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From: The 1919 Eclipse on 14 Jan 2010 08:16

What's the equation for the length of day, as a function of latitude
and calendar date?
From: Greg Neill on 14 Jan 2010 08:33
The 1919 Eclipse wrote:
> What's the equation for the length of day, as a function of latitude
> and calendar date?

You can calculate the local times of sunrise and sunset
for a given date, then take the difference between the two.

Keep in mind that there are different definitions for
sunrise and sunset for different disciplines.

Take a look at this:

http://williams.best.vwh.net/sunrise_sunset_algorithm.htm


From: Sam Wormley on 14 Jan 2010 11:55
On 1/14/10 7:16 AM, The 1919 Eclipse wrote:
>
> What's the equation for the length of day, as a function of latitude
> and calendar date?

There's an app for that.
http://emeraldsequoia.com/h/

From: tadchem on 15 Jan 2010 05:39
On Jan 14, 8:16 am, The 1919 Eclipse <eclipse-1...(a)sbcglobal.net>
wrote:
> What's the equation for the length of day, as a function of latitude
> and calendar date?

The length of day also varies with the date:
http://en.wikipedia.org/wiki/Equation_of_time

Tom Davidson
Richmond, VA
From: Mike Dworetsky on 19 Jan 2010 04:40
tadchem wrote:
> On Jan 14, 8:16 am, The 1919 Eclipse <eclipse-1...(a)sbcglobal.net>
> wrote:
>> What's the equation for the length of day, as a function of latitude
>> and calendar date?

You would have to look up the variation of solar declination over the course
of a year. Any formula for this would involve orbital eccentricity and
axial tilt, and take leap years into account.

Given the declination (N or S of the celestial equator) from an almanac or
formula, the hour angle H of rising or setting at some latitude (N +; S -)
is

cos H = -tan (lat) tan (dec)

If cos H comes out with absolute value greater than unity, the sun is
circumpolar and either doesn't rise or doesn't set, depending on
circumstances. Convert answer from radians to hours and multiply by two for
total length of a day.

The above does not take into account horizontal refraction. This typically
lengthens the day by 8 minutes or more. Does the OP need this refinement?

>
> The length of day also varies with the date:
> http://en.wikipedia.org/wiki/Equation_of_time
>
> Tom Davidson
> Richmond, VA

Equation of Time describes the varying difference between apparent noon and
mean noon (or clock noon) caused by the Earth's axial inclination and
orbital eccentricity. The Sun transits earlier or later than noon as read
by a clock. The length of the day is unaffected.

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

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