As everyone knows, June 5 saw the last transit of Venus across the face of the sun until December, 2117. Scientists kept telling us this event won’t be seen again in our lifetimes, and for me, that’s almost certainly true. I doubt very much I’ll live to be 170. It’s almost certainly true for just about everyone living today. A newborn of today would have to live to be 105 to see the next transit, and the newborn wouldn’t have been part of the audience who could understand the phrase “for the last time in our lifetime.” On the other hand, a list of the verified oldest people on Wikipedia lists ages ranging from 115 years to 122 years for the top ten. So it’s just barely possible that a 10 - 17 year old amateur astronomer of today could be around for the next one. Wouldn’t that be cool? Imagine a very old woman telling her great-great-whatever granddaughter, “The last time I saw this happen I was your age.” And great-whatever answering, “Yeah, Gammy, it’s a spot on the sun, okay?”
Since we’ve established I’m not going to be around, I spent most of the evening of June fifth glued to my computer monitor watching the live feed from Oahu on the Exploratorium website. Not that there was much to watch. It was a spot on the sun, moving so slowly that it was impossible to detect actual motion at all. It was kind of like watching hour hand on your watch: you know it’s changing position but you can’t make your eyes see it move. So when I say “watching”, I mean mostly sitting reading a book, looking up every so often, listening to the kickass background music ( a sound composition being created from the video of the transit in real time) and to the audio commentary every thirty minutes or so. From the commentary I discovered that the transit is marked by four points of contact: first contact being when the leading edge of Venus appears to touch the outer surface of the sun, second contact being when the entire disk of Venus is first visible against the sun, third contact being when the leading edge of Venus touches the opposite side of the sun, and fourth contact being just before Venus appears to have completely cleared the sun, when only the trailing edge is making contect. You can see videos of first and second contact and third and fourth contact here (just look down at the index).
What I found most interesting about the Mauna Loa feed is that it gave three views of the transit: in white light, and filtered through a calcium filter, giving the sun a blue-violet look, and through a hydrogen alpha filter, giving it a red look. Each view revealed a somewhat different surface: the sun as seen through the calcium filter looked somewhat smaller, because you saw below what is considered the surface, and so third and fourth contact appeared sooner than they did when the sun was viewed in white light. The hydrogen filter allowed us to see more of the gases off the surface, making the sun appear larger and delaying the onset of third and fourth contact.
Since I am not really an astronomy person and definitely a words person, knowing this leads me to muse about the definition of the word “surface”. We ordinarily don’t think of “surface” as having a vague meaning. We live with surfaces all around us, and graceless as I am, I bump into them a lot. When I’ve stubbed my toe or banged my hip or barked my shin on a surface, I think it’s pretty clear where that surface is.
The sun, however, is a ball of fiery gases. What we call the surface is really the photosphere, the part we can see. What we see depends on how we look at the sun. Since looking at it with the naked eye is harmful, that generally means looking through a filter, and a filter changes where we perceive the surface. So what is the surface? The outermost area we can see in white light? The outermost area we can see through a filter? Which filter?
Furthermore, when Venus transits the sun, it is not at any time making contact. Venus is a little over 67 million miles from the sun. The “contact” is apparent, not real, as Venus comes between Earth and the sun. Even if it were real, however, just how much contact counts as contact? How many degrees of arc must be touching for us to perceive contact?
If “surface” and “contact” are vague, “lifetime” is even vaguer. An individual’s lifetime is easy to measure, from birth to death, at least if allow a little wiggle room in the times of “birth” and “death”. When we use the term “in our lifetime”, however, whose lifetime do we mean? The average life expectancy of a poor child in a developing nation? The natural lifespan of a person in good health who does not meet with any accidents or violence?
I think there’s a good chance that one of the world’s many newborns kicking in their cradles on June 5, 2012, unaware of any astronomic events of note, will make it to December, 2117, possibly still unaware at that great age of any astronomic events of note. For myself, however, I think it’s a certainty - never again in my lifetime.