Astronomy can often offer a chance to observe a unique celestial event that once it is gone, will never be seen again in a lifetime. A transit of Venus across the face of the Sun is a rare event with only seven having occurred since the time Galileo was observing, so we are lucky to be living at a time when we can witness a rare Venus transit next month.
Known as inferior conjunction, Venus passes between the Sun and Earth every 584 days, but the orbit of Venus is tilted 3.4ļ with respect to Earthís orbit so Venus usually passes north or south of the Sun. A transit can only happen when Venus is at inferior conjunction and very close to where the orbital planes of Earth and Venus intersect. This is very similar to the geometric arrangement for our lunar and solar eclipses. If the Moon orbited in the same plane as Earth we would have lunar and solar eclipses every month. If Venus orbited the same plane as Earth we would have a transit of Venus across the Sun every 584 days.
Here in New York State the transit will be an evening event as it begins on June 5 at 6:10 P.M. This is the most interesting part of the transit, the moment when Venus first makes a tiny dent in the edge of the Sun. It takes Venus about 18 minutes to fully enter onto the Sunís disc, but will it be a perfect black disc as soon as it is fully on the Sun? Many have observed and photographed a thin, dark filament connecting Venus to the edge of the Sun known as the Black Drop Effect. This filament does not last long, but it is real. Another phenomenon is the bright rim of Venus visible against the blackness of space while it is crossing the limb. This is also real and both phenomena are due to the thick atmosphere around Venus that refracts and bends light. Once Venus has made it onto the Sunís face it is a casual event and one can simply admire the fact that they are witnessing an event that will never be seen again in their lifetimes. The Sun will have set about an hour before Venus reaches the middle of its transit at 9:30 pm.
This will be a fun transit to watch with several of the MVAS members having more sophisticated equipment than in 2004. Keep in mind that safe solar observing is a must! NEVER even think about using sunglasses or staring at the Sun to observe this transit even if it is hazy as the Sun grows lower. The Sun is still dangerous! If you are in doubt, please ask or team up with a club member who knows what they are doing. Do not take any chances as the slightest error could cause blindness. Simple pinhole projectors will show the transit very well and indeed whatever you used during the 2004 Venus transit will work just the same for this one. A simple solar filter for viewing sunspots is perfect for this event and will be the number one choice of many. It might be possible to see Venus against the Sunís chromosphere (lower atmosphere) or maybe even against a prominence before the transit begins by using a hydrogen alpha filter. A pair of solar eclipse glasses will also work fine if you are at work or where you cannot readily set up a telescope. Binoculars with solar filters or #14 welderís glass will fit the bill if you are in a pinch for time and still want a quick glimpse to say that you saw it.
Weather will be the potential big spoiler for this rare event and a late-spring afternoon in early June has the potential for cumulus clouds and thunderstorms. These clouds tend to dissipate as the Sun sinks lower, the air cools down and stabilizes. If luck holds this will be the second Venus transit in our lifetimes. There will be no third chance unless there is a way we can live until December 11, 2117, so go all out and make the most of it!