The Northwest of England, through the Jodrell Bank observatory, contains a UNESCO World Heritage Site for the pioneering of radio astronomy. North West Bylines references this though our logo, inspired by the Lovell Telescope. What is less well known, is that about 300 years before the building of the Jodrell Bank observatory, two young men from Lancashire, Jeremiah Horrocks and William Crabtree, were pioneers in the use of optical telescopes. With the aid of William Gascoigne, a Yorkshireman, their work is regarded as one of the foundations of modern quantitative astronomy.
Their most important work was carried out in late November – early December 385 years ago. These three young men, from relatively humble backgrounds, all died young around the age of 30 during the turmoil of the 17th century civil war. Their collective passing was before their work was published, distributed, and recognised. Although largely forgotten in the popular history of British science, Horrocks and Crabtree are commemorated by a lunar crater and a minor planet, respectively as well as by local memorials.
Astronomers
The history of astronomy is often presented as a succession of important men (no women appear until the eighteenth century). From the Renaissance until the scientific revolutions of the Enlightenment, the key figures are normally considered: Nicolas Copernicus who proposed a heliocentric cosmology, Tycho Brahe with his meticulous observations, and Kepler who refined the Copernican model. They are followed by Galileo, who first used the telescope for astronomy and finally, Isaac Newton, who produced his theory of gravitation and the System of the World.
Our pioneers sit within this timeframe, between Galileo and Newton. Their greatest achievement was to predict and observe the transit of the planet Venus across the face of the Sun on 24 November 1639 (sometimes reported as 4 December in the Gregorian calendar that was not used in England). This was the first time the transit was conclusively observed and more importantly recorded. From this, the first calculation of the distance between the Earth and the Sun that is close to the accepted value, was achieved.
Jeremiah Horrocks
Jeremiah Horrocks was born in Toxteth Park near Liverpool in 1618, where his father worked as a watchmaker. The family was moderately prosperous and wealthy enough for Jeremiah to attend Emmanuel College at Cambridge University, although as a Sizar (a student who worked part time to fund his studies). He matriculated (recorded as attending) in 1632 but had not completed a degree by the time he left Cambridge in 1635, which may be because the family were puritans, and the university had strong ties to the Church of England.
Attending university at this young age (13 or 14) was not unusual at the time and there is no indication that he was considered in any way exceptional in his ability. He would have received a basic training in mathematics, arithmetic, geometry, and classical astronomy. Returning to Liverpool, by 1637 he was reported to have had a telescope and be in correspondence with astronomers in Europe.
Around this time, he moved to Much Hoole near Preston, where he lived in Carr House, which still stands today. It is possible he was a tutor to the children of Carr House, owned by the wealthy Stones family.
William Crabtree
William Crabtree was born in 1610 in Broughton, Salford. He too was from a modestly prosperous family of merchants, who could send him to Manchester Grammar School. It is not clear where his interest in astronomy originated from, but he was in correspondence with Horrocks. He was sufficiently mathematically literate enough to have had a copy of the Rudolfian astronomical tables, produced by Kepler, and checked them for accuracy. Crabtree was also in correspondence with astronomers elsewhere in Europe and England, although little of his communication, and that of Horrocks, has survived.
William Gascoigne
The final young astronomer was William Gascoigne, who was from Leeds and had a slightly more elevated position in the English class system. His father was a minor country gentleman or landowner and associated with the Royalist faction in the later civil war. Gascoigne, despite his higher social status, seems to have been the most practical of the three and is best known for the invention of the screw micrometer.
His key contribution to astronomy was to transform the telescope from an observational tool to a measuring instrument. He is credited with inventing the crosshair sight that locates the centre of the field of view in a telescope. Legend has it, that an enterprising spider had started to construct a web at the combined focal point of the two lenses in a telescope and this formed a sharp image superimposed upon the field of view.
Gascoigne understood sufficient optics to be able to recreate the effect and this allowed a telescope to accurately track the motion of a celestial object or to measure the width of a planetary disc. It is unclear whether Gascoigne ever met Horrocks, but he and Crabtree did meet shortly before their early deaths and Crabtree used the telescopic sight to make measurements to validate Horrock’s predictions.
