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Modified explanation for twilight variations across latitudes

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Old October 16th 18, 08:10 PM posted to sci.astro.amateur
Gerald Kelleher
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Default Modified explanation for twilight variations across latitudes

The Polar twilight and its presence for around 6 weeks after the Equinox at the North and South Poles is a separate issue with a separate rotational cause.

The variations is twilight length at habitable latitudes and especially the difference between Equatorial latitudes and those further North occupy the theme and resolution here - the slower the latitudinal speed away from the Equator, the longer the twilight when the Sun comes into view and turns out of sight each 24 hour day.

The present explanations using the angle of the descent of the Sun is so pathetic that those who follow them should be allowed to be left to their own devices and not troubled but neither should they interfere in the proper explanation.

The distance from horizon to horizon or indeed the circumference surrounding an observer, at least at sea level, is only a number of miles so the Sun coming into view at one horizon and disappearing from view at the opposite horizon can be unhelpful when demonstrating the global view of twilight/dawn lengths and their variations across latitudes with different rotational speeds.

As the Sun disappears from view, the astronomer should look in the opposite direction as their location approaches and turns through the circle of illumination. By looking towards the right (as the Northern hemisphere observer faces the Equator ) and ignoring the Sun, the observer can now concentrate on how fast they turn through the circle of illumination. Presently the Moon and Mars provide a wonderful reference as the contrast becomes sharper as the distance of observer and their location to the circle of illumination increases after they pass through the terminator.


It is also why the transition from light to darkness is more rapid at the Equator than at any other location North or South as any person who travels between latitudes will know. The leap from local horizon to global perspective takes a bit of effort but the value is not in the direction of the setting Sun as regards twilight lengths but as a location approaches the fixed circle of illumination daily.

Old October 16th 18, 08:11 PM posted to sci.astro.amateur
Gerald Kelleher
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Posts: 1,382
Default Modified explanation for twilight variations across latitudes

* By looking towards the left (as the Northern hemisphere observer faces the Equator )
Old October 17th 18, 09:32 AM posted to sci.astro.amateur
Gerald Kelleher
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Posts: 1,382
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 -


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..


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