sundial  

Re: Longest day calculations

Willy Leenders
Thu, 01 Jul 2010 13:58:33 -0700

Using the formula of John Schilke I become 244,74 hours (above the horizon), for latitude 51° and declination 23,44°
Using my formula (see picture) I become 16,32 hours.


Willy LEENDERS
Hasselt in Flanders (Belgium)

Visit my website on the sundials in the province of Limburg in Flanders (Belgium) and on worthwhile facts about sundials


Op 1-jul-2010, om 22:39 heeft John Schilke het volgende geschreven:


On 2010 July 1, at 10:12, John Goodman wrote:
I'm looking for a simple formula which calculates the number of  
hours that the sun will be above the horizon on the summer solstice  
for any given latitude. I don't need to worry about refraction, or  
take into account the sun's diameter. I'm just interested in the  
simple geometric case, using the center of the sun's disk for  
determining the start and end times.
For my purposes, these simple relationships will be true: If the  
answer for summer solstice hours is SSH, then winter solstice hours  
will be 24 - SSH, and equinox hours will be 12, regardless of the  
latitude.

John,

It would seem simplest to use the old formula

cos T = – tan φ tan δ
(latitude and declination, respectively)

then, H (hours above the horizon) = 2 arc cos T, approximately.

That is, in fact, how I usually do it, quickly and easily.

Your other statements are correct.

Best wishes,

John
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