Credit: J W Lavender (1905)
Jeremiah Horrocks was born in Toxteth, Liverpool in 1619, and is considered by many to be the father of British astrophysics. He was the first person to observe a transit of Venus (where Venus passes in front of the Sun) in 1639, and was the first to prove that the orbit of the Moon was an ellipse. In his short life - sadly he died when he was aged just 22 - he made a number of remarkable discoveries that where later used by Sir Isaac Newton to formulate his fundamental laws of Physics.
Although his early years were spent on a farm, by the age of thirteen Jeremiah Horrocks had entered Emmanuel College, Cambridge as a 'sizar' or poor scholar and taught himself Astronomy. In 1635 he returned to Toxteth and used Kepler's Laws of Planetary Motion to prove that the Moon orbited the earth elliptically. Following Kepler's prediction that Venus would transit the Sun in 1631, Horrocks calculated that these transits were not isolated, but occurred in pairs just eight years apart.
On the 24th November 1639, the date of the second transit Horrocks, who was now working as a curate in the village of Much Hoole, near Preston, prepared his equipment for the observation. Using a simple telescope set on a wooden beam, he was able to project the Sun's image onto a piece of paper marked with a six inch graduated circle. He then says:
I watched carefully on the 24th from sunrise to nine o’clock, and from a little before ten until noon, and at one in the afternoon, being called away in the intervals by business of the highest importance which, for these ornamental pursuits, I could not with propriety neglect. But during all this time I saw nothing in the sun except a small and common spot… This evidently had nothing to do with Venus. About fifteen minutes past three in the afternoon, when I was again at liberty to continue my labours, the clouds, as if by divine interposition, were entirely dispersed, and I was once more invited to the grateful task of repeating my observations. I then beheld a most agreeable spectacle, the object of my sanguine wishes, a spot of unusual magnitude and of a perfectly circular shape, which had already fully centred upon the sun’s disc on the left, so that the limbs of the Sun and Venus precisely coincided, forming an angle of contact. Not doubting that this was really the shadow of the planet, I immediately applied myself sedulously to observe it.
Horrocks had about 35 minutes to observe the transit before apparent sunset at 3.50 pm, making three careful measurements at 3.15, 3.35 and 3.45pm. From those measurements he was able to calculate the transit path, angular size, and orbital velocity of Venus. He also derived a value for the solar parallax that was smaller than previously recorded, and so concluded that the Sun was further away from the Earth than previously thought. In 1640 he returned to Toxteth, wrote his "Venus in Sole Visa" and started working towards his next investigation of solar dimensions. However, on the 3rd January 1641 Jeremiah Horrocks died suddenly.
During the Victorian age, the importance of Horrocks' work was finally recognised and a plaque in his memory was placed in Westminster Abbey.