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On Wednesday 6th June, 2012 in Eastern Australia and New Zealand (as well Pacific islands across to Alaska) Venus was seen to pass between the Earth and the Sun; appearing as a small circular spot crossing the sun’s disc; for around six and a half hours.

This is a very rare astronomical event that has been the cause of great change to our world.

This is not because, as the astrologers would have it, that human events are governed or predicted by the disposition of the stars or planets.  It is because the event has served to significantly advance scientific knowledge and our understanding of the Universe.

We know in advance when the Transit will happen and where it will be visible from (unless it is cloudy) because we now understand that the Earth, and other planets, orbit the Sun; and how these orbits lie in relation to each other.  This was the view of heavens famously proposed by Copernicus at the beginning of the Renaissance but his insight was then refined and given verifiability by Galileo, Kepler, and Newton. 

Venus passes between the Earth and the Sun only very occasionally because Venus and Earth do not orbit in exactly the same plane.  Instead of catching up at regular intervals (of 8 years) as you might imagine as two planets revolve in the same direction at different speeds; long gaps of 121.5 years and 105.5 years separate each 8 year pair of transits.

This misalignment would have appalled Copernicus, a monk, who firmly believed the heavens to be perfect and sublime, the realm of God, in contrast to the imperfect earth; sullied by original sin. It was the implication of heavenly imperfection that brought Galileo before the Inquisition.

Many intelligent people doubted the Copernican model.  The principal argument against a heliocentric model of the solar system was that the Earth would need to be speeding through space and we would be thrown off; whereas if it was stationary at the centre of the Universe the apparently tiny planets, Moon and similar sized Sun could easily revolve around the Earth; as it seems obvious they do to the naïve observer.  

Arguments that the sun was a long way away would mean that it was ridiculously big; and it would be unlikely, or impossible, to be orbiting the Earth that fast; giving support to the view that the earth orbits the sun.

Soon after Kepler published his laws astronomers realised that the transit of Venus could be used to discover how far away the Sun is and, correspondingly, how big it is. 



Kepler's Laws Diagram from Wikipedia

  1. The orbit of every planet is an ellipse with the Sun at one of the two foci.
  2. A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.
  3. The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit. 

Illustration of Kepler's laws from Wikipedia (link)


Just as your eyes determine distance using the stereoscopic (parallax) effect, two different observers on different latitudes will see Venus cross the sun on different lines.  And because its disc is circular these two paths can be timed to determine their relative length.  Using Kepler’s laws, simple observation of the time Venus takes so orbit the Sun tells us the ratio of Venus’s orbit to that of the Earth.  We can then use the transit data from two observers a known distance apart to determine the distance value (size) of those ratios; hence the distance from us of both Venus and the Sun.



Surpassing the Ancients

This was an observation and calculation impossible before Kepler used the data of Tycho Brahe to formulate his laws.  But just twenty years after the last law was published Jeremiah Horrocks was able use the 1639 transit to roughly calculate that the Sun is at a very great distance and the Solar System is immensely large.  In doing so he also demonstrated that human understanding of the Universe had at last convincingly surpassed that of the ancients.  We could no longer rely on the Greeks, Romans, Egyptians, or indeed Hebrews, Indians, or Chinese, for our knowledge of the world.  We were now 'out front'; and on our own.

At each successive transit scientific knowledge, ability and clocks improved and the observations gained in accuracy; showing that the earliest observations had been inaccurate and thus significantly underestimated the true distance. This distance is now known as an Astronomical Unit and is known with great accuracy (1 AU = 149,597,870.70 km). 

It became correspondingly obvious that the Sun is huge; not the about same size as the moon as the ancients had thought.  We now know it to be about 110 times the diameter and 332,950 times the mass of the Earth.

Thus each transit put one nail after another into the model proposed by the Greek/Egyptian mathematician and astrologer Ptolemy of Alexandria and absorbed as a matter of faith into the philosophically sophisticated arguments of the medieval theologians.  As a consequence the Transit became a driving force of the enlightenment and the scientific revolution.  In the 18th Century observing the Transit was judged to be of singular scientific importance; justifying significant planning and expense; on a contemporary scale similar today’s space missions; giant telescopes; and particle colliders.

