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When Venus Transits the Sun: A Q & A with astronomer Richard Strom

Understanding Science is a new event series of scientific seminars for the general public, brought to you by The Institute of Physics (IOP), RSC and Euraxess: Researchers in Motion. This Wednesday, Richard Strom (NAOC, ASTRON & University of Amsterdam) discusses “When Venus Transits the Sun: Heroic efforts to observe a rare event.” This event is brought to you by The Institute of Physics (IOP), RSC and Euraxess: Researchers in Motion.

Here, event speaker astronomer Richard Strom gives some insight into the once in a lifetime phenomenon. 

Mengfei Chen: What actually happens when Venus transits the Sun? What does it look like?

Richard Strom: Only the inner planets (Venus and Mercury) can pass between the earth and the sun, blocking a tiny amount of sunlight. During a Venus transit the sunlight is dimmed by about 0.1% (it’s a mini- mini- mini-solar eclipse). In that sense not very impressive. But a transit of Venus can be seen with the unaided eye (Mercury cannot, it’s too small), provided solar glare is dimmed (for example by haze, smoke, observing when the sun is near the horizon, or through a solar filter). It looks rather like a small sunspot, and in fact many medieval reports of transits were actually observations of sunspots. Someone I know who saw the 2004 transit of Venus said, it looks like someone punched a hole in the sun.

MC: Have you seen it? Could you describe the experience?

RS: I have seen it, with the aid of a telescope. While nowhere near as impressive as a solar eclipse, it was a special moment to think that no one then alive had seen the previous Venus transit (in 1882). Unlike a sunspot, the shadow of the planet was perfectly round, with a clean, unfuzzy appearance; beautiful in its modest way.

MC: When was the transit first predicted? Observed?

RS: Kepler in 1630 predicted that there would be a Venus transit in 1631, but it could not be seen from Europe. In 1639, the brilliant English amateur Jeremiah Horrocks predicted – just weeks before the event – and then observed the first transit on 24 November (old style). He and his fellow amateur William Crabtree were the only observers of the 1639 transit.

MC: Why were the attempts to see the transit heroic?

RS: To achieve the scientific aims, a transit had to be observed over a wide range of latitude, so teams were sent to remote locations. The heroism in the eighteenth, and to a lesser extent nineteenth, century lay in getting there. Many expedition members suffered hardship, and a not insignificant number lost their lives.

MC: Scientifically, why was the transit important?

RS: For two centuries it was seen as the best method for determining the distance between the Earth and the Sun, which is technically called the astronomical unit. It is the fundamental unit of distance in astronomy, our measure of scale from the nearest stars to the furthest galaxies.

MC: When will the next transit occur? 

RS: 105 years and 6 days after my talk, on 11 December 2117. If it’s cloudy where you are, no fear. There’ll be a repeat in December 2125.

Correction: An earlier version of this post credited the answers to Professor Richard de Grijs.

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