> Long ago, in the days of North Atlantic weather ships, a meteorologist > told me that he had managed to see the green flash at sunset twice at > one setting. He did this first from the main deck of the weather ship > and then scampered up to the upper deck to see it again. I imagine that > the green flash presents a more precise instant of time than the > terminator and hence it would be possible to measure the precise > interval between the two events and also the precise distance between > the decks. A bit difficult to organise, though, although the conditions > for a possible green flash are well known. > Frank 55N 1W
Yes it is indeed possible to see a green flash more than once in one setting. I'm fortunate in having a low western (land) horizon and one one memorably occasion managed to observe a green flash at ground level and then run upstairs to see it again - the stuff of heart attacks! However, it is questionable whether green flash timings would be a reliable source of measurement. The time that the last sliver of sun disappears is quite dependent on local atmospheric conditions and therefore the magnitude of atmospheric refraction. Detailed studies have been made of actual sunset times versus astronomical predictions under fairly normal conditions and correlations obtained with balloon measured local atmospheric temperature profiles. The differences between observed and predicted can be significant. A traditional (inferior mirage type) green flash requires a warm air layer near the earth's surface (ocean or land) coupled with rather strong vertical temperature gradients. These conditions are more extreme and can be even more variable and unpredictable. But for all that, green flashes are sights of a lifetime! Les Cowley Atmospheric Optics - www.atoptics.co.uk --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
