The Venus Transit in multiple wavelengths
I missed the very beginning of the transit of Venus last evening, most notably the first and second contacts, due to my daughter’s dance recital. But immediately afterward, the sun poked its nose out and we raced to a pre-determined meeting spot for my local club (ASNNE). Everyone was taking down their scopes by the time we arrived since the sun had gone behind the trees across the street, but we were streaming it indoors and some of my friends managed to get some pretty cool shots.
Instead, we headed home and I found the absolutely outstanding feed provided by The Exploratorium who had a telescope set up at around 11,000 ft. on Mauna Loa in Hawaii. It was far better than the feeds that provided nothing but endless chatter and very little in the way of actual views. The Exploratorium’s stream moved between three different filters – hydrogen-alpha, calcium (I don’t know if it was the H or the K line) and white – and moved around a bit so as to pick up views of some of the sunspots, prominences, and flares (quantum phenomena at work!). Commentary was intelligent but not overly technical (so my kids could understand it) and only lasted a few minutes every half-hour. Plus, in the 20-25 minutes between commentary they streamed spontaneously composed (likely by computer) ambient music in surround sound. So I hooked up my computer to my home theatre system and we watched for a couple of hours on the wide screen with ambient music surrounding us! It was rather Floydian, I must say…
At any rate, I stayed up late to catch the end and was able to witness some really interesting things. In particular, take a look at the image I created below using a partial screen capture (I’m not sure about the rights to these images since they came from The Exploratorium’s telescope via the Internet, but as I’m using them in an ostensibly educational manner, I am hoping they will forgive me!):
Notice that “third contact” (the point at which Venus’ disk reaches the edge of the Sun’s disk after crossing the latter) occurs in white light before it does in H-alpha. This is because, in a sense, the hydrogen-alpha filter allows us to look “deeper” into the atmosphere. Or, rather, there are portions of the Sun’s atmosphere that are not visible in the white light filter image. In fact, third contact in H-alpha didn’t occur for another few minutes (this is a tad bit later than third contact in H-alpha, but close):
Looking deeper still (and about 30 seconds earlier) into the atmosphere, here is roughly third contact in calcium:
What’s interesting is that the contact points in the transits of 1761, 1769, 1874, and 1882 were used to calculate the astronomical unit based on a suggestion made in 1663 by future St. Andrews University (my alma mater) professor James Gregory. If any of these data values were undertaken using different filters (which they were not, since nothing of the kind existed then), a different value for the AU (or perhaps just a greater uncertainty, depending on how the data was taken) would have been reached. Keep in mind that when Lalande calculated the value of 153 million kilometers (±1 million kilometers) in 1771, the precision was apparently less than what had been hoped. When Newcomb calculated the value of 149.59 million kilometers (±0.31 million kilometers) after the 1882 transit, this approximately 2% increase in precision was enough to nail down the value (and thus eliminate the need for parallax calculations).
Now, I seriously doubt there is a 2% difference between the H-alpha and the white light third contact points. Nevertheless, the difference might have a measurable effect on the calculation of the AU. Of course, these days we use more sophisticated techniques to measure the AU that don’t rely on a transit. But it is nevertheless interesting to think about.
Incidentally, there were also a number of huge sunspots (larger than Earth) and a prominence on the lower portion that was bigger than the disk of Venus which means it was way bigger than Venus in actuality. In fact it was likely larger than Neptune.
All in all, an amazing, once-in-lifetime (actually twice, since they come in pairs separated by eight years) event!