Once in a blue moon, Venus passes in front of the Sun. It’s then a black dot crossing the solar disk, an event called a transit. If two observers far from each other – one in Peking and one in Brooklyn, say – note the moment when each sees Venus touch the inner edge of the Sun’s disk at the start of the transit, they can compare the times that this happens at each place; and if they know the distance between observers, they can obtain a precise distance to both the Sun and Venus.
Edmund Halley, of comet fame, pointed all this out two centuries ago. Result: More than a dozen countries launched expeditions to far-flung places to observe the transit of 1761 – and to each of the next five thereafter.
Transits are rare. No, rare isn’t quite the right word; it’s more like bizarre. Once a transit happens, another will follow in exactly eight years minus two days. But then 105½ years must pass, followed by another pair of transits eight years apart. Then 121½ years. Then eight. So that’s the curious pattern: 8, 105½, 8, 121½, 8, 105½, 8, 121½…and on and on forever. They’re always in December or June.
There had been none – not one – in the entire 20th century, whose full eventful span fell by bad luck for the likes of my parents and grandparents during that longest possible transitless gap of 121½ years. Indeed, even now, only six transits of Venus have ever been observed. They happened in 1639, 1761, 1769, 1874, 1882, 2004 and now this Tuesday, June 5.
You don’t even need a telescope – just a solar filter, so you won’t turn your retina to charcoal. In 2004 the sky was cloudless around here. And there it was: a respectable black dot moving leisurely across the Sun’s face. Now again our region is on the lucky Sun-facing part of the world. Venus takes six hours to strut across the solar disk, and the fact that the Sun will set here before Venus finishes its transit hardly matters. We’ll get a few hours’ worth, starting just after 6 p.m., and that’s more than enough.
You don’t want to miss it. Get hold of eclipse glasses from eBay, or else shade number #12 welders’ goggles, and look at the Sun late Tuesday afternoon, June 5. Nothing to it. Miss it and you have to wait 105½ years for the Christmas transit of 2117.
Or join me at Mohonk Mountain House. We’ll have an exciting late-afternoon program and supply the correct eye protection, and even have special solar telescopes set up; call (845) 255-1000 for more info. Or, go to http://events.slooh.com on your laptop to join a million others to watch free live coverage from multiple sites and hear narration from several top solar astronomers. I will be hosting about half of that six-hour program as well, starting at 6 p.m. on Tuesday. Or you could attend the program given by SUNY-New Paltz at its Lecture Center starting at 5 p.m. Lots of ways to see the event. If it’s cloudy, definitely go to http://events.slooh.com.
Celestial “spectacles” that grab media attention can be visually stunning and also common – like the Aurora, for people who live in and around Fairbanks. Others are visually stunning but rare – like total solar eclipses, or the amazing dense meteor shower that we had here on November 18, 2001. Some are visually stunning and not truly super-rare, but uncommon, like gorgeous all-sky auroras visible from here, or a dramatic comet like 1997’s Hale-Bopp.
Others are visually just okay or rather subtle, but are don’t-miss events solely because they’re so very rare. That’s the case with this Venus transit. Don’t expect to be knocked backward. You should check it out only because nobody alive today will see Venus cross the Sun’s face – ever again.