Catch Venus in transit

In my three years writing this column, I've often described routine rundowns of steady and unchanging observation targets. I'd argue there's nothing routine in the endless wonders of astronomy, but most nights the sky can offer only its usual selection of stunning stars, nebulae and planets, with the occasional meteor shower thrown in. But every once in a while, the stars align and something truly special happens.

I've been looking forward to writing this column all year because one of the rarest astronomical events is upon us. On Tuesday, June 5, Venus will transit the sun for the last time until December 2117.

With the correct equipment - which could be just binoculars channeling the sun's reflection off of a white piece of paper or solar filters if you have a telescope or binoculars and want to look directly at the sun - you can watch the first part of Venus's six-hour trip across the sun's face.

For observers stationed throughout most of North America, the transit will begin in the evening with Venus still marching across the sun at sunset. First contact, when Venus's disk reaches the sun's edge, occurs a couple of minutes before 6:10 p.m.

Just before 6:28 p.m., Venus moves into the solid circle backdrop of the sun. This is when we will be able to see something good. Venus will reach the center of the sun around 9:30 p.m. and cross the outer edge around 12:32 a.m., three hours later. Yes, the sun will be on the other side of the world by then, but we'll have seen enough to be satisfied.

Although it will undoubtedly create a better observing experience, you don't need a telescope or binoculars to see the transit of Venus. It will look like a small black dot moving across the top half of the sun. For a safe naked-eye view of the transit, look through either shade-14 welder's glass (not the more common shade-12, which isn't strong enough), which you can buy for cheap at many welding supply stores, or get some eclipse glasses, which resemble (but aren't!) 3-D movie glasses and are inexpensive.

Planetary transits as seen from Earth are so uncommon because only Mercury and Venus are between us and the sun. Earth, the planet and the sun have to be perfectly aligned, and because Venus's orbit tilts 3.4 degrees to the plane of Earth's orbit, it usually passes slightly above or below the sun from our vantage point.

Venus only transits the sun twice a century in pairs eight years apart. Tuesday's transit is the second of the current pair - the first was in June 2004. Mercury, which hugs the sun tighter and orbits it faster, transits twice a decade, give or take, because there are more opportunities for everything to align just right. The next transit of Mercury visible from Earth will take place in 2016.

Pray for clear skies and face west this Tuesday evening (with proper solar filters, of course). It's not every day you get to watch a planet travel across the sun - and the next people to watch Venus do so haven't been born yet. Neither have their great-grandparents.

localuniverse@msn.com

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