Sunday, July 22, 2012

Venus Dancing on the Sun

It's been almost two months since the last Venus transit this century. Fortunately, I collected a huge stack of imaging data. I posted a few key images in my "Black Drop" post last month, but all along I planned to make a time lapse video; and I finally uploaded that hand assembled video. Here it is, but I recommend following the link and viewing it at the YouTube site. The JPEG artifacts of this embedded version are not as bad there. More about this video below.

So what's going on? Of course the big yellow ball is the sun. (See my earlier posts about how I photograph the sun.) The smaller, fuzzy spots near the center of the sun are sun spots, and the perfectly round black spot is Venus. When the video starts, Venus is not there. It first contacts the sun just to the right of the top edge, then works its way down and to the right. By the time video finishes, Venus is about half way down. I have more frames that I could have added to the video, but the sun (and Venus) sets before Venus exited from the Sun, so I didn't see a point to extending the video any further.

You'll also see some flashing near the beginning of the video. That is clouds getting in the way. A few clouds caused some headaches near the beginning of the transit, but they passed soon enough and most of the transit was cloud free. A few of the early frames were completely ruined by clouds, though, so I replaced them with filler frames so that time advances correctly. That's part of the reason it took me so long to get this video assembled.

You might notice in the video that Venus is going down at an angle of roughly 52°. The camera was right side up, so that is what people with eclipse filters were seeing too. We were at around 38°N latitude on the surface of the Earth, so 90°−38°=52°. Hey, that's interesting! (It's also slightly wrong because I'm ignoring the seasonal tilt of the Earth. The Earth's tilt adds a curve to Venus' path across the sun.) Venus crosses the sun in roughly the same plane as the orbit of the Earth around the sun, so for people standing on different latitudes on the Earth would see Venus following a different path. People standing near the equator would see Venus go straight down (±23° depending on the season). OK, so that's very approximate because I didn't put any effort into leveling the camera or accounting for axial tilt, but hopefully you get the idea.

The camera took an image every 20 seconds, and the video displays 10 frames per second. Therefore, the video appears about 200 times faster then real life. On the actual day, I was spending a lot of time standing around chatting with visitors and listening for the periodic click from the camera as it collected data. And I would check the tracking of the telescope and occasionally make adjustments. Of course the tracking was not perfect, so once I got home with the data and started working with it, I had to align each frame by hand, one at a time. After about 20 frames (2 seconds of video) my eyes would start watering, so that's all I did at a sitting.

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