Archive for July 4th, 2012

h1

Observing report: the transit of Venus in Claremont

July 4, 2012

Here, just one day shy of being one month overdue, is my post on the transit of Venus on June 5. As promised, I took scopes downtown and did some sidewalk astronomy, and eventually some rooftop astronomy. As with the solar eclipse on May 20, the primary instrument was my son’s Astroscan with a homemade sun funnel, and once again it performed beautifully.

My partner in this enterprise was fellow Claremont paleontologist Matt Benoit. He was there for the whole thing, and helped keep scopes on target and help people understand what they were seeing. We hit a grocery store beforehand for soda and snacks and basically made an extended party out of the event.

I wanted to see the transit, period, but I was especially keen to watch the entry of Venus onto the solar disk to see if I could spot the “black drop effect” that bedeviled transit-timers in previous centuries. Sure enough, as Venus started to pull away from the limb of the sun there was a persistent dark blob or zone that seemed to connect the planet to the black space beyond, like surface tension keeping a drop of water from falling off the faucet. The black drop effect was once thought to be an effect of the atmosphere of Venus, but it’s not, because airless Mercury shows the same effect during its transits (the next of which is coming up in 2016, by the way). It’s nothing to do with our visual perception, either, since it’s easily recorded photographically, as you can see above. It’s now understood to be an effect of diffraction when a vanishingly thin line of light separates two darker spaces or silhouettes. You can see it by holding your finger and thumb up to the light and bringing them together–just before they touch, the black drop effect will seem to bridge them.

Along with the Astroscan and sun funnel, we had along the Celestron Travel Scope 70 with the aperture mask and solar filter described in this post, for direct viewing. Here’s my friend Marcy, who was there with friends for about half of the transit, getting her first look.

Although we both put in time on both scopes, for the most part I drove the Astroscan while Matt minded the Travel Scope. He also helped people get some photos through the eyepiece, as he did here with Marcy’s DSLR.

The view through the filtered scope was not as detailed as in the sun funnel, but the warm yellow color was more aesthetically pleasing, and many of our visitors appreciated both views.

Like the eclipse, the whole effect of the transit was a little unreal. In addition to the scopes, we also had eclipse glasses and a piece of welder’s glass. Every few minutes we would look up with our naked eyes and see a little black dot on the sun, and know that it was a whole world. And not just a world, but a twin of Earth. Someone on Mars watching a transit of Earth would see something very similar–our whole planet, all our evolutionary and human history, everything we’ve done or built (except for the handful of tiny things we’ve sent away)–all shrunk to a point, no larger, to the naked eye, than the period at the end of this sentence.

We had a steady stream of visitors downtown until a little after 6:00, when the theater blocked the view of the sun from the public square. So we decamped to the top of the parking garage across the street. Some people followed us over from downtown, and some found us up there on their own. One guy said that he found us because he had Googled for Venus transit events in Claremont and found my morning-of invitation post, which is nice, because that’s exactly why I put it up. In all, about 85 people saw at least some part of the transit through one of our scopes.

My son, London, watching the very tail end of the transit with the welder’s glass.

Venus was still crossing the face of the sun when they set together. As with the eclipse, I managed to get a shot right when the world crossing the sun touched Earth’s horizon. A moment later, it was gone, and the last transit of Venus until 2117 was over. I’m glad I got to see it, and to share it. I hope you had the opportunity to do the same.