November 24, 2010

First observation of transit of Venus – 1639

Every once in a while, one of the planets gets between us and the Sun. These planets are too far away to cast a shadow on our planet or to block much of the sunlight—in other words, they don't eclipse the Sun like the Moon occasionally does. Instead, from our point of view, they appear as tiny black dots that travel across the disc of the Sun. This is called a transit.

Only two planets can be seen in transit across the Sun from the Earth. Do you know which ones they are?

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ANSWER: Mercury and Venus are both closer to the Sun than we are here on Earth, so they are the only planets that can get between us.

Anyway, Jeremiah Horrocks was an English astronomer. He was the only person known to have predicted the transit of Venus on this date in 1639, and he is one of only two to have observed and recorded it. (His friend William Crabtree was the other.)

You may be wondering how he—or anybody—could watch a transit across the blindingly bright Sun. Well, he focused the image of the Sun through a simple telescope onto a piece of cardboard. He then watched the not-so-bright image on the cardboard. People still make this simple kind of helioscope these days, although astronomers have of course figured out more sophisticated systems with which they study the Sun.

NOTE: Never look at the image of the Sun EVEN through a helioscope without adult supervision.

A Rare Treat!

Transits of Venus are very rare. They happen in pairs that are eight years apart—and these pairs are separated by more than 100 years! The last transit of Venus occurred in 2004 (pictured here), and the next will occur in June of 2012. That will surely be the last of our lifetimes!

So mark your calendar now for June 5 or 6, depending on your location, 2012... Hmm. I don't have a calendar that far ahead, do you? In the meantime, here is a website called "Transit of Venus." Notice that North Americans will be able to watch the 2012 transit at sunset; most Europeans and some Africans, Asians, and Australians will be able to watch it at sunrise. Alaskans and eastern Asians and Australians will be able to watch the entire transit. 

You can watch a transit using a pinhole camera or by focusing the image of the Sun through one lens of a pair of binoculars onto a sheet of paper or cardboard. See more specific directions here. 

Again: Don't look at the sun directly, or through binoculars or camera lens or telescope. Allow the light from the sun to pass through the lens or pinhole onto another surface. With an adult supervising.