Posted on Monday 7 June 2004 to Quintessence

This animation is based on photographic plates taken of the Transit of Venus in 1882 by David Peck Todd at Lick Observatory in California [more]

Staying with the Venusian theme, the planet will transit across the face of the sun tomorrow.

While the earth, sun and Venus line up quite regularly (every eight years in fact) Venus only travels across the sun's disc (i.e. transits) twice every 121.5 years. Normally it passes either above it or below it. While this event will be completely invisible over the Americas it will observable over the entire African and Eurasian landmasses where the vast majority of humanity resides. If it's a sunny day tomorrow, I'll be outside after 3pm (eastern Australian time) trying out the pinhole lens approach. If not I'll be checking out one of the many webcams: from Iran (1); from Iran (2); from Iran (3); from the Canary Islands; from the Netherlands; from Denmark; from Turkey; from Greece; from Hong Kong; from Norway; from Australia; from Sweden; from Macao (near Hong Kong); from India; from Brazil; from Earth orbit; from GONG.

Obligatory warning: Don't look directly at sun, kids. It's really, really bad for your eyes.

In years past this event held considerable interest for astronomers because Edmund Halley (the one for which the comet is named) produced a paper demonstrating that the transit was a particularly useful event because it could be used to determine the precise distance between the earth and the sun. By measuring the times that this event occurred at two distantly placed points on the earth, the distance to the sun could be calculated geometrically using a process of triangulation. Students of Australian history will no doubt recall (unless they were home sick that day) that observing the transit of Venus from Tahiti was James Cook's primary mission for his voyage across the Pacific (his secondary and top-secret mission was to chart the hitherto unknown east coast of the Australian mainland).

Why was important? It was needed in order to calibrate Kepler's Third Law, i.e. the period of a planet's orbit is equal to the cube of its radius from the sun (actually this was its average radius because Kepler also knew that planetary orbits are elliptical rather than purely circular). By knowing, as we now do, that the average radius of the earth's orbit is about 150 million kilometres and that its period is 365.25 days, we can easily calculate the size of the solar system and the distances between all of the planets solely from the periods of their orbits.

Unfortunately for Cook and others, the exact timing of the start and end of the transits was obscured by an optical effect know as the black-drop effect caused by refraction in our atmosphere. This experimental uncertainty greatly reduced the usefulness of the transit of Venus and the correct distance was eventually found by a different route, by calculating the speed of the planet earth in its orbit from an anomaly known as the aberration of starlight.

More recently this figure has been refined to a very accurate degree through the use of radar.

(tip o' the hat to Pete)

Update:

Well the pinhole approach was a complete failure in Melbourne's feeble winter sunlight but Peter did a remarkably good job with a pair of binoculars (following the method outlined here). This image was taken from Melbourne at 6:40 am UTC

The time difference required in order to take these two quite similar images is a function of the solar parallax. I'll leave it as an exercise for the reader to use this to calculate the size of the solar system.