The Transit of Venus
What is it that makes an event in nature special? Is it the rarity of its occurrence, the immensity and sheer wonder of the spectacle, or possibly the effect it has on mankind? There are arguments for all these criteria and there is also personal significance. There will be a Transit of Venus on June 6th this year and while it is
not a visual spectacle that excites the imagination of the general populace the way a Total Solar Eclipse does, it is an event that holds great significance for us all.
Firstly, it is a truly rare event. Transits of Venus occur in pairs eight years apart but then do not recur for 105.5 years and on the next cycle 121.5 years. Total Solar Eclipses occur at the rate of three every two years. Few people alive to see this transit will see another. Transits of Venus were unknown until Johannes Kepler, the great planetary mathematician, predicted an occurrence on December 6th, 1631. Venus appears as a small round black dot when crossing the face of the Sun, invisible to the naked eye. As Galileo had turned the newly invented telescope on the heavens for the first time in 1609 it had never been observed before. Due to a small error in parallax mathematics Kepler did not predict the second of the transit pair occurring eight years later but it was independently predicted and observed by English amateur Jeremiah Horrocks on 4 December 1639. As he was not well known and did not publicise his calculations it is believed he and his friend were the only two people on the planet to see this one.
Right – The Author’s photo of the Jun 8, 2004 Transit
In the eighteenth century, advances in telescope design and instrumentation continued at breakneck speed. Planetary positions and orbits were plotted more accurately and predictions could be mathematically refined. Around this time we see the great nations of Europe exploring the world by sea for trade riches and power. The Spanish, French, English, Dutch and Portuguese had been sending explorers to all corners of the globe for a hundred years. Enormous riches and influence, not to mention new territories to claim, were the spoils for the adventurous, however many of these explorers and crew perished because they became lost. Unable to accurately ascertain their longitude they could miss their target by many miles or hundreds of miles, running out of water and food or becoming becalmed in unknown waters. It was easy to find your latitude, simply measure the angle of the Sun above your horizon at local noon but to find your longitude was more difficult. The story of the search for the best method is brilliantly told by Dava Sobel in her book Longitude. If a navigator could know accurately the time at a given point on the globe, say Greenwich or Paris, he could compare it to noon at the meridian of his location.
Every hour difference was equal to 15 degrees of longitude. Many solutions were proposed but ultimately those who had accurate timepieces, unaffected by temperature, barometric pressure or the motion of a ship, would be the most astute navigators. The chronometer and its use for accurately plotting one’s position was obviously a momentous advance for mankind but what has it got to do with the Transit of Venus? As you will see both our own history and the development of astronomy are connected by this event.
After observing a transit of Mercury in 1677, the brilliant English astronomer, Sir Edmund Halley, proposed that the next transit of Venus could be used to determine the distance of Venus from the Sun, and by simple trigonometry the distance from Earth to the Sun.
Why was this important to astronomy?
Astronomers had noticed that some stars, when measured at different times of the year, appeared to move slightly against the background stars. They had known of this effect for quite some time as the superior planets (outside Earth’s orbit) would appear to move in reverse against the stellar background for a short period when Earth went past them in its own orbit. This effect was known as parallax. See the diagram below.
By checking a stars position at six monthly intervals an astronomer would be measuring the baseline of a triangle the diameter of Earth’s orbit or twice the length of the Earth – Sun distance. Knowing that distance accurately meant the parallax distance to some stars and perhaps measurement of the scale of the visible universe would be within their grasp.
Above: The transit appears differently from separate locations on Earth – (courtesy of Exploratorium TV)
Halley proposed to use a smaller measure of parallax to find this Venus-Sun/ Earth-Sun distance. He posited that two observers timing the Transit from distant locations on Earth could create a long enough baseline using parallax measures to create a heoretical angle from Earth through Venus to the Sun. Johannes Kepler, who formulated the three Laws of Planetary Motion, had also calculated, with his third Law, the ratio of a planets distance from the Sun compared to the time taken for its orbit. Kepler had proven that the ratio of Venus’ distance to the Sun compared to Earth was 0.72. By multiplying the apparent Venus /Sun angle by 0.72 we arrive at the Earth/Sun angle. Let’s not worry about the actual equation here but remember we have now worked out the angle and the length of one side of our triangle (the distance between our observers on Earth) and we can use our High School maths (remember Sine, Cos and Tan?) to calculate the distance accurately. This method is the same as used by all surveyors to determine distance with a theodolite.
Diagram of the Timing of a Transit – (courtesy of Quasar Publishing)
Of course, unless the observers in different parts of the world knew their positions accurately they could not determine the distance between each other. Most of the advanced seafaring powers and their respective scientific societies equipped and commissioned Transit expeditions to all corners of the globe. This is where our recent history and the event of the Transit
Venus black drop effect
intersect. For the Transit of 1769 the English Navy purchased a coal carrying ship called the Earl of Pembroke , and joined with the Royal Society in commissioning a Research survey to Tahiti and the South Pacific. As it was nominally a Naval vessel it could only be commanded by a ranking Naval officer so James Cook, chosen for his navigational skills and his surveying prowess,
was duly promoted to Lieutenant and commissioned to observe the Transit of Venus from Tahiti on June 3, 1769. As the Commander of the ship he was entitled to be called Captain. From Tahiti Cook was to continue to New Zealand to observe a Transit of Mercury on 9 November and map the North and South islands while there. Here he was to open his sealed orders that instructed him to continue west where he discovered and mapped the east coast of Australia. On this first tour of duty Cook navigated with the use of accurate Moon and star charts supplied by the Royal Observatory at Greenwich, using a sextant to calculate angles and lunar distance. On his second voyage of discovery in 1771 he had the benefit of a replica of Harrison’s famous
chronometer to calculate his longitude, but no Transit to time on this occasion. This chronometer cost £400 or approximately £59,000 in today’s currency.
When all the timing, positional, and angular measures were calculated the results from the earlier Transit of 1761 and those of 1769 gave a figure for the Earth/Sun distance that only varied from our modern measure by about 3%. Inaccuracies crept in because of poor seeing, poor timing, poor calculation of locations, and an anomalous effect of the Transit known as the Black Drop that made it difficult to see the exact second the planet entered or exited the disc of the Sun. Around a century later in 1874 and 1882, the research was conducted primarily with the new technique of photography but was again slightly marred by the discovery that Venus had an atmosphere which generated further inaccuracies in timings.
So, is the event rare? It certainly is! Is it an immense and wondrous spectacle? Perhaps only when you consider the implications. Does it have a profound effect on mankind? The event itself, perhaps not, but the observation generated an enormous tide of activity in the affairs of men. It pressured advances in technology, navigation, timekeeping, measurement and the science of Astronomy. Is it personally significant? To us, who will not see another, it may be. If you have an inkling of the history it must affect you. Personally, it is one of many aspects of the Universe above our Earth that astonish and compel me.