Musings on optics, physics, astronomy, technology and life

Archive for June, 2012

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Tomorrow: the transit of Venus!

I put an exclamation point at the end of the headline because, well, I’m excited!

Tomorrow, part of planet Earth will experience a “transit of Venus,” meaning that we’ll see the second planet from the Sun pass across the disk of the Sun. We see transits of Jupiter’s larger moons across the face of Jupiter all the time, but a transit of Venus across the Sun is something quite special. Usually when the Earth, Venus and the Sun line up in that order, Venus is above or below the face of the Sun because of the inclination of its orbit. Once or twice in a really great while, Venus actually crosses in front of the Sun from our viewpoint on Earth.

I say “once or twice” because the complicated orbital mechanics of the solar system makes the transits come in relative pairs, with huge time gaps in between. Sky & Telescope magazine goes into much more detail in its press release on tomorrow’s event.

The first transit of the current pair took place on June 8, 2004 — almost eight years ago. That transit was at sunrise from the perspective of the East Coast of the USA, so one had to rise fairly early in order to catch it. This year’s event will be at sunset instead of sunrise, and for the East Coast, the Sun will go down while the transit is still going on. All the same, the event’s still going to be spectacular.

If you want to see the transit, the first rule is: DO IT SAFELY! Do NOT look directly at the Sun. You probably won’t see Venus anyway, and you will certainly damage your eyes. Again, Sky & Telescope has a complete guide to viewing the Sun safely. In 2004, I used a pair of 10 x 25 binoculars to project the solar image onto a piece of white cardboard. I did NOT look through the eyepiece — it’s easy enough to tell when the binoculars are pointing at the Sun. You could try doing this projection method with two pieces of thin cardboard — make a pinhole in one and use it to project onto the other. This works for partial solar eclipses, but I am not sure whether this will provide enough detail to see the small dot that is Venus.

Another safe method of viewing the transit of Venus is to attend an event staged by people who know what they’re doing when it comes to the combination of optics and the Sun. In the area where I live, I have a choice between the Astronomical Society of Greenbelt’s viewing at the NASA Goddard visitors’ center or the University of Maryland astronomy department’s public viewing atop a campus parking garage. The University of Maryland page has a list of links to other public viewing sites in and around Washington, D.C. Various U.S. observatories are also planning webcasts, so you might be able to continue watching the transit even after your local sunset — or even if your local area is overcast. (Drat those clouds!)

Notice that I’ve been throwing around words like “tomorrow” and “sunset” from the point of view of my personal location. Northwestern Canada, Alaska, much of Asia and eastern Australia will see the whole transit. Some places, like western Africa and most of South America, won’t see it at all. And from the perspective of other locations, the transit will take place at sunrise on Wednesday, August 6. See the NASA Eclipse Web page, curated by Fred Espenak, for an excellent explanation of these details.

Finally, if you’re seeking some music to play while watching this fabulous celestial event, may I suggest John Philip Sousa’s Transit of Venus March?

The blankness of the common thread

At many times in my past, I’ve felt a bit whipsawed as I careen from one writing and editing assignment to another: from elder affairs to school board meetings, from dying kids to budgetary minutiae, from higher mathematics to apartment fires.

In recent months, I went straight from writing about photography in the American Civil War — the pre-light-bulb era — to putting together a feature on modern-day light pollution — which has a lot to do with poorly placed electrical lighting fixtures. So what could these topics possibly have in common?

Blank skies.

The blankness is obvious in this famous Crimean War photograph by Roger Fenton (which actually predates the American Civil War) and this image of a dead Confederate soldier at Gettysburg by Alexander Gardner. Even when there’s a sharply defined shadow, as in this Mathew Brady photo, you can’t distinguish between blue sky and white clouds. (The line at the top of that photo looks like an issue with the collodion plate coating or other chemicals.)

Yet when I got my first 35-mm SLR camera as a high school graduation present and started learning about photography, I read that, in order to increase the contrast between puffy white clouds and beautiful blue skies, one should use a red filter in black-and-white photography and a polarizing filter in color photography. So, why the blank skies in the Civil War?

The answer: prior to 1873, photographic chemicals were sensitive only to blue light. So the wet photographic plates picked up the blue light coming from the sky (Rayleigh scattering and all that) and the blue component of the light reflected off the clouds and recorded them all the same. Photography studios of the era often had walls painted in a sort of robin’s-egg blue, yet they looked lighter on the resulting ambrotype or tintype than they would on modern panchromatic black-and-white film (or digital images turned monochromatic in an image-editing program).

More blank skies are readily visible in this amazing set of Western American images by Timothy O’Sullivan, one of the Civil War photographers who kept on plying his trade after the fighting ended. It makes you wonder about some of the gorgeously detailed skies in the works of some 19th-century Romantic painters — were they trying to one-up the photographers?

Meanwhile, light pollution makes today’s urban night skies look just as blank to the naked eye as the daytime Civil War skies looked to the wet-collodion plates (see top and bottom images here). Different effect, same blah outcome.

My feature article on photography in the American Civil War just came out in the June 2012 issue of Optics & Photonics News. My story on light-pollution will appear in that magazine later this year. If you are an OSA member or can get to a library that subscribes to OPN, please read my article and let me know what you think.