The Transit of Venus in 1639
Astronomers realised that the planets Mercury and Venus lay between the Earth and the Sun for some part of their orbit. A transit occurs when the planet’s motion is such that it appears to physically cross the disc of the Sun, and its presence is seen as a black disc travelling across the Sun. Because the Earth and Venus orbit the Sun at slightly different angles of inclination (see figure), this coincidence is a rare event, occurring in a 223-year cycle with pairs of transits eight years apart separated by gaps of 121.5 years and 101.5 years.
Kepler was the first astronomer to prepare sufficiently accurate tables describing the relative motion of the planets in the solar system to allow the prediction of when transits of Venus or Mercury occur. He had predicted that a transit would occur in 1631 but unfortunately the rotation of the Earth was such that this could not be observed in Europe. Although it would have been visible in North America, no one there knew about this transit to make an observation.
Kepler predicted the next transit would not be until 1761. There is some confusion as to whether Kepler also realised that there would be another transit in 1639 and the interval he stated was between the pairs. However, it is known that Horrocks independently consulted the Rudolfian tables and used his observations combined with those of Crabtree to predict the transit in 1639. He only made this discovery about one month before the event. He corresponded with Crabtree and the two of them prepared for independent observations of the transit.
It was 3pm on a late November evening, the weather in Lancashire was typical of the time of year and there were intermittent clouds. William Crabtree was in Salford, and he was able to make a single fleeting observation of the transit as the clouds parted before sunset.
Horrocks confirmed Crabtree’s observation
Jeremiah Horrocks was more fortunate in Much Hoole, and he was able to make three different sitings to track the progress of the planet across the face of the Sun and thus confirmed he was not merely observing a sunspot.
Horrocks compared his data with Crabtree’s to prove they were observing the same phenomenon. His three measurements and the time between them allowed him to determine the velocity of Venus and its apparent size relative to the sun. He used this to determine the distance between the Earth and the Sun to be about 95 million km, which was about 10 times greater than previously thought and close to its actual value of 150 million km.
We now know Horrock’s method, based on assumptions on the relative size of the planets, was flawed. However, the result was a milestone in our understanding of the scale of the solar system. Horrocks presented this in his treatise Venus in sole visa (Venus seen on the Sun) in Latin (the international language of science at the time). However, this was not published until 1662, many years after his death and first in Danzig (now Gdansk) by the Polish astronomer Hevelius. This publication, some 40 years after the deaths of the astronomers, led to their achievements being recognised and celebrated.
Jeremiah Horrocks died on 3 January 1641, barely one year after these observations. Gascoigne and Horrocks continued their correspondence after Horrock’s death but in 1642 Gascoigne received a commission in the Army of King Charles I during the civil war and he died at the Battle of Marston Moor on 2 July 1644. Crabtree made his will on 19 July 1644 and is recorded as buried in Manchester Collegiate Church (now the cathedral) on 1 August 1644. The turmoil during and after the civil war led to the loss of their original manuscripts and much of their correspondence.
Legacy and memorials
Thus ended a brief period of intense astronomical activity carried out chiefly in Lancashire but with important contributions in instrumentation from Yorkshire. We know that after Horrock’s death and before his own demise, Crabtree used instruments designed or supplied by Gascoigne to make a more precise measurement of the Moon’s orbit. These confirmed Horrock’s original observations that first demonstrated that the Moon orbited the Earth in an elliptical orbit. These observations were used and acknowledged by Newton and Halley in their later work on gravity and planetary orbits.
Modern Legacy
Liverpool has adopted Horrocks because he was born in Toxteth and is believed to be buried near the Unitarian Chapel. Preston also lays claim because of his transit observations in Much Hoole. Manchester commemorates Crabtree with one of the town hall murals (reproduced above), even though he was born across the Irwell in Salford. The Walker Art Gallery in Liverpool has a similar vintage Victorian representation of Horrock’s observations that he apparently carried out in classical puritan clothing and hat (reproduced above).
