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Old October 17th 18, 09:32 AM posted to sci.astro.amateur
Gerald Kelleher
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Default Modified explanation for twilight variations across latitudes

Twilight duration variations across latitudes is important as it draws attention to the changing relationship of latitudes to the orbital plane and the central Sun as moves away from the unhelpful 'tilt' which is actually fixed inclination.

The variations in the orientation of the Earth's Equatorial latitude to the orbital plane and central Sun explains why day/night length (minus the orbital variation) is always equal regardless of where the Earth is in its orbit -

http://afewbitsmore.com/img/2015_ecliptic.png

While the originator of that graphic got the orientations for the Equinoxes reversed that is only a small matter and one I will correct in future, it does accurately show how the orientation of the Equator bows to the orbital plane whereas on the Equinoxes, the differential is made up of a direct motion across the full illuminated face of the Earth.

A sufficiently developed astronomer would leave behind local observations of horizon to horizon and take the leap to translate local observations into global observations. This allows astronomers to explain twilight duration variations across latitudes to local latitudinal speeds rather than ridiculous notions of the 'Sun's angle of descent'.

Polar twilight at the North/South Poles is a separate issue and cause insofar as the 6 week twilight since the September Equinox at the North Pole is due to a distinct and separate rotation of the Earth, originally Copernicus got it right but jettisoned the explanation due to technical issues he faced in his era -

" The third is the motion in declination. For, the axis of the daily rotation is not parallel to the Grand Orb's axis, but is inclined [to it at an angle that intercepts] a portion of a circumference, in our time about 23 1/2°. Therefore, while the earth's center always remains in the plane of the ecliptic, that is, in the circumference of a circle of the Grand Orb, the earth's poles rotate, both of them describing small circles about centers [lying on a line that moves] parallel to the Grand Orb's axis. " Copernicus, circa 1514

Of course all this wouldn't appeal to those who can be bullied into silence but for everyone else, the facts are there to be developed and appreciated..