Categories: Astronomy

Journal Club – Transit of Venus


According to Wikipedia, a journal club is a group of individuals who meet regularly to critically evaluate recent articles in scientific literature. Being Universe Today if we occasionally stray into critically evaluating each other’s critical evaluations, that’s OK too. And of course, the first rule of Journal Club is… don’t talk about Journal Club.

So, without further ado – today’s journal article under the microscope is about the 2012 transit of Venus.

Today’s article:
Sigismondi Solar diameter with 2012 Venus transit.

The 2012 transit of Venus will proceed for nearly 7 hours over 5 and 6 June (UTC). It’s not likely that we are going to squeeze a huge amount of ground-breaking science out of this event, which was closely monitored by 21st century technology the last time it happened in 2004. But Sigismondi argues that a more exacting observation of this transit should enable us to clean up some of the historical data from previous transits by kind of reverse-engineering some of the inherent inaccuracies that plagued earlier measurements.

The point of such an exercise may become clear by considering a claim made back in 1979 that the Sun was shrinking – based on an analysis of 120 years of Greenwich Observatory solar measurement data. Apparently this finding has since been hijacked to support a young Earth hypothesis – as in if the Sun is shrinking so fast, then how can it possibly be billions or even millions of years old and yada, yada.

Shapiro was able to quickly counter the shrinking data finding in a 1980 publication (in Science), demonstrating that transits of Mercury data, going back to 1736, indicated that the solar diameter had remained constant to within 0.3 arcseconds. This was then followed up by Parkinson et al, also in a 1980 publication (in Nature), demonstrating that changes in the Greenwich solar data correlated closely with changes in instrumentation, atmospheric conditions and in the people taking the measurements (and thanks to Matt Tiscareno for this story).

Anyhow, Sigismondi outlines how the solar diameter can be measured from the transit of Venus’ outline when it contacts each edge of the Sun – and then discusses a method whereby the ‘notorious’ black drop effect can be eliminated. The black drop effect involves the black shape of Venus seemingly to elongate as it approaches the edge of the Sun – which had confounded all measurements taken prior to 2004.

Transits of Venus generally happen in pairs separated by 8 years, with either 105.5 or 121.5 years separating the last of the pair and the first of the next. Apparently Kepler was the first person to predict a transit of Venus in 1631 – but he failed to predict that it would not be visible from Europe. So it fell to Jeremiah Horrocks and William Crabtree to make the first scientific observation of a transit 8 years later in December 1639. The next two were in June 1761 and June 1769, the latter famously observed from Tahiti by Lieutenant James T Cook (OK, kidding about the T) and then there were two more in December 1874 and December 1882.

Then another 121.5 years passed until June 2004 – now to be followed by this year’s June 2012 transit, being the 7th ever scientifically recorded transit. And BTW here’s an original drawing by James Cook of the June 1769 transit, showing the black drop effect.

So… comments? Is it OK to get a little bit excited about ‘just another’ transit of Venus – since it’s only the 7th we have ever recorded data about? Did you know that the plural of ephemeris (the position of something in the sky) is ephemerides? Want to suggest an article for the next edition of Journal Club?

Steve Nerlich

Steve Nerlich is a very amateur Australian astronomer, publisher of the Cheap Astronomy website and the weekly Cheap Astronomy Podcasts and one of the team of volunteer explainers at Canberra Deep Space Communications Complex - part of NASA's Deep Space Network.

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