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Q: Term needed: the sun fully passing a single point on the horizon ( Answered ,   3 Comments )
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 Subject: Term needed: the sun fully passing a single point on the horizon Category: Science > Astronomy Asked by: johnwjames5-ga List Price: \$3.00 Posted: 07 May 2005 23:19 PDT Expires: 06 Jun 2005 23:19 PDT Question ID: 519089
 ```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?```
 Subject: Re: Term needed: the sun fully passing a single point on the horizon Answered By: hedgie-ga on 09 May 2005 11:00 PDT Rated:
 ```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?```
 johnwjames5-ga rated this answer: ```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.```