Twilight length

On 2/28/10 8:15 AM, oriel36 wrote:


Seasonal Variations of the Duration of Sunrise and Sunset

Definitions:

duration of sunrise = time elapsed from the first appearance at the
horizon of the upper limb of the rising Sun, until the first moment when
the entire solar disk is visible

duration of sunset = time elapsed from the last moment that the entire
solar disk is visible until the moment of the last appearance of the
upper limb of the setting Sun at the horizon

Variations of the duration of sunrise and sunset are related to the
duration of twilight and depend mainly on the angle made by the path of
the rising or setting Sun with respect to the horizon, known as the
solar parallactic angle. Although Sun rises at the true east direction
and sets at the true west direction on the day of an equinox, on that
day the solar parallactic angle would be 90° only at the equator.

The solar parallactic angle can only be 90° on days when the solar
declination equals the observer's geographic latitude, which occurs
twice per year for locales between the Tropic of Cancer and Tropic of
Capricorn, but never occurs at latitudes further north or south of the
tropics. Locales that are on the Tropic of Cancer see a 90° solar
parallactic angle only when sunset or sunrise occurs near the moment of
the north solstice, and locales that are on the Tropic of Capricorn see
a 90° solar parallactic angle only when sunset or sunrise occurs near
the moment of the south solstice.