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Serendipity and a Spider

William Gascoigne (c.1612-44) and the Invention of the Telescope Micrometer

As the French astronomer Adrien Auzout penned a letter to the Royal Society of London on 28th December 1666 he could not have imagined the consternation that it would cause. Nor could he have guessed that his action would almost certainly save from oblivion the work of a young astronomer from northern England, who had suffered a violent death almost a quarter of a century earlier.

Full of enthusiasm, Auzout wanted to tell English scientific friends of his invention of the telescope micrometer: an entirely novel device that, together with Jean Picard, he had used to measure the angular diameter of the Sun, Moon and planets. “We can take diameters to seconds”, he wrote, “…. and we can be almost certain that we cannot be mistaken by more than 3 or 4 seconds …. I can well assure you that the diameter of the sun was hardly smaller at its apogee than 31' 37" or 40"” 

When Richard Towneley, a wealthy Lancashire patron of science, saw Auzout’s letter in the Philosophical Transactions of the Royal Society, he was dismayed. He contacted the Society: “I am told I shall be looked upon as a great wronger of our Nation, should I not let the World know, that I have out of some scattered Papers and Letters that formerly came to my hands, of a Gentleman of these parts, one Mr Gascoigne, found out, that before the late unfortunate wars, he had not only devised an Instrument of as great a power as Monsieur Auzout’s, but had also for some years made use of it …. The very Instrument he first made, I have now by me”.

Text Box:  Towneley’s letter appeared in the next issue of the Transactions. Auzout, though keen to have more details, took the news with good grace.

Of the life of William Gascoigne (1612-44), the Yorkshire astronomer whose micrometer and observations had fallen into the hands of Towneley, little was known. He was born into a wealthy land-owning family and lived at New Hall in the tiny Yorkshire village of Middleton, near Leeds. He made his own lenses and telescopes, and invented a telescopic sight and a telescope micrometer. Before his untimely demise, he had a Treatise on Optics ready for the press.

Though by his own account spending on astronomy only the same time as friends and neighbours spent on “hawks and hounds”, by the age of 28 Gascoigne had made some remarkable innovations. He had devised experiments to show how an inverted image was cast onto the retina of the eye. He had verified the sine law of refraction – recently published by Descartes – and had used the same law to calculate the angular size of the Sun, from the size of its image projected by a Galilean telescope.

One day, returning to the open case of a Keplerian telescope with which he had been experimenting, Gascoigne found that a spider had spun its web inside the case – a few centimetres from the convex eyepiece. With characteristic curiosity, rather than sweep the web aside, he bent to look through the telescope. To his astonishment, he saw that, not only was the web in focus, so too were distant objects.

“This is that admirable secret, which, as all other things, appeared when it pleased the All Disposer, at whose direction a spider’s line drawn in an opened case could first give me by its perfect apparition, when I was with two convexes trying experiments about the sun, the unexpected knowledge.”

In the Keplerian telescope the objective lens was convex and unlike in the Galilean model so too was the eyepiece lens. With this arrangement any object placed at the focal point of the objective lens, could be magnified and brought into focus alongside the image of the distant object being viewed. This ‘incredible rarity’, as Gascoigne called it, immediately suggested a number of possibilities. Firstly, it was feasible to introduce a simple marker, to act as a telescopic sight.

if I .... placed a thread where that glass [the eyepiece] would best discern it, and then joining both glasses, and fitting their distance for any object, I should see this at any part that I did direct it to ...” 

Secondly, one could introduce a measuring device into the common focal point and thereby measure the size of the observed image. At first he used simply a scale. Then he introduced a pair of brass pointers. These were mounted on a screw and could be moved towards and away from each other by turning the screw (Fig.1). The change in separation was proportionate to the number of turns, as also was the angular separation of the two distinct points enclosed by the pointers. With this ingenious device he measured the diameters of the sun, moon and planets.

Figure 3 - Gascoigne Micrometer.jpg

Fig.1  Gascoigne's telescope micrometer

Gascoigne was in regular contact with the young Lancashire astronomers, William Crabtree and Jeremiah Horrocks and, for a remarkable, though lamentably brief period, these three blazed a trail for precision astronomy in Europe.

Whilst William Crabtree, however, was using something similar to a cross-staff, fitted with needles, to observe the sun through thin cloud and measure its angular diameter, Gascoigne, by contrast, was using the “wheels and screws” of the micrometer to make similar measurements with unprecedented accuracy. The whole purpose was to test Kepler’s idea that shape of the Earth’s orbit was an ellipse, rather than the eccentric circle argued by Copernicus.

From ancient times it had been assumed by astronomers that the distance between the Earth and the Sun varied over the course of the year (Fig.2). Hipparchus had deduced this from the observed difference in the length of the seasons and an assumption that the Sun moved around the Earth at a uniform speed. Copernicus, though he placed the Earth in circular motion around the Sun, was obliged to offset the centre of the circle from the Sun to get the right length for the seasons. Kepler introduced the notion of an elliptical orbit of the Earth, with the Sun at one focus of the ellipse. All these theories implied a variation in the apparent angular diameter of the Sun throughout the year. The variation of this apparent diameter and hence the ‘eccentricity’ of the orbit, however, was far too slight to be measured reliably - until, that is, the invention of the telescope micrometer. Herein lay the importance of the device invented by Gascoigne and Auzout. The precise value of the eccentricity could be calculated from the semidiameters at the furthest and nearest approach of the Earth to the Sun. What is more, they would differ, according to whether Copernicus or Kepler was correct.  

