Venus in Transit

I wanted to call your attention to a solar system event taking place today. It isn't very showy, but it is very unusual.

The planet of Venus is going to pass in front of the Sun. The event is called the Transit of Venus, and it takes place twice every 120 years or so. It happened in 2004, and won't happen again until 2117. Very few people who are now alive will get another chance to see it.

The transit had immense scientific importance 250 years ago. All our measurements of distance in space, from the late 1700s until the space age depended on observations made by a few scientists during the 1769 transit of Venus. Up until then scientists understood the proportional size of planets’ orbits, but not the absolute size of any of them. Then Sir Edmond Halley suggested using the passage of Venus in front of the Sun as an opportunity to use trigonometry to determine the radius of Earth’s orbit around the Sun. He did this with absolute knowledge that he would not live to see the day the measurement was made.

Heeding Halley’s call, several French and British scientists spent years traveling to far flung places to make measurements of the transit. In fact, the scientists had left intending to make the measurements during the 1761 transit, but were universally thwarted by enemies, and the elements, spending an additional 8 years at sea before finally make measurements of the second transit eight years later. Once they did return home, it still took years more to reduce the data and determine what we now know as the AU: the Astronomical Unit, which is the mean distance from the earth to the sun, a value we now know to be 92,955,807.3 mile. Once we knew that number, we knew the size of all the other planets’ orbits, and we were able to directly measure the distance to the nearest stars.

With that basic knowledge, and a much later discovery called Red Shift, we have been able to infer the distance to everything else in the Universe. The struggle to measure Venus’ position as it passed in front of the Sun is truly one of the great untold stories of human scientific exploration.

Now for your attempt to see the event. When viewed from Winchester, the transit will start at around 6 tonight and last until after the sun sets around 8:30 p.m. When looking at the transit you will be looking right at the Sun, so you must take the same precautions that you would take to look at a solar eclipse: either make a pinhole camera, or get No. 14 arcwelding filters. Looking straight at the Sun will permanently damage your eyes.

In the weeks after the event you can check out various NASA websites for movies of the transit. Hinode XRT, a telescope that I helped build, should have a very nice movie.

For more information related to viewing the transit, and for movies of the 2004 Transit, take a look at links from Sky and Telescope, Fourmilab and Transit of Venus.

-Peter Cheimets

Smithsonian Astrophysical Observatory