If one watches the sun set on the horizon, it takes about six minutes
(at least where I live in Northern California) from the time the
lowest point of the sun makes contact with the horizon to the time the
highest point drops below the horizon. Is there a name for this time
measurement, and is it a consistent value based upon the sun's
position in the sky, or the observation point wrt distance from the
equator?

Hi johnwjames

 This question is more complex then it seems.

One term which has something to do with this time interval is

Angular Size

It happens to be about .5 degree doe sun
http://www.studyworksonline.com/cda/content/article/0,,EXP789_NAV2-78_SAR712,00.shtml

and since sun is traveling along the speed of 15 degrees/hour, ir should take

  (.5/15  )  * hour = 2 minutes

to cross a point on sky, when far from the horizon.

Question then is why it is more near horizon, why sun (and moon) look
larger near horizon.

I suspect it has to do with bending of light, which is also causing
other effects during sunset:
http://amos.indiana.edu/library/scripts/greensky.html

making  Sun oval-shaped at sunset
http://www.physics.utoledo.edu/~lsa/_color/12_mirages.htm

This effect  would depend on weather a bit and on the latitude too,
since layers of air would have different temperature profiles.

Here is one video from your backyard - does it look like this?

http://www.goldengatephoto.com/earthsky/diablosunset.html

Looks pretty

Hedgie

---

Clarification of Answer by hedgie-ga on 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:
http://tafkac.org/science/moon_larger_at_horizon.html
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 

  Twilight duration 

which is defined and described here
http://www.jgiesen.de/daylight/
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 ).."

http://www.astro.uu.nl/~strous/AA/en/antwoorden/zonpositie.html

http://hia-iha.nrc-cnrc.gc.ca/sundef_e.html
 
However 

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
.."
http://www2.abc.net.au/science/k2/stn/november1999/posts/topic8738.shtm

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. .."

http://www.astro.uu.nl/~strous/AA/en/antwoorden/zonpositie.html


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...." 
in

http://mintaka.sdsu.edu/GF/papers/Zenit/GF.html



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.

Hedgie

---

Clarification of Answer by hedgie-ga on 31 May 2005 17:01 PDT

John

       You asked: 

1) Is there a name for this time measurement, 
2) observation point wrt distance from the equator?
answer says
1)  Sunset Duration  (which is the term sought)
   also called:    apparent duration of sunset 

2) yes, it depends on latitude via factor 1/cos(lambda)
 "..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"

I do not argue with your rating, but wonder if you did read whole answer.
Perhaps it was too verbose?

Interesting info. No answer to the question posed, but interesting
info all the same.

The answer to this question may be of interest to you:
http://answers.google.com/answers/threadview?id=495777

It doesn't give a name for the period you mention.  I believe this can
very due to differences in the refraction.

>Question then is why it is more near horizon, why sun (and moon) look
larger near horizon.

Conventional wisdom is that it's an optical illusion - high in the
sky, with nothing to compare them to, they appear smaller.  I've never
tried this, but I'm told that if you bend forward and view the moon
through your legs it destroys the illusion (honestly!).

Ian G.

Hedgie-ga, that was great.
About 1/3 of the way down your answer, the quotation explains
(indirectly) the very significant factor about the duration of the sun
or moon's setting.  It is not setting perpendicular to the horizon, as
it does at the equator, where it disappears in 2 min 8 sec  - the
"interval" (barring the effect of refraction). Rather it is sloping
down, and then also extending the time that the influence of
refraction occurs. Hadn't thought about that before.  Thanks.