Figure 4 - eccentric orbits.jpg

Fig.2 Three alternative models for the orbital eccentricity in the Earth-Sun system.

a=maximum distance of the orbiting body from the centre of the of orbit

e= the eccentricity. The offset of the Earth (geocentric theory) or the Sun (heliocentric theory) from the centre of the orbit, expressed as a fraction of a

SS= summer solstice, WS=winter solstice, AE=autumnal equinox, VE=vernal (or, spring) equinox

Gascoigne’s values of 15' 53" and 16' 25", respectively, implied an orbital eccentricity of 0.01651 (compared with the current value of 0.01672), and helped to confirm the elliptical orbit of Kepler.

In 1642 England was plunged into a bloody civil war, which brought an end to Gascoigne’s astronomical pursuits. We will perhaps never know what impelled William Gascoigne to join the King’s side in the conflict. Whatever the case may have been, he abandoned his telescopes and – despite the impediment of a lame leg - threw in his lot with the Cavalier army of Charles I.

Within two years he was dead - killed by a bullet, according to Christopher Towneley, at the battle of Marston Moor near York on 2nd July 1644: His naked body probably thrown into a mass burial pit along with up to 10,000 other unfortunate young men.


Even after Richard Towneley’s belated account of Gascoigne’s inventions  it appeared that his contributions were destined to fade into oblivion.  A handful of letters between Gascoigne and Crabtree were in the hands of the Towneley family, until 1711 when they were handed to the Reverend William Derham.

Shortly after getting them, Derham happened to notice an obscure passage in the Preface to 1687 tables of Philippe de La Hire (1640-1718), in which the invention of the telescopic sight was attributed to Picard. Derham promptly wrote to the Royal Society in London, saying “Monsieur de La Hire … robs our Mr Gascoigne of the honour of first applying telescopic sights to mathematical instruments and ascribes it to Monsr Picard …. Now since Mr Gascoigne’s papers are in my hands … I am fully able to do Mr Gascoigne justice”.

When Derham’s paper was published, it demonstrated beyond doubt, with extracts from letters of Gascoigne and Crabtree, that Gascoigne had not only invented the telescopic sight and micrometer, but had also taken many measurements with them.  

Meanwhile, de la Hire was busy compounding his ‘misdemeanour’. In a new memoir concerning the date of the invention of the Micrometer he did not even mention Gascoigne. Instead, he said “we are indebted to Monsieur Huygens for the invention of the micrometer”.

Across the Channel it took nearly four decades for a response to come. By then John Bevis – the discoverer of the Crab Nebula – had found a new piece of evidence confirming Gascoigne’s achievements. In the library of the Earl of Macclesfield he had found and copied a letter written by Gascoigne to the mathematician, William Oughtred, in 1640-01. “It consists of several sheets of paper”, he reported to the Royal Society, “all about his invention for measuring small angles to seconds; where he not only gives the geometrical and optical principles of his contrivance, and the construction of the instrument, but also a series of observations actually taken therewith.”

Eventually, the true role of Gascoigne was acknowledged on both sides of the Channel. In his authoritative History of Modern Astronomy (1821) the French astronomer, Jean-Baptiste Delambre devoted several pages to Gascoigne and the priority dispute. He acknowledged the Middleton astronomer’s micrometer measurements of the solar disc and compared them favourably with the theoretical values given in the French national almanac, Connaissance des Temps.

                                                    Gascoigne                           Connaissance des Temps            

    25 October (OS)                16' 11" or 10"                      16' 10"

    31 October                          16' 10"                                   16' 11".4

    2 December                       16' 24"                                   16' 16".8


Horrocks and Crabtree achieved lasting fame because of their observation of the 1639 transit of Venus. A fame which is reinforced with each fresh transit ‘season’, including that through which we are fortunate to have lived (2004-12). Their friend, William Gascoigne – according to Flamsteed, “as ingenious a person as the world has bred or known” - has been consigned, however, to a mere footnote in the history of astronomy.

The original letters that Derham acquired have long since been lost. Ironically, it is largely due to the unintentional ignoring of Gascoigne’s achievements by French astronomers (Auzout, de La Hire) that Gascoigne was rescued from total oblivion. Without the resulting priority dispute, we would probably be unaware of the tragic story of that remarkable young man.



Note: David Sellers is the author of The Transit of Venus: the Quest to Find the True Distance of the Sun (Leeds, 2001), co-author of Vénus devant le Soleil (Paris, 2003, Ed. A Simaan), and the author of In Search of William Gascoigne: Seventeenth Century Astronomer (Springer, New York, 2012)



Sellers, David, In Search of William Gascoigne: Seventeenth Century Astronomer, Astrophysics and Space Science Library, Volume 390. ISBN 978-1-4614-4096-3. Springer Science+Business Media New York (2012) - see publisher's flyer

Sellers, David, A letter from William Gascoigne to Sir Kenelm Digby, Journal for the History of Astronomy (ISSN 0021-8286), Vol. 37, Part 4, No. 129, p. 405 - 416 (2006)

Chapman, Allen, Three north country astronomers (1982), Dividing the Circle (pp. 35-45) (1995), Oxford Dictionary of National Biography (pp. 591-3) (2004)

Willmoth, Frances, William Gascoigne, in Encyclopedia of the Scientific Revolution (ed. Wilbur Applebaum) (2000)

Thoren, Victor, William Gascoigne, in Dictionary of Scientific Biography (ed. C.Gillespie) (1972)

Taylor, R.V., Biographia Leodiensis (pp.86-7) (1865)

Wheater, William, William Gascoigne - the astronomer, The Gentleman's Magazine (pp.760-762) (1863)