This week is the last chance for all but the very young to observe the transit of Venus. It is a striking event that occurs when Venus passes in front of the face of the sun as we see it from Earth.
Venus’s and Earth’s orbits around the sun tilt at slightly different angles, so they line up exactly only four times every 243 years, in pairs separated by eight years. The last time was in 2004 and the next one will be on December 11, 2117.
The transit will be visible from Tuesday evening across North America, and from Wednesday morning in Europe, northeast Africa and most of Asia. Hundreds of people across the globe will be recording it using filters or projection techniques, observing a small black dot moving across the sun for about six hours. Venus is roughly one-thirtieth of the width of the sun so it should be visible without magnification, although you should never look at the sun directly.
It is an important event, both in terms of what it has meant in history for the science of the solar system and what it means for the future.
The most notable transits were in 1761 and 1769, because they led to great developments in astronomy. Edmond Halley, the English astronomer famed for his work on the orbits of comets, realised that by recording the transit of Venus you could answer the most important question in science at the time: how far away is the sun? Without an answer, it was impossible to know the size of the solar system.
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Johannes Kepler, a German mathematician, had worked out the relative spacing of the planets but nobody had discovered their absolute spacing in terms of distance. It was Halley who in 1716 proposed how measuring a transit of Venus could be used to work this out.
He applied a method we call parallax. We demonstrate this by telling people to hold up their thumb at arm’s length, close one eye and open the other. You see your thumb shift against the background, revealing how when you look at an object from different locations its position seems to shift. By recording how long Venus took to move across the sun from the northern and southern hemisphere, scientists were able to triangulate the sun’s position.
Remarkably, if you average out the scientists’ observations, they were accurate to what we now know is 4% of the distance between Earth and the sun, which is just under 93m miles. At this point we did not even have a full map of the Earth, yet this simple observation gave us the map of the solar system.
Transits are the only time that scientists can use solar telescopes to study Venus, as it is normally too far away from the sun. So this week offers a rare chance for us to use different techniques to observe the planet, and we hope to collect important scientific data.
One question that we want to answer is an old, technical one of why the “black-drop effect” occurs. This is an optical phenomenon that occurs when Venus is just inside the sun: its black disc seems to elongate so it’s very hard to time accurately the exact moment that Venus crosses the edges of the sun. This is probably what caused the 18th-century scientists’ slight inaccuracies. There has been a proposal that it is to do with how light spreads out inside telescopes, and a group of scientists will be testing this.
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Scientists will also be hoping to gather more information on the effect that the solar wind has on Venus’s atmosphere. We know that solar wind has a very strong effect on all the planets in the solar system, but the difference between Venus and the Earth is that Venus doesn’t have a magnetic field, so this is an opportunity to study the wind in a different environment. The solar winds hit the top of its atmosphere, which causes it to glow in ultraviolet light, so we will be measuring this and taking images of the effect of the wind.
Most importantly, perhaps, scientists will also be hoping the transit will help us to understand planets around other stars. Nasa’s Kepler mission has developed a new area of science in discovering planets by the brief dimming of starlight as they pass in front of the stars they circle. As Venus transits in front of the sun, it will dim the light very slightly. Some sunlight passes through the planet’s atmosphere, and in this case the chemical composition of the atmosphere is imprinted in the sunlight and its colour changes. A telescope fitted with an instrument called a spectrometer is needed to observe this.
By observing we can test our techniques and then apply these to the other planets transiting in front of distant stars in other solar systems and, we hope, use them to discover accurately the chemical composition and atmosphere of those planets. Crucially, we hope that we can use this technique to find out which of these planets might have atmospheres that could harbour other life.
Lucie Green is a solar physicist at the Mullard Space Science Laboratory, University College London’s Department of Space and Climate Physics. The Transit of Venus will be on BBC2 at 9pm on Tuesday