On Wednesday 6th June, we have the chance to witness a rare astronomical event - a Transit of Venus. The Earth, Venus and the Sun will fall into line and we will see (with the appropriate equipment) Venus as a small black dot moving across the bright yellow Sun. The first Transit observed was in 1639, and there have only been five since, in the years 1761, 1769, 1874, 1882 and 2004.
"I recommend it therefore again and again to those curious astronomers who (when I am dead) will have an opportunity of observing these things, that they would remember this my admonition, and diligently apply themselves with all their might in making this observation, and I earnestly wish them all imaginable success …."
Edmund Halley, the astronomer made famous by Halley's comet, wrote those words in 1716. He was sixty years old at the time and was well aware that he would not live to see a Transit in his lifetime. But he had discovered that this rare event would unlock the scale of the Solar System and so he urged future astronomers to make good use of his findings and wished them " immortal fame and glory."
You see, back then we knew the relative distances of the planets – Mercury is almost 3 times closer to the Sun than Earth, Saturn is 10 times more distant – but we didn’t know their true distances. The key was the Earth-Sun distance, astronomers call it the Astronomical Unit, and Halley had realised that this could be measured during a Transit of Venus.
Observations of the transit from different locations across the world would differ slightly – some would see Venus travel a short path, moving onto the Sun later and leaving earlier than would be seen elsewhere. By timing the planet's journey and adding in some trigonometry (the mathematics of triangles) the Earth-Sun distance could be measured and everything else would fall into place.
The path of Venus across the Sun varies slightly when viewed from different locations on Earth. Image is not to scale.
Source: Museum Victoria
Astronomers of the 18th Century took up Halley's call but the world was a much bigger place back then. The southern hemisphere was largely unexplored – Captain Cook observed the 1769 transit from Tahiti then went on to undertake the historic mapping of Australia’s east coast.
And no one can forget the tenacious efforts of the French astronomer Guillaume Le Gentil. The Seven Years War was underway and after more than a year of traveling across treacherous seas Le Gentil was unable to reach land in India because it had been taken over by the enemy, in this case the British.
In fact, Le Gentil’s story is heartbreaking. He had brilliant viewing conditions for the 1761 transit, but because he was stuck out at sea with no means of determining his location (ie. longitude) nor an accurate clock for timing the event, his observations didn't mean a thing. What he would have given for a smartphone with GPS!
Amazingly, Le Gentil decided not to go home but to wait out the next 8 years for another Transit. He built an observatory, survived a severe illness, and was fully prepared for the day, only to be beaten by the weather. When he did return to Paris eleven years later, he had been presumed dead – his wife had remarried, his estate was gone and he’d lost his seat at the Royal Academy of Science. Not exactly the fame that Halley had imagined.
Those early astronomers by solving the scale of the Solar System, were also helping us to understand the Sun. By knowing its distance, we could confirm the Sun's size, mass and intrinsic brightness. What’s more, they were also setting us up to determine the extent of the entire Universe. The Earth-Sun distance is the baseline for measuring the distances to nearby stars. A series of stepping stones then takes us distance hopping across the Universe – all the way from star clusters to galaxies near and far.
So next month, when we have the chance to witness the last Transit of Venus for this century, I urge you to heed Halley’s words. It may not be a glitzy show but it’s our connection to both the Universe around us and a piece of our history. And just like those astronomers of the past, we can take a moment to wonder what the world will be like by the time the next Transit rolls around for that far-off December in 2117.
UPDATE: Scienceworks' special Breakfast with Venus from 8am to 10am on Wednesday 6th June is now sold out.
Venus transiting the Sun in 2004
Image: Hugh Gemmell
Source: Hugh Gemmell
Transit of Venus