By Mike Glazier
A short background article in readiness for the June 5-6 2012 transit of the planet Venus.
Jeremiah Horrocks was the first recorded person known to have observed a transit of Venus across the face of the Sun. He was born at Toxteth park Liverpool somewhere around 1619, then a leafy sparsely populated part ofEngland, a far cry from the sprawling built up area we know today. A young man of slight build, he was on a mission. His ambition was to study astronomy and whilst doing so had meticulously studied the works of Tycho Brahe and Johannnes Kepler, the man who had gone into the annals of history by publishing his laws of planetary motion, the man who also had become Horrocks’ literary and scientific mentor.
Horrocks had won a place at Emmanuel collegeCambridgeat the very early age of 14 and studied amongst other subjects, maths, Greek and Latin, two languages he reportedly spoke fluently. He was also and more importantly, fortunate enough to have been born just a few short years after the invention of the refractor telescope for astronomical purposes by both Galileo 1609 and Thomas Harriot a year earlier , and it was a replica of this very instrument which enabled him to become the first person to observe and record a transit of Venus across the face of the sun.
To get a flavour of the times in which Horrocks lived it is important to remember that in those far off days people more were more readily disposed to accept witchcraft, devils and magic than they were to try and understand the kind of astronomy that had filtered through to them . The ‘wheeling’ motion of the night sky was and remained a complete mystery to all but a few educated members of society.
To give a prime example of these attitudes, it has been estimated that over 1000 women were hanged as witches in England during those times and indeed it was Henry VIII ( he outlawed witches) and the Witch Finder general who were directly responsible for this terrible state of affairs. Women were mostly accused of witchcraft since they were regarded as being the most susceptible to demons. The small fenland villageofWarboysin Cambridgeshire is still home to a hanging post and a ducking pond where the ducking stool was used to drown women who were accused of being a witch.
InScotlandhowever it has been estimated the situation was worse in that over 4000 people, mainly women were burned alive at the stake. It is also a fact that at the time of Bruno (1600) who was also burned at the stake for stating his belief that the Earth went around the Sun, people believed the biblical explanation of the way the Earth and the universe came into being, was the way it actually happened. Heresy was an unforgiveable sin punishable by death!
Horrocks’ family were religious and followed the protestant faith. Horrocks himself was a church lay reader and after leavingCambridgehe went to live at Carr House in Much Hoole nearPreston. He had a number of important daily duties to perform and these included taking the family’s children to school during the week and accompanying them to Sunday bible reading classes at the local St, Michael’s church which was situated a short distance away at the edge of the village.
It was while he was living in Much Hoole that he realized after doing his own calculations that the periods of Kepler’s predictions of Venusian transits were in error; Kepler had correctly predicted a transit in 1631 but Horrocks’ own predictions showed that rather than occurring singly they did in effect pair together by a period of 8 years. Only after that would a transit occur many years later, in fact either 105 or 121 years later. He did other work and used Kepler’s laws of planetary motion to prove the moon’s orbit is not circular but very slightly elliptical, somethingNewtonhimself gave him credit for. He was also able to roughly calculate the sizes of the bodies in the solar system and in doing so found the sun to be gigantic.
He didn’t have a massive amount of astronomical material to work with because of course Uranus wasn’t discovered until 1781,Neptunewas unknown until 1846 and the now debunked planet Pluto wasn’t found by Clyde Tombaugh until 1930.
However continuing on with his own careful calculations, he came to realize that a transit of Venus was due on Sunday 24th of November 1639 and that by an amazing stroke of luck it was going to be observable fromEurope . He wrote to his friend an astronomer, mathematician and merchant William Crabtree (1610-1644) advising him not to miss the historical and once in a lifetime event which was about to pass. Whether or not Crabtree took heed of the advice or indeed even received the letter is open to conjecture, what is known is the transit took place on the very same day as Horrocks was attending church for biblical reading classes with the family’s children and, as he later discovered began just a mere 35 minutes before sunset . Before leaving the house to attend to his charges he had set up what he called a modest ‘half a crown telescope’ by the living room window with the instrument ready to focus any sort of an image onto a readily prepared 6” solar circle mounted on a thick piece of board.
During the day Horrocks had watched with great concern the state of the sky. The sun at times was heavily obscured by cloud and at times he must have wondered if by some divine misfortune, he was to miss the once in a lifetime event. Later that afternoon and just as soon as the church service was finished he rushed at great speed from the church with his cape billowing out behind him, his right hand holding frantically onto his hat, and the children following behind towards the house. Then after bursting into the heavy oak front door of the house and making a few small adjustments to his instrument he scrutinized the white projected disc for anything that would have resembled a small black dot.
He later recorded his endeavours ” I watched carefully on the day of the 24th from sunrise to nine o’clock and from a little before ten ‘till 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 had evidently nothing to do with Venus. About 15 minutes past three in the afternoon when I was again at liberty to continue my labours, the clouds as if by some divine interposition were dispersed and I was once again invited to the grateful task of repeating my observations. Later I 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 to 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 Venus I immediately applied myself sedulously to observe it”.
He was able to make three extremely important estimated scientific measurements, Venus’ transit path, its angular size and its orbital velocity, something which had never been done before.
Most of his other work such as that concerning the Moon had been carried by the use of a home made ‘astronomical radius’ a combination of two pieces of ‘marked ’ wood which moved independently to each other for measuring purposes and which whilst being used caused a degree of attention from the children who watched from their bedroom window with great amusement. But it was with this piece of equipment that he cleverly established the apogee and perigee differentials of the moon’s apparent size thus establishing the fact the orbit of our satellite was not constant. Combined with this ingenious piece of carpentry he made another ‘measuring stick’ with which to measure the angular diameter of Venus .He managed to predict by other means of observations the positions of the planets , in particular that of Venus and over a 4 years period predicted the 1639 transit date.
Unfortunately he died at a very early age, in fact on his way to collect some money owed to him by a friend. He was only approximately 24 years old. Had he lived to fulfil his dreams of astronomer-hood, his name would have ranked high amongst those such as Newton.
However the story doesn’t end there because more refined measurements were needed and by more than just one observer to determine the dimensions , distances and ultimately the scale of our solar system, and that of course included the sun.
We follow on with that famous voyage toTahiti otherwise known as Venus Point by that famous mariner Captain James Cook. He was chosen for his experience, not only for his sailing history but also for the number of solar eclipses he had recorded.