Gordon Simons wrote:
> Some idle questions:
>
> 1. What is the length of daytime -- time between sunrise and sunset -- for
> someone living on the equator?
>
> 2. Does it stay precisely the same year round?
>
> 3. Is the length of daylight slightly perturbed by the revolving of the
> earth around the sun?
>
> I would guess that daytime at the equator is about 12 hours year round but
> is actually a couple of minutes longer than 12 hours because of the
> earth's orbital travel.
>
> What do the experts say?
1. 12 hours - ignoring effects due to twilight, the fact that the sun isn't a
point source, & the effects noted under 3.
2. At the equator, yes - again with the noted qualifications.
3. The solar day - the time between two successive transits of the sun across
the meridian - differs from the sidereal day - the time between two successive
transits of a "fixed" star. This is because the orbital motion of the earth
makes the sun appear to move relative to the fixed stars by ~1 degree a day.
Thus the difference between the two days is about 24 hours/360 ~ 4 minutes.
But since the earth's orbit is elliptical & its orbital speed varies, the
length of the solar day varies slightly with respect to the sidereal day. For
ordinary timekeeping the solar day is averaged over the year to give mean
solar time.
Shalom,
George
George L. Murphy
http://web.raex.com/~gmurphy/
"The Science-Theology Interface"
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