Why observe the transit of Venus?


This is the second post of Suzanne’s mission to Sweden for the Transit of Venus”. The first post is here.

Edmund Halley famously said: “This sight […] is by far the noblest astronomy affords.” Had he had access to modern telescopes and instruments, he might have reserved this accolade for another astronomical vision. After all, seeing a small black dot cross a big bright disk, very slowly, might not sound like much. But anyone can observe the transit, with basic precautions and minimal equipment, and it brings home the mechanics of the solar system in an immediate way, as captured in the photograph of the 2004 transit by Turner-prize winning photographer Wolfgang Tillmans. For someone like me, who spends their entire professional life searching for and studying planetary transits, the symbolic significance of the event is all the more obvious, and I’m looking forward to watching how it inspires the other participants of the expedition, and to helping them observe and understand it.

Venus transit 2004, from Vol. 3: Truth Study Center © Wolfgang Tillmans

But symbolism isn’t all there is to it. Observations of the transit of Venus have been important historically in a number of ways. The duration of the transit was used to measure the scale of the solar system, starting with the first documented observation by Jeremiah Horrocks in 1639, with consecutive refinements up until the beginning of the 20th century, when other more precise techniques were developped. It is also by observing a transit of Venus in 1761 that the Russian astronomer Mikhail Lomonosov first discovered that Venus, like the Earth, had an atmosphere. During the early part of the transit, he noticed a bright ring around the part of Venus that was not yet super-imposed on the Sun, and correctly supposed that it must be due to refraction of the Sun’s rays by the atmosphere. Ironically, though, the first real evidence that Venus had an atmosphere was long held to be the “black-drop effect” – which marred attempts to time the start and end of the 1769 transits precisely. It turns out that the black-drop effect is purely instrumental – it is caused by diffraction and other aberrations in the telescopes used, and completely disappeared in observations of the 2004 transit made with more powerful telescopes. Lomonosovs’ discovery, though, was based on a different effect, and has stood the test of time. This re-interpretation of the black-drop effect is somewhat reminescent of recent (even ongoing) debates surrounding exoplanet observations, where disentangling atmospheric and instrumental effects is often very difficult.

Composite image of the 2004 transit showing light refracted by the atmosphere of Venus © D. Kiselman, Royal Swedish Academy of Sciences

I was finishing my PhD when the last transit occurred in 2004, and I remember anxiously running the transit detection algorithms, which were the mainstay of my thesis, on measurements of the Sun’s total brightness taken by the SoHO satellite in the month around the transit. Would I be able to see it, despite all the other variability caused by sunspots and such like? The answer was yes – but only if I knew where to look first. Nonetheless, more serious analyses of the signature of the 2004 transit in similar data helped establish confidence in the transit-search missions like CoRoT and Kepler, which were then in the late stages of their preparation. Kepler is now discovering terrestrial planets by the bucket-load, and spectrally resolved transit observations are routinely being used to probe the atmospheres of larger exoplanets. The 2012 transit offers an opportunity to test and calibrate this type of observation on a planet whose atmosphere is much better known. Indeed, the Hubble Space telescope itself will observe the transit, although of course it will not point at the Sun directly. Instead, it will point at the Moon, in an attempt to measure the how the spectrum of moonlight – namely sunlight reflected by the Moon – will change during the transit. This will be a difficult measurement – the wavelength dependence of the transit will be very small, as Venus’s atmosphere covers only a very small fraction of the solar disk – but at least there will be no shortage of photons.


Schematic of the HST observation of the transit (S. Aigrain)

The coming month will also be a busy time for exoplanet transits with Hubble, with observations scheduled on June 8th and June 10th as part of our large programme on transit spectra for hot Jupiters in the visible.


In my next post I will describe my own plans for observing the transit – more Heath Robinson than state-of-the-art, I’m afraid…




About Author

I work on the detection of extrasolar planets (planets outside the solar system) via the transit method, and on the exploration of the time domain in astrophysics in general. I am particularly interested in finding and studying small (terrestrial, or even Earth-like) planets, and young planets (to understand how they form, and how their early evolution is influenced by their environment).