NavList:

   

Reply

It has long been known that under Arctic conditions, extreme values of 
refraction can occur. Nicolas de Hilster has drawn our attention to 
observations by Barents' navigator, de Veer, forced to overwinter in Novaya 
Zemlya in 1513.

Here are some occurrences related by Shackleton in "South", from Endurance, 
trapped in ice in the Antarctic: terminally so, as it turned out.

They start in April 1915-

"Worsley was in the crow's-nest on the evening of the 15th, watching for
signs of land to the westward, and he reported an interesting phenomenon.
The sun set amid a glow of prismatic colours on a line of clouds just above
the horizon. A minute later Worsley saw a golden glow, which expanded as he
watched it, and presently the sun appeared again and rose a semi�-diameter
clear above the western horizon. He hailed Crean, who from a position on the
floe 90 ft. below the crow's-nest also saw the re-born sun. A quarter of
an hour later from the deck Worsley saw the sun set a second time. This
strange phenomenon was due .to mirage or refraction. We attributed it to an
ice-crack to. the westward, where the band of open water had heated a column
of air."
............

"We said good-bye to the sun on May 1 and entered the period of twilight 
that
would be followed by the darkness of midwinter. The sun by the aid of
refraction just cleared the horizon at noon and set shortly before 2 p.m. A
fine aurora in the evening was dimmed by the full moon, which had risen on
April 27 and would not set again until May 6."

.........................

"The ship's position on Sunday, May 2, was lat 75� 23'S., long. 42� 14'W."

.........................

"The sun, which had made "positively his last appearance" seven days 
earlier,
surprised us by lifting more than half its disk above the horizon on May 8.
A glow on the northern horizon resolved itself into the sun at 11 a.m. that
day. A quarter of an hour later the unseasonable visitor disappeared again,
only to rise again at 11.40 a.m., set at 1 p.m., rise at 1.10 p.m., and set
lingeringly at 1.20 p.m. These curious phenomena were due to refraction,
which amounted to 2� 37' at 1.20 p.m. The temperature was 15� below zero
Fahr., and we calculated that the refraction was 2� above normal. In other
words, the sun was visible 120 miles farther south than the refraction 
tables
gave it any right to be. The navigating officer naturally was aggrieved. He
had informed all hands on May 1 that they would not see the sun again for
seventy days, and now had to endure the jeers of friends who affected to
believe that his observations were inaccurate by a. few degrees."

==============================

de Veer's observation (the Novaya Zemlya effect) would amount to a 
refraction of over 5�, and his account was widely disbelieved for many 
years, but has been rehabilitated in more recent times. Some polar observing 
stations now regularly record when refraction is over 2�, which is not 
particularly unusual; though I doubt if any have reached de Veer's value.

It's rather easy to imagine how such effects can be caused under Arctic 
conditions, of prolonged night. In clear weather, heat from the Earth's 
surface radiates away into space, with no compensating input from the Sun. 
Where there's water, convection within the liquid can replace that heat lost 
at the surface. As soon as ice forms, that ice-body remains trapped at the 
surface, and can only get heat from the ocean below by means of conduction 
through the solid ice: a much less effective process, particarly as the ice 
thickens. So then, the surface temperature of that ice can get extremely 
cold, and under windless conditions can cool the layer of air just above it, 
correspondingly. This gives rise to a powerful temperature inversion, where 
the lower air becomes much colder, and correspondingly denser, than the air 
above. That gradient causes light to bend, tending to follow the Earth's 
curved surface, rather than being nearly a straight-line tangent to it, 
allowing an observer to see much further than usual around the curve of the 
Earth..

The required conditions would then be-

Prolonged Arctic night.
Clear skies.
Over land-ice, or thickish sea-ice.
Windless conditions.

And the effect would become much more noticeable around the times of 
appearance, or disappearance, of the Sun.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.






--~--~---------~--~----~------------~-------~--~----~
NavList message boards: www.fer3.com/arc
Or post by email to: NavList@fer3.com
To , email NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply