(A technical follow-up to earlier material. Skip it if you want to!) On a number of occasions, particularly in my monograph "The Sun's Path at Night," I referred to Rabbeinu Tam's endorsement of the view of the Chachmei Yisrael that the sun, upon setting, changes direction to travel horizontally through the firmament, whereupon it changes direction again, traveling up and behind the sky. Since this is clearly incorrect, some people wanted to know if this means that following Rabbeinu Tam's view - with its ramifications regarding the time of the departure of Shabbos - has no basis.