On 8 June 2004, Venus crossed in front of the face of the Sun for the first time in living memory. This was truly an impressive spectacle to behold, but despite its rarity, another transit will occur on 5/6 June 2012.
The reason is that Transits of Venus occur in pairs separated by eight years, but there is a gap of over 100 years between each pair. This means that after the 2012 transit, the next one won’t be until 2117.
The transit of Venus is more than just a pretty sight; it allows some important science to be performed. Historically it allowed the distance to the Sun to be determined. This was done using a technique called parallax, where multiple observers at different locations on Earth used trigonometry to calculate the Sun-Earth distance. Measuring the distance was first attempted during the 1639 transit and the measurement was refined in later centuries.
Nowadays astronomers are using the Venus transit to make important measurements of the planet’s thick atmosphere. As we see Venus touch the edge of the Sun’s disc during a transit, a glowing arc known as the aureole is produced. First observed in 1761, this arc of light is caused by the Sun’s light refracting through Venus’ atmosphere, and it can be used to determine the density and temperature of the atmosphere depending on the brightness and thickness of the aureole.
Much is still left to be learned about this phenomenon. “2004 was essentially a verification of historical records and the first quantitative study by CCD imaging,” says Paolo Tanga from the Observatoire de la Côte d’Azur in France. “During the 2012 event high resolution imaging and spectroscopy will be performed. Impressive instruments will be used for this task, such as the Dunn telescope or SODISM on board the PICARD satellite.”
While ESA’s Venus Express orbiter can also study Venus’ atmosphere, the craft cannot determine if variations in the atmosphere occur over time, or if they are dependent on latitude. Venus Express can only probe the atmosphere at a single position at a time, while observing the transit will allow the atmosphere to be observed at all latitudes simultaneously.
Observing the transit of Venus will also allow astronomers to test exoplanet transit models. We are only just beginning to sample terrestrial planets with NASA’s Kepler mission, and observing the transit of Venus will show what the detection limits are when a planetary atmosphere can only be observed using this method.
“Exoplanet transits of telluric planets should provide spectral signatures of their atmospheric compositions,” Tanga tells Skymania News. “This is very weak and out of reach today, but it could/should become accessible in future, especially from space telescopes devoted to exoplanets. So, the idea is to check simple absorption models of Venus’ atmosphere and see if one can find the expected signatures during the transit. This is the closest and easiest way to emulate exoplanet transits.”
The Venus transit will be visible in its entirety from parts of Australia and Asia, at sunset on 5 June for North America, and at sunrise on 6 June for most of Europe.