Exactly 243 years ago, Captain James Cook made the sketch you see above. On June 5–6, 2012, you could make your own drawing of such a rare celestial event (provided you have the right eye-protecting equipment). But don't miss the chance, as you won't get another one for 115 years.
Cook's drawing shows the Transit of Venus as it appeared on June 3, 1769, from Tahiti. During first voyage around the world, Cook, astronomer Charles Green, and the crew of the HMS Endeavour set up observing equipment on what is now known as Point Venus. In fact, the Transit was one of the motivations for the expedition, as it would provide valuable information for determining the size of our solar system. The explorers made many measurements of the event, and Green added his own sketch (see the downloadable large image).
During a transit, Venus passes directly in the path between the Sun and Earth, taking the appearance of a small black spot moving across the face of the Sun. Venus appears in silhouette against the solar backdrop because of the geometric alignment of the three bodies. (Other objects, such as the International Space Station, can make similar silhouettes.) The differing speeds and inclinations of the orbits of Venus and Earth mean that these configurations come infrequently—every hundred years or so—and in pairs. The first transit in the current pair occurred in 2004; the previous transits took place in 1874 and 1882; the next will occur in 2117 and 2125.
When the transit occurs this week, do not look directly the Sun. Venus covers too little of the solar disk to block the blinding glare. Be sure to use some type of projection technique or a solar filter. Many astronomy clubs will have solar telescopes set up to observe the event, so contact your local club for details. The transit will be visible on every continent and will last roughly seven hours.
NASA’s Goddard Space Flight Center will provide high-definition images and movies of the transit as viewed from the Solar Dynamics Observatory (click here). Astronauts on the International Space Station also will be snapping photographs of the event. You can also post and compare your own images in a shared flickr space. And NASA staff will send answer questions about the event via Twitter at hashtag #VenusTransit.
Image scan provided by the Museum of the History of Science, Oxford. Originally published as part of â€œObservations Made, by Appointment of the Royal Society, at King Georgeâ€™s Island in the South Sea; By Mr. Charles Green, Formerly Assistant at the Royal Observatory at Greenwich, and Lieut. James Cook, of His Majestyâ€™s Ship the Endeavour,â€ in Philosophical Transactions, 61 (1771), 397-421. Caption by Michael Carlowicz.