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DISPLAYING POSTS TAGGED: transit of venus (2)

Seeing the Transit

Author
by Tanya
Publish date
29 June 2012
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It may just have been a little dot – but what a dot it was!

Breakfast with Venus was held at the Melbourne Planetarium as a special event for the Transit of Venus, selling out in just three weeks.

We were treated to a glorious morning, which was a great surprise as the days leading up to the event were dreadful, with constant cloud and rain.

Visitors watched Venus move onto the Sun via a live feed from Mauna Loa in Hawaii, made possible through a partnership with the Exploratorium, San Francisco. It was incredible to have a room full of silent people in our planetarium foyer, just waiting for the moment to see Venus' dark shadow appear. And it was just so brilliant when it did!

We then moved out to the Scienceworks arena where five telescopes were set up, including one projecting a large screen image. Everyone was able to see the moment again, but this time directly for themselves. We all had our eclipse glasses too and we were surprised at how easy it was to see Venus through them.

After getting our fill of Venus and some light breakfast, we headed into the Planetarium for a presentation describing the geometry of the transits – particularly why they come in pairs before having to wait over a century for the next one – followed by the highs and lows of previous transit expeditions.

Path of Venus across the Sun Transits of Venus come in pairs, one either side of the "sweet spot" where Venus' orbit crosses the ecliptic plane. By 2020, when Venus and the Sun are lined up again, Venus will fall short and miss the Sun.
Source: Museum Victoria
 

The human stories drew much collective laughter and sighs from our audience. Over the centuries astronomers have dedicated years of their lives to see this event. None more so than Jean-Baptiste Chappe d'Auteroche who saw the Transit of 1769 from a Spanish Mission in Baja California (what is now part of Mexico) but then passed away six weeks later as an epidemic spread through the area.

In the lead up to the 2012 transit, Guillaume Le Gentil became a bit of a 'poster boy' for the event. He was the one who saw a brilliant transit in 1761, but because he was stuck out at sea, he wasn't able to make any meaningful measurements. He managed to set up an observatory in India for the 1769 transit ...

"only to be the spectator of a fatal cloud which came to place itself before the Sun at the precise moment of my observation, to carry off from me the fruits of my pains and fatigues."

Fortunately for our transit, we were able to continue viewing the event throughout the day. A few hundred people saw Venus, with many commenting that they had taken time off work or kept children home from school to do so. I joined in too, and two of my sons were able to get out of school for a short while to share the moment with their mum.

Tanya with her sons A happy astronomer shares the Transit of Venus with her sons.
Source: Tanya Hill
 

I was amazed by the dedication of our visitors who were happy to wait for just another clear patch of sky so they could catch one more glimpse of Venus. And my final thought – what will the world be like when Venus next meets up with the Sun, in that far off December of 2117?

Transit of Venus

Author
by Tanya
Publish date
29 May 2012
Comments
Comments (2)

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.

 

Transit from space 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.  

2004 Transit Venus transiting the Sun in 2004
Image: Hugh Gemmell
Source: Hugh Gemmell

Links:

Transit of Venus

Transit of Venus app for iPhone and Android

Transit of Venus: 1631 to the present by Dr Nick Lomb, published by Sydney Observatory.

Science in the South Seas exhibition at the National Museum of Australia

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Updates on what's happening at Melbourne Museum, the Immigration Museum, Scienceworks, the Royal Exhibition Building, and beyond.

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