Clarification of Answer by
09 May 2005 22:56 PDT
Why is the 'moon larger' question, mentioned in comment by ing, is a FAQ,
and concensus is that it is an optical illusion,
which disppears when distance cues are eliminated:
by bending, or using a tube (which is much easier :-)
If that's true and only reason, that would NOT explain the
discrepancy of 6 minutes observed vs about two minutes theoretical value for
Sunset Duration (which is the term sought)
also called: apparent duration of sunset
which is not to be confused with
which is defined and described here
and which can last 20-70 minutes.
Twilight refers to the light which appears
before sun becomes visible (or after it sets).
Both durations depend on the latitude, but Sunset Duration should not be more
then four minutes, as discussed in many quantitative detail here:
"..For example, at a latitude of 40 degrees (either North or South),
the fastest sunset takes about 128/cos(40°) = 167 seconds (2 minutes
47 seconds), and the slowest one about 142/cos(1.14*40°) = 203 seconds
(3 minutes 23 seconds). At a latitude of 50 degrees, the sunset lasts
approximately between 199 and 261 seoonds (3 minutes 19 seconds and 4
minutes 21 seconds). At the equator, the sunset lasts between about
128 and 142 seconds (2 minutes 8 seconds ).."
The effect of light bending acting as a magnifing lens, as mentioned here
Subject: re: Moon larger near the horizon thing
"In my humble opinion the moon apears larger due to refraction, As any
diver knows items veiwed under water apear 25% larger. The atmosphere
also refracts light although to a lesser extent. Therefore while the
moon is low on the horizon you are veiwing it through much more air
then when it is overhead thus more air ,more refraction, Larger moon
cannot be completely discounted even though it is denied here:
"..The duration of sunset and sunrise is independent of the refraction
by the atmosphere that slightly lifts up things near the horizon so
that they appear to be higher in the sky than they would have done
without any refraction, because you compare two instants of time when
different parts of the Sun are at the same altitude in the sky. The
same altitude means that the refraction is equally strong (except if
the conditions of the atmosphere in that direction have changed in the
meantime), so both instants are delayed by the same amount and their
difference remains the same. .."
So, in conclusion,
I would attribute the 6 minute duration to the mix of the optical effects,
such as Tyndall cone (scattering), stool and green flash:
Light from the setting sun, particularly on a hazy day, has to pass through
a thick, nonhomogenous and oftern turbid layers of air, which will separate
wawelengths (green flash) and distort the shape of the image (stool).
The different shapes one can see, when obesrving sunset. are shown on
the computer simulation:
"The [[transfer-curve]] graphics following the simulations illustrate
how light of these two wavelengths are refracted differently by the
atmosphere; the true altitude above the horizon is plotted against the
apparent altitude. The divisions on the axes are in arc minutes...."
So: the point in time when 'sun disappeared' is not well defined
and depends on the color and intensity of the light.
The 'sunset duration' is not sharply differentiated from different
degrees of the twilight duration. Hence certain scatter of data
obained by subjective observations is to be expected.