In 1769 there would be another transit visible in the Pacific and scientific teams from several countries were dispatched to different latitudes in a time zone (longitude) that would allow the observation to take place throughout the day.  The Royal Society sent a young lieutenant together with a team of scientists and technicians in a converted coal ship (His Majesty's Bark the Endeavour) to set up an observatory in Tahiti.

Having successfully completed the observation James Cook and Joseph Banks then sailed West in search of the eastern shore of ‘Terra Australis Incognita’ (the unknown southern land).  They first encountered New Zealand, which they circumnavigated and mapped.  They then continued west reaching the east coast of Australia.  Sailing north along the coast they then discovered the Great Barrier Reef, by running into it, before returning to England.

Banks was an independently wealthy young gentleman and scientist who went on to great influence as President of the Royal Society during its 'golden age'.  It was he who was influential in the choice of Botany Bay as a British settlement and penal colony.  Cook went on to become Britain’s most famous naval explorer of the age.

Thus the transit was directly responsible for the timing and nature of the European settlement of Australia.

The transit observations greatly enhanced astronomical science and it was soon realised that the Sun is but one of the visible stars; many with their own system of orbiting planets.  And indeed the Earth does orbit the Sun at a great speed.  We now know this to fluctuate around 107,280 km/hr.

Astronomers soon went on to realise that our sun is but one of billions in our galaxy and that it is, in turn, just one of billions of galaxies.  The Sun too travels at great speed but not around the Earth.  It orbits the galactic centre around once every 240 million years (one galactic year); at a speed of 828,000 km/h.  When we are orbiting in the direction of the Sun’s orbit we are travelling through space at close to a million km/hr.

This is a long way from being stationary; and we are nowhere near the centre of the Universe.  

In the interim we have also realised that universal time is on a scale that dwarfs human existence.  Our earth is around 19 galactic years old but we modern humans have existed on it for just over a galactic hour and will certainly disappear again, as do all species, in perhaps another hour; or even a day if we are very very lucky.

This effectively put an end to the anthropocentric myth that mankind is the object of creation or that we collectively, or individually, hold a special place in the considerations of the creator.



Speaking French

In Australia it is popularly said that had the first fleet not arrived with exactly the timing dictated by the Transit of Venus we would all be speaking French. 

A few days after the first fleet arrived in Botany Bay in 1788 two French Ships appeared, commanded by Jean François de Galaup, comte de Lapérouse; known to the English as La Perouse. 

His mission was scientific, to explore and map new territories, as Cook’s had been. 

There was no attempt to claim Australia or any part of it.  The French were greeted politely but kept at some distance.  They had been attacked by natives in Samoa with a number of casualties and the loss of a longboat. They were allowed to build an observatory, a stockade and plant a garden on Lapérouse peninsula in Botany Bay, where they stayed for several months making repairs, building a new boat and recuperating.  In March they sailed for New Caledonia and were never seen again.  Fortunately their scientific journals and logs, to that point, were left to be dispatched to Paris on the ship returning to England, the Sirius.  This disappearance remains a mystery that three separate expeditions have unsuccessfully attempted to solve.  Sirius was herself lost, not mysteriously, some years later.  Her anchor was recovered and has pride of place in Macquarie Place in Sydney.

A second incident occurred in 1802, three years after the French Revolution and just before the Napoleonic wars, when the French were seen as more of a threat.  Prior to circumnavigating the continent Mathew Flinders had been charged with mapping the southern Australian Coastline and on 8 April he sighted the Géographe, a French corvette commanded by the explorer Nicolas Baudin.  Flinders and Baudin met in an inlet and exchanged discoveries, Flinders named the place Encounter Bay.  But there was subsequent speculation that Baudin had been surveying on behalf of the French Government with sinister intent.

Of course speculations on 'what might have been' are meaningless.  Had not the events around the Transit of Venus occurred just as they did you and I would not be here to speculate; we would never have been born.  To learn why follow this link. Possibly some French speakers would inhabit Australia but they would be quite different French speakers to any on the planet today. 

So it is true that a heavenly event, the Transit of Venus, is responsible for your, and my, very existence.



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