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Re: Venus
From: Frank Reed CT
Date: 2005 Dec 11, 23:04 EST
From: Frank Reed CT
Date: 2005 Dec 11, 23:04 EST
George H, you wrote: "I hope Frank will back me in warning sky viewers about the dangers of peering so near to the Sun." Yep. If you project the Sun onto a sheet of paper with a typical six-inch reflector and hold a piece of paper --even white paper-- right at the normal distance where you would place your eye, the paper ignites in a fraction of a second. I wonder how long it takes to ignite an eyelid? And: "Ideally, it should have the ability to be locked to an automatic motor-drive, as I imagine was the case with the instrument that Frank used." No operating clock drive the few times I've done this, but the axes on most equatorial mounts can be clamped, and it's wise to do that when observing close to the Sun. "As we have discussed before, the telescope doesn't make a Sun image intrinsically brighter; only bigger, on the retina. Nevertheless, the brightness of any Sun image on the retina is enough to quickly cause irreverseable damage. Be warned!" I agree with that. Isn't that a bit of a change of heart for you? I thought that you had previously argued that this fact that surface brightness can't be changed by an optical system (which is very true) meant that the threat of eye damage from looking at the Sun through a telescope (specifically a sextant telescope, in previous discussions) was no greater than the threat of eye damage when looking at the Sun for the same period of time with the naked eye. There's another approach to observing Venus near inferior conjunction that reduces the risk quite a bit. Set up the telescope near the edge of the shadow of a house or other large object. Aim at the Sun, as before by projecting its image, to get the instrument aligned. Set the setting circles to the Sun's RA and Dec. Wait 20 minutes, or as long as it takes for the Sun to pass behind the house so that the telescope is now completely shaded. Offset to the RA and Dec of Venus adding those 20 minutes, or however many minutes, you've waited (assuming the clock drive hasn't been running). Of course, modern computer driven telescopes can do all this with the click of a button, but where's the challenge in that??! So that you have some evidence that other people do this, too, I went hunting for examples of inferior conjunction observations of Venus on the web. Here's a nice account from a guy in Australia: "On the morning of August 20th early cloud cleared at about 11am to a sparkling pre-Spring day. Using the shadow line at the edge of my house, I carefully blocked out the Sun (aware of the 3,000 in Britain reported suffering from vision problems after the August 11th solar eclipse). Using extended fingers to mark degrees and natural markers on guttering I was soon able to explore the right hunting area. Five minutes brought no success. Then I noticed the rising gibbous Moon and took a look. My binoculars were out of focus! Back to the search. Within minutes the silver glowing eyelash of the crescent Venus jumped out at me. Carefully, I moved away the glasses and after several attempts I just glimpsed the planet with the unaided eye. This was Venus within about 12 hours of Inferior Conjunction. Leaving my seat in position I easily found the planet again the following day. " That "silver glowing eyelash" is a nice description. But best of all, since Venus has a thick refracting atmosphere, the crescent of Venus extends beyond the usual 180 degrees from cusp to cusp when it's very close to inferior conjunction. It's a unique sight. [complete URL for the above quotation: _http://www.asnsw.com/universe/1999/cvas-1099.htm_ (http://www.asnsw.com/universe/1999/cvas-1099.htm) the site includes some excellent advice on daylight Venus hunting] -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars