John Carmichael wrote: > >My new Celestial Products moon calendar says that there will be a total >lunar eclipse at 4:41 UT on January 21 (the evening of Jan. 20th in North >America). > >We have already discussed on the list the fact that moonlight should >indicate the correct time on a sundial during totality if corrected for EOT >and longitude and if its light is bright enough to cast a shadow. But what >happens to the declination readings on a sundial during a lunar eclipse? >I'm thinking that the shadow of the nodus should indicate a date which is >exactly six months from the date of the eclipse. In this case a sundial >with declination lines would show the date to be July 21. Is my theory >correct about this?
You are correct for the date but because the Earth's shadow is approximately 4 times the moons diameter there can be quite a variation in declination of the moon even during a total lunar eclipse. I haven't checked the tables or programs for this one. > >Those of you who have sundials with declination lines might want to check >this out during totality. I'm also curious if there will be enough >moonlight to even see a shadow. > During the total eclipse no. With the full moon on the the Swensen sundial I can really see the shadow but I can photograph it during the summer when the moon is low in the sky (the Swensen dial is vertical). You can see this at http://www.uwrf.edu/sundial/images.html I'm sorry that this page contains quite a few images so it'll take a few minutes to download on a regular modem. The last image is one taken by Dick Swensen after he saw one taken by me. I put his in because his wife had the great idea of including the North star. Even though this is only a week after the Summer Solstice the moons shadow is noticbly below the solstice line due to its declination being of from the sun's at the winter solstice. The exposure is about 15 seconds at f 2.0 with 200 ASA film. I nearly froze to death at about 0 F trying to take a similar picture at the winter solstice when the moon is very high. The light intensity on the vertical wall was too feeble to show up even with a 30 second exposure. Of course you cant go much longer as the moon moves approximately like the sun, 1 degree every 4 minutes. Cheers, John Professor John P.G.Shepherd Physics Department University of Wisconsin-River Falls 410 S. 3rd. St. River Falls,WI 54022 Phone (715)-425-3196, eve. (715)-425-6203 Fax (715)-425-0652