Venus passed in front of the Sun

To observers on Earth, Venus appeared as a black dot moving across the Sun for several hours on 5 and 6 June. Its next transit will not take place before 11 December 2117!

 

Precautions such as observing the Sun with an astronomical instrument equipped with suitable protection or with special certified glasses need to be taken when watching such events as Venus’ transit. Credit: NASA

Venus’ transits have always been of great interest to astronomers. In fact, prior to modern means, they enabled the distance between the Sun and the Earth to be determined by comparing the difference in time from the beginning to the end of the phenomenon in accordance with various positions on the globe.

Observed on Earth and in space
We now have other much more efficient techniques for determining this distance (an average of 150 million km), but the event remains rare as it occurs in “pairs” 8 years apart, each pair separated by just over a century. The previous pair of transits took place on 8 June 2004 and 5-6 June 2012. The next pair is scheduled on 11 December 2117 and 8 December 2125.
The 2012 vintage did not favour Europe as the transit came to an end just as the Sun was rising (it began on 5 June at 22:05 Universal Time and ended at 04:50 on 6 June). Nevertheless, other parts of the world were more fortunate and events were organised, notably by NASA who has set up an album of the photographs taken on this occasion on this Flickr page.

One of the photographs of Venus’ transit from NASA’s special Flickr page. Credit: NASA/budc1958

Although the determination of the Earth-Sun distance is no longer a priority, professional astronomers nevertheless scrutinise the phenomenon as it enables, for instance, the examination of the atmosphere of Venus, this “pressure cooker” version of a twin to our planet (ground temperatures on Venus go up to more than 400°C!). As a result, even some space means were put to the task. The European Venus Express probe could not “see” the transit as it is currently orbiting the second planet in the solar system. However, it did examine how light from the Sun was altered when it was filtered through the Venusian atmosphere. The data gathered will be compared with that collected by observatories on Earth which were to try to analyse the slight amount of solar light that passed through the Venusian air during this transit.
The Hubble space telescope was unable to turn towards the Sun: its sensitive instruments and its lens could not support the luminous flux (and the same goes for us: never look at the Sun without the appropriate equipment). This orbiting observatory was therefore trained on the Moon! With our natural satellite reflecting the light from the Sun, Hubble was able to measure the minute drop in luminosity provoked by the passage of Venus (which blocked part of the luminous flux) and also the alterations caused by the fact that part of the light was filtered by the Venusian atmosphere. Astronomers are hoping that the results obtained will provide markers for one day doing the same (but via direct observation this time) with planets that orbit stars other than our own. By passing in front of their sun, these distant worlds could reveal the composition of their atmosphere. Obviously, Venus’ transits are highly topical.

Published on 6 June 2012