Observing the Transit of Venus from Wisconsin
by John Rummel
It’s not ideal, and it might not even be good, but for those of us not lucky enough to be traveling overseas next June 8th, our only option is to try to observe the tail end of the transit of Venus from right here in Wisconsin.
The transit will be visible in its entirety from most of Europe, central and western Africa, the middle east, and much of Asia. Observers in the United States, east of a line from Montana to east Texas (the farther east, the better), will be able to see only the very end of the event, with the sun rising just as Venus is about to leave the western edge of the sun's disk. We effectively have a chance to observe just the final minutes of a 5.5 hour event, as the disk of Venus makes 3rd contact, then diminishes to a notch, and finally nothing.
Given that no transit of Venus has occurred since 1882, this is an event well worth attempting, whatever the possible limitations. My primary constraint that day is that I can’t miss work. Wherever my observing location, I have to be able to get back to Madison at a reasonable time that morning.
My possible observing scenarios:
1. From Madison: Find a place with a nice flat eastern horizon and observe at sunrise
2. Get permission to observe from the roof of a downtown tall building to improve the eastern horizon
3. Travel east to Milwaukee: Observe from the lake shore, taking advantage of the water front for a clear eastern horizon
4. Observe from the roof of a downtown Milwaukee high rise near the water front.
5. Travel north and east to find a location with suitable eastern horizon.
Local Circumstances for the Venus Transit in Madison and Milwaukee
June 8, 2004 Madison Milwaukee Sunrise (CDT) 5:20 am 5:13 am 3rd Contact 6:06 am Same Sun’s altitude 6 degrees 7 degrees 4th Contact 6:25 am Same Sun’s altitude 9 degrees 10 degrees
As the circumstances table shows, one gains 7 minutes of observing time by traveling east to Milwaukee thanks to the slightly earlier sunrise there. Since there is no significant parallactic change in the circumstances of the transit between Madison and Milwaukee, you effectively can see 7 additional minutes of the transit by going to Milwaukee.
The benefits of observing from the roof of a tall building are a little less clear cut. There is an obvious benefit in terms of avoiding a cluttered horizon. The time advantage is slight but real. You can probably see the sun's disk break the horizon about two minute earlier from 100 meters up (from a 2 meter elevation, the horizon is 5 kilometers distant; from a 100 meter elevation, the horizon is about 35 kilometers away. Since the sunrise time between Madison and Milwaukee is about 7 minutes, with distance of 123 kilometers, the additional 35 kilometers gained by increasing your elevation would be roughly equivalent to getting yourself 35 kilometers further east, or about two minutes).
Whatever the time benefits of the additional altitude, the real advantage of this scenario may be avoiding the surface haze that could ruin viewing of an event like this that requires a view of the sun so low on the horizon, and again cost valuable time. Any morning mist or fog over the lake will delay the appearance of the sun until it’s already several degrees above the horizon. It is then that a lofty vantage point may come in handy. According to Milwaukee climate almanacs, fog and haze are not uncommon in June. In fact, Milwaukee averages just 8 cloudless days in June (Madison fairs a bit worse with an average of 7 cloudless days). What about lake haze? I can find no climatological reference that records such a statistic. My best bet would have been to travel to Milwaukee several mornings last June and see for myself. Not having done that, I’m a little short on facts.
The final way to buy more observing time is to travel north. The farther north you go, the earlier the sunrise occurs (if you go as far as the Nunavut province, above the arctic circle, you could observe the whole transit). For every degree of latitude you travel north, the sun will rise approximately 4 minutes earlier. A degree of latitude is approximately 69 miles, so the gain is modest but again, every little bit could be crucial. For example, from Green Bay, sunrise is at about 5:10 am.
So this is where I need your help fellow transit observers:
1. What are your plans for June 8th 2004?
2. Does anyone have any firsthand knowledge of the pros and cons of observing over Lake Michigan early in the morning?
3. What do you think of the idea of observing from the roof of a tall building, either in Madison or Milwaukee?
4. What considerations are most crucial (other than cloudless conditions) for a successful transit observation from Wisconsin?
5. What other observing locations east or north might have bigger upsides?
Please contact me at the address below and let me know your feedback, answers, comments, corrections, or intentions. If reasonable, I’d love to organize an Wisconsin expedition of folks like me who want to observe this historic event, but are stuck here at home for the day. Send me your feedback, I look forward to hearing from you.
(thanks to Neil Robinson for his comments while preparing this piece. Neil plans to capitalize on the altitude variable by observing from an aircraft)
Starry Night Pro simulation of sunrise June 8 2004. Venus' size is exaggerated slightly.
Courtesy Fred Espanak, NASA
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