Jeremiah Horrocks and the transit of Venus

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.

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