Frank Evans wrote: > Regarding Fernando Cabral's multiple observation of sunsets at one > setting I recently wrote to the "Marine Observer" (British Met. Office) > concerning a meteorologist who claimed to have seen the sunset green > flash three times at one setting by climbing to successive decks of an > ocean weather ship as the sun sank. This elicitated the following note > to the same journal (Marine Observer, April 1999) from Dr. R. J. > Livesey, Director of the Aurora Section, British Astronomical > Association, whose text I acknowledge: > > "I was in Holyrood Park, Edinburgh, on the lower slopes of Arthur's Seat > with a good clear sky. The sun was setting behind the top of David Hume > tower block of Edinburgh University, which is about a mile away, when I > saw a green flash. Realising the geometry of the situation I ran back > up the slope until the sun was again visible above the tower block. > Again as it set behind the building there was a green flash. I was able > to repeat the phenomenon by a second run up the slope. One was required > to move only so much as would bring the upper limb of the sun just above > the tower block to cause a repeat as the sun sank down."
This surprises me greatly. I thought the green flash was a phenomenon related to the passage of the sun's light through long distances of atmosphere. Can it really be observed when the sun goes behind buildings?! Then it should not be such a rare phenomenon after all. It should also be visible if you move your head so the sun disappear behind the side of a building. I expect myriad reports by tomorrow morning. The vertical motion of the sun at sunset is about omega ~ 1e-4 rad/sec. If the sun is setting behind an object only 1 km away, one must only climb at a leisurely 0.1 m/sec to keep up with it. In contrast, the height required to see the setting sun above a flat horizon is h = R*omega^2*t^2/2 ~ 0.03 m/s^2. If your top climbing speed were 1 m/sec, already at 8 m above sea (horizon) level, you will not be able to keep up any more. If you climbed twice as fast but stopped now and then to observe the flash, which takes 3 or 4 seconds, you cannot have time for more than 3 or 4 observations. I am currently living in San Diego, where bluffs overlook the ocean to the west. I will try the trick myself, when circumstances permit. --Art Carlson
