An upcoming solar eclipse may attract more attention, but some parks are also planning for visitors who would like to observe an even more unusual celestial event. The Venus Transit on June 5 will be a literal "last chance in your lifetime" occurrence.

Many of us aren't familiar with the term "Venus Transit," so we'll begin with an explanation of what to expect … and why it's so unusual.

A transit of Venus occurs when the planet Venus passes directly between the sun and earth. Observers in the northern hemisphere will see Venus appear to move from left to right across the upper half of the Sun at a slight downward angle.

According to experts at the NASA Goddard Space Flight Center, "Transits of Venus across the disk of the Sun are among the rarest of planetary alignments." They're so rare, in fact, that only seven such events have occurred since the invention of the telescope (1631, 1639, 1761, 1769, 1874, 1882 and 2004.)

If this is of interest to you, it would pay to mark your calendar for June 5. The next similar occurrence won't roll around until December of 2117!

As we mentioned in yesterday's Traveler, the upcoming solar eclipse on May 20 is getting a lot of attention in parks, primarily because more people are familiar with the term "eclipse"—and that phenomenon is also easier to see, with proper eye protection, without a telescope.
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Bryce Canyon National Park[/url], for example, holds an annual astronomy festival, and decided to move this year's event to take advantage of the May 20 solar eclipse. That doesn't mean, however, that the Venus Transit will be overlooked.

Kevin Poe is the "dark ranger" at Bryce Canyon—the park's key contact for programs focusing on the night sky. For 2012, however, both the May 20 solar eclipse and the June 5 Venus Transit offer daytime opportunities to help visitors enjoy and understand unusual natural events visible from the park.

“The Venus Transit is an event that’s several hours long," Poe explained. "Unlike [the May 20 eclipse], which is two hours long, with only five minutes of the annularity where the moon is almost completely in the way, the Venus Transit lasts for a longer time. So for that event, on June 5, we’re just going to make it an open house thing. It won’t be a grand festival, but we’ll let everybody know that they can come to Bryce on June 5 and look through a bunch of telescopes and watch it …as it slowly moves across the face of the Sun.”

Another park with an active astronomy program is Great Basin National Park in Nevada. A spokesperson says the park will have activities all day long leading up to the beginning of the transit at 3:00PM (PDT) on June 5. Participants can learn how astronomers used earlier Venus Transits to calculate the size of the solar system, and try it out for themselves. Special safe solar telescopes to view the transit will be available, along with a chance to build your own solar viewing telescope. Special safe solar viewing glasses will be available for purchase in the park bookstores.

As time for the event draws closer, other parks are expected to schedule similar activities. The National Park Service has created a web page for the event, which notes, "In the hours before sunset, every park in the contiguous United States, Hawaii, and the Virgin Islands will be able to view most of the transit in the few hours before sunset."

Best views will be in "parks located in the Pacific such as Alaska, Hawaii, Guam and American Samoa," where observers will be able to see the entire event. (On the U. S. mainland, sunset will occur before the transit has finished.) "For serious photographers, the Pacific parks offer an amazing setting to photograph the entire sequence of Venus racing across the face of the Sun."

As is the case for viewing a solar eclipse, getting a safe look at Venus crossing the face of the sun will require some special precautions or equipment, although those items need not be expensive. Another option that won't require anything but a home computer will originate from a unique site located on the island of Hawaii, not far from Hawaii Volcanoes National Park.

The Mauna Kea Observatory, located at the 13,796-foot summit of Mauna Kea houses the "world's largest observatory for optical, infrared, and submillimeter astronomy." It's the highest point in the Pacific Basin, and normally a reliable spot for clear skies in June. For the Venus Transit, a cooperative effort by NASA, the Sun Earth Day Team and the University of Hawaii Institute for Astronomy will offer a live 'remote' webcast from Mauna Kea, showing the Venus Transit in its entirety in real time. You'll find more information at this link.

If you'd like to find out what time the Venus Transit will be visible from your home, a park or any other location in the country, you can use a "Transit computer" provided by the U. S. Naval Observatory. You can enter a city and state and determine the beginning and ending times that the transit will be visible, along with the all-important sunset time. Note that times are given in "Universal Time"; a chart at this link will allow an easy translation into your time zone.

The length of time the transit will be visible before sunset becomes longer the further west you travel in the U. S. In Hilo, Hawaii, the transit will be visible for 6 hours and 34 minutes. The viewing time in San Francisco lasts for 5 hours and 22 minutes, but in the New York City vicinity, it shrinks to 2 hours and 19 minutes.

A bit of trivia connects an earlier Venus Transit with a well-known American composer. John Philip Sousa, who was reported to be very interested in the 1882 transit, wrote his "Venus Transit March" in 1882-83 at the request of The Smithsonian Institution. Despite its name, the march wasn't written specifically in commemoration of the transit itself, but rather to honor Joseph Henry, a highly-regarded American physicist who died in 1878.

If you're interested in night sky and other astronomy-related programs, events and information from national parks around the country, you'll find lots of useful details on the website for Night Sky Parks.