LETTERS TO THE EDITOR The downloaded PDF for any Letter in this section contains all the Letters in this section. Letters are selected for their expected interest for our readers. Some letters are sent to reviewers for advice; some are accepted or declined by the editor without review. Letters must be brief and may be edited, subject to the author’s approval of significant changes. Although some comments on published articles and notes may be appropriate as letters, most such comments are reviewed according to a special procedure and appear, if accepted, in the Notes and Discussions section. (See the “Statement of Editorial Policy” at http://ajp.dickinson.edu/edPolicy.html) Running controversies among letter writers will not be published.

THE TRANSIT OF VENUS: periods and different inclinations of opportunity to increase public aware- JUNE 5/6, 2012 their orbital planes. The exact alignment ness of astronomical phenomena and of Venus, Earth, and the Sun required the history and nature of science. With For most living humans June 5, 2012 for a transit of Venus only occurs twice international cooperation (including will be their last chance to see Venus in (roughly) a century, as a pair of tran- inter-American cooperation) it will be cross infront oftheSun,to sharethe fasci- sits about 8 yr apart. For an observer on possible to combine your own measure- nation felt by astronomers in past centu- Earth, Venus will usually pass above or ments with those of distant observers. ries, and to appreciate their observations below the Sun and be invisible during You have the chance to repeat histori- and measurements. It is also an occasion its conjunction with the Sun. cal measurements with modern equip- to organize public viewings and to partic- When being observed from distant ment, make your own measurement of ipate in international projects to publi- sites, the transit will appear slightly the AU, and experience the fascination cize, view, and measure the transit. different: of scientific discovery and the difficul- On June 5th/6th between 10:09 pm ties that had to be conquered in order to • Different observers will see Venus and 4:50 am Universal Time, the black, enter onto the Sun’s disc at times dif- achieve a satisfactory accuracy. circular silhouette of Venus will slowly fering by up to 15 min, and they will The 2012 transit will begin in the pass across the Sun’s disc from east to measure similarly different exit times. afternoon on the North American conti- west (see Fig. 1). Such a passage is • The positions of Venus on the Sun’s nent but will end after sunset. People in 1 called a transit of Venus. disc on photos taken at different pla- Asia and parts of Europe and Africa The transit can be observed without ces at exactly the same time will dif- will be able to view the second part of any magnification using only a pair of fer by up to half of the diameter of the transit, but only a few people (e.g., Solar filters. You can also project the Venus’ disc (see Fig. 2). in Japan and Australia) will be able to Sun with field glasses, you can view it These parallax effects occur because observe the entire transit. through a telescope equipped with a so- Venus is not as far from Earth as the Several international projects have lar filter, and, of course, you can photo- Sun. From measurements of the paral- been organized to observe, photograph, graph it. The last transit of Venus lax, one can determine the distances of and measure the transit. In addition, occurred in 2004, but after this June the Venus and the Sun from Earth. This there are numerous websites that offer world will have to wait until 2117 for Earth-Sun distance, called the Astro- information about the history, the im- 4 the next one. nomical Unit (AU), is the basic bench- portance, and the details of transits. Transits of Venus are rare because mark for determining the size of our Such websites also help bring people Venus and Earth have different orbital solar system. In the 18th and 19th centu- together from all over the world so they ries, several countries organized expedi- can compare their observations and tions to locations all over the world to determine the distance from Earth to view transits of Venus. At the time, this the Sun. was the best way to determine the AU.2,3 U. Backhaus By organizing public viewing events, Universitaet Duisburg-Essen, the upcoming transit offers an excellent Germany

1More information on transits of Venus can be found online at 2D. Hudon, “A (not so) brief history of the transits of Venus,’’ J. R. Astron. Soc. Can. 98(1), 6–20 (2004), available online at 3E. Maor, Venus in Transit (Princeton U.P., Fig. 1. Reconstruction of 2004 transit of Venus Fig. 2. Parallax of Venus during 2004 transit as it Princeton, 2004). across the Sun as photographed from Essen, appears in the superposition of properly rescaled 4See, for example, and Germany. and rotated photos from Germany and Namibia. .

361 Am. J. Phys. 80 (5), May 2012 http://aapt.org/ajp VC 2012 American Association of Physics Teachers 361