Journal Club – Transit of Venus

Article written: 28 Jan , 2012
Updated: 24 Dec , 2015


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?

18 Responses

  1. Anonymous says

    Is the black drop effect due to atmospheric effect? If it is the Solar Dynamic Observatory should be able to observe this quite well without that optical effect.


  2. Black drop effect is from point-spread function and limb darkening. See TRACE results and other black drop details at

    • Anonymous says

      Thanks for that.


      • Member
        Steve Nerlich says

        I think it’s a bit about atmospheric effect anyhow. To reverse engineer past inaccuracies there is sense in doing it from inside the atmosphere again.

    • Anonymous says

      Okay – maybe no ‘black drop’, then. While we are myth busting Venus, how about the ‘ashen light’?

      • Anonymous says

        In thinking about this my initial conjecture is that this light is due to all the starlight reflected off the top of the Venusian atmosphere.


      • Member
        IVAN3MAN_AT_LARGE says

        Er… that might be the case if the Solar System was within a globular cluster, but I think that the starlight in our ‘neck of the woods’ is too faint to be reflected off the Venusian atmosphere to any noticeable effect.

        According to the authors of this (PDF) paper, the phenomenon of ashen light is probably due to lightning in Venus’ atmosphere; this hypothesis is the consensus among astronomers.

      • Anonymous says

        I looked this up on Wiki-p and found this as well.


      • Member
        Steve Nerlich says

        If we are still on black drop here – I understand it has nothing to do with Venus’ atmosphere – as you get much the same effect with Mercury (which has virtually no atmosphere). The problem involves blurring by Earth’s atmosphere.

      • Anonymous says

        I find it a lot easier to explain the ‘ashen light’ as an effect within the eye-brain system. If the arc of light scattered from the backlit atmosphere is soft on one edge and hard on the other then we will get an apparent contrast from the Cornsweet illusion. A decent set of photometric measurements from TRACE should settle this.

  3. Gadi Eidelheit says

    I am very interested in the transit, It is one of the rarest events, but Since only the end of it will be visible from my location I will just enjoy the view

  4. Anonymous says

    Cook’s expedition is a story in itself and akin to going to the moon with a 40% loss of crew, NASA has a good article

  5. ITSRUF says

    What does “critically evaluat[ing] recent articles in scientific literature” mean? Is this useful in some way?

    Also, what does the photo intend to convey?

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