and other celestial bodies. The micrometer, as it became known, was to lie at the heart of astronomical measurement down to the twentieth century.

Mr Sellers concluded his presentation by showing one of only three modern working replicas of the Gascoigne micrometers in the world, and society members had the opportunity to hold and operate the device.



Mr Sellers work has been published.



Mr Sellers opens his presentation.



A schematic of the type of telescope William Gascoigne was using when he saw the spiders web inside at the focal point, marked ‘5’ on this illustration.





Robert Hooke’s illustration and description of the telescope micrometer invented by William Gascoigne for “Dividing a Foot into Many Thousand Parts, and Thereby Measuring the Diameters of Planets to a Great Exactness”.



Mr Sellers holding a replica of William Gascoigne’s Telescope Micromiter



Adrien Auzout’s (1621-1692) Micrometer published in his book (1662)



Mr Sellers holding a replica of William Gascoigne’s Telescope Micromiter



Mr Sellers explaning to a young society member how Gascoigne’s Telescope Micromiter operated