Hello,
An interesting article, _Prolonging the Green Flash_ in the May, '97
issue of Sky & Telescope, speaks to the issue of witnessed event time
and the elevation(s) of the observer(s).
Enjoy,
Luke Coletti
Roger Bailey wrote:
>
> Hello Frank and all,
>
> I am surprised that no-one has yet give a definitive answer to your puzzle.
> The clues have all been posted. The dip correction (0.97 * sqrt h)gives the
> change in altitude with height. The path of the setting sun is described by
> the angle Psi( where Cos Psi = Sin Lat / Cos Dec). From these two
> relationships and some guesses on your friends location, I conclude that he
> had not just 4, but over 7.5 seconds to climb to the next deck. His feat of
> witnessing the instant of sunset three times in one evening now becomes
> plausible, if not quite credible.
>
> The weather ship that he was on must have been a typical coast guard ship.
> I have assumed decks at 16, 25 and 36 feet above the water line. This makes
> the square roots easy (4, 5 and 6). Approximating the 0.97 factor in the
> dip equation as ~1.0, the difference in dip correction between the decks is
> 1 minute of altitude. If the sun sank straight down as it does at the
> equator, this would give the time angle of 4 seconds per deck as you
> mentioned.
>
> But this ship was not likely to be on the equator, but somewhere near the
> British Isles. I am assuming a latitude of 55 degrees and a summer solar
> declination of 15 degrees. Solving Cos Psi = Sin Lat / Cos Dec with these
> inputs gives the angle Psi as ~32 degrees. This is the angle the sun's path
> makes with the horizon as it crosses at sunset. From simple geometry of the
> triangle with the altitude of 4 seconds, the hypotenuse (the suns's path)
> is 7.5 seconds. (4 sec / sin 32). This would allow sufficient time to climb
> to the next deck but little time for viewing the green flash phenomenon.
>
> The green flash is a new tangent to go off on. Jules Verne used it for the
> story line in his novel "Le Rayon Vert". The physics, although complex, are
> now better understood. I have enjoyed many Great Lakes and ocean sunsets
> but I must confess to never consciously observing the phenomenon. When I
> first heard of it while vacationing in Hawaii, I dismissed it as an urban
> legend. Now I know what to look for.
>
> A web search on "green flash" will bring up lots of interesting URLs. A
> couple are: <http://pw2.netcom.com/~flzhgn/grnray.htm> for pictures, or
> <http://mintaka.sdsu.edu/GF/index.html> for the Green Flash Home Page.
>
> How many of you have seen the phenomenon?
>
> Roger Bailey
> Walking Shadow Designs
> 51 N 115 W
>
> (and surrounded by the Rocky Mountains which give local "sunsets' when the
> solar altitude is 13 to 17 degrees)
>
> At 03:55 PM 1/30/99 +0000, you wrote:
> >Hi John and others in the golden west,
> >
> >You make the case that the sun truly sets earlier than it appears to do.
> >If, as you say, 34 minutes of arc represents 2.26 minutes of delayed
> >sunset time due to refraction, we are getting into seconds. In that
> >case we must do what the Nautical Almanac does not, and consider the
> >height of the observer's eye.
> >
> >The late Mr. E.W. Barlow, a meteorologist in the British Met. Office
> >once told me that while serving aboard one of the ocean weather ships he
> >saw the green flash (visible under very clear conditions at the moment
> >of sunset) three times at a single setting. This he said he achieved by
> >moving to successively higher decks as the sun went down.
> >
> >Pondering this, I have reckoned that if the decks were, say, ten feet
> >apart he would have about four seconds to move from one deck to the
> >next. I wonder if, being prepared, such a feat would be possible and
> >whether anyone else has ever attempted it. Or was he just hauling my
> >leg?
> >Frank
> >
> >
> >--
> >Frank Evans
> >
