Last Wednesday, the 6th June, despite cloudy weather, Sydney was witness to one of the rarest astronomical events-The Transit of Venus. The transit occurs eight years apart, every 105 years or so, so the next is 2117.

Although Venus passes between the sun and earth five times every eight years, it needs to intersect earth’s orbit within two days of earth intersecting Venus’s orbital plane for us to see the transit.  Since it was first predicted over 400 years ago, there have only been eight, of which only seven have been witnessed.  So it’s rare.

Wanting to be a part of a great astronomical event, I travelled 1 ½ hours north of Sydney to Koolang Observatory in the hope of witnessing it.  The drive up the F3 did not look promising; clouds dominated the sky with little sign of the sun.

But enough about me, what does it have to do with Sydney’s history?  Well, for one thing if it wasn’t for the transit, Captain Cook would never have been in the neighbourhood in 1770 to sight and land at Botany Bay and so set in motion the English plans for a new colony.

The transit was big news in the 18th and 19th century.  The first predicted transit to happen after the advent of the telescope occurred in 1631, but no-one saw it (at least no-one recorded seeing it).  The second, in 1639, is viewed by two people (astronomers Jeremiah Horrocks and William Crabtree) in England.

The point of observing the transit is that through the observations made it was hoped to be able to calculate the size of the known universe and earths place in it.  By using trigonometry, astronomers could triangulate to calculate the distance of Earth from Venus and then deduce Earth to Sun distance.  With this measurement, or Astronomical unit (the basic measure for astronomy) they could turn the other way and begin measuring the universe.

But to get accurate readings, you need lots of comparative points.

The first opportunity to do this, on a large scale after astronomers had worked out it was going to happen, was 1761.  Sadly the expeditions planed were hindered by wars, monsoons, dysentery and all manner of other 18th century mishaps and so few got good readings.

So for the 1769 transit governments pull out all stops.  151 official observers are sent off in ships all over the known and unknown world.

One of these is the HM Bark Endeavour, Captain Cook in charge.  Cook is dispatched to Tahiti and had perfect weather to observe the event.  Once completed he set out on the next part of the mission, to search for the great southern land, which he did, sighting the east coast of NSW in April 1770.  And so began Sydney’s European history.

But the transit wasn’t finished with Sydney.

The next transit occurred in 1874.

By this time the Sydney and its Observatory were well established.  The Government Astronomer in charge was Henry Chamberlain Russell who had reorganised the observatory and already sent a number of scientific missions out to make observations of such things as the 1871 total eclipse of the sun.

Powerhouse Museum Collection
One of Russell’s temporary observatories at Woodford in the Blue Mountains

Russell was also able to use new technology-photography.  Russell had been preparing for the transit since 1871, adapting telescopes to handle cameras and modifying others to act as cameras.  To better his chances of success Russell set up temporary observatories at Goulburn, Woodford, Eden and Bathurst, sending teams and camera equipment to each.

In Sydney, Russell took delivery of a new Equatorial telescope made in Hamburg, Germany with a 11.4 inch aperture and a focal length of 12 feet 6 inches that could magnify the sun’s image to four inches across.

Russell and his 1874 crew-the transit team

It was this telescope, which is still on display at the Sydney Observatory, that he fitted his best photographic equipment too, with the end passing into a dark room in the dome.

With the sky nice and clear, Russell captured 180 photographs of the Transit (with well over 1300 from his other temporary sites), with some showing what appeared to be a halo around Venus in the early stages of the event.  Russell described seeing the halo in his report as follows:

It [the halo] was very remarkable and beautiful, like a fringe of green light, through which the faintest tinge of red could be seen; it was brightest near the planet, and seemed to shade off to nothing.

Russell had captured the first images that confirmed Venus had an atmosphere.  His photos, presented to the Royal Astronomical Society in London, were considered the best and most complete collection and contributed vital information to the final analysis.

The halo effect as recorded by Russell’s assistant in 1874

Russell’s work had put Sydney and its observatory on the world stage.

As for me, at about 1.23pm, the clouds over Koolang cleared for a few minutes, and I too saw Venus transit in front of the sun.  I was lucky enough with the weather to have a make a few observations and saw its last contact, the moment it slips off the face of the sun, not to return for us for another 105 years.

PS: if you missed it on Fbi you can hear it streamed through Soundcloud here

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