Re: Happy equinox to all


canopus56 wrote:
Florian wrote:
Well, to quibble, equinoxes are an instant in time. They don't really begin/end. But fall began in the north and spring in the south! Happy equinox Canopus56!! ;-)

True enough, you have to make a distinction between astronomical
autumnal equinox that occurs when the Sun reaches ecliptic longitude
270 degrees and the local "civil" equinoxes that occur at local noon
for the following 24 hours.


I like your misplaced geocentric courage and good for you.

The Earth did not tilt towards the Sun today to cause the Equinox,the
change in orbital orientation of the Earth split the poles in two. As
the polar axis is determined by terrestial rotation you can enjoy the
Equinox event as a local effect between axial and orbital orientations
but be sure to gauge what is moving and what is not.



Autumnal equinox - Sundial gnomons only trace a straight line between
10 and 4 on equinoxes, here animated between Sept. 23, 2006 between
12:24MDT and 2:01MDT

http://www.utahastronomy.com/view_photo.php?set_albumName=album40&id=20060923SDialAnimate
(234kb)

in

http://www.utahastronomy.com/view_album.php?set_albumName=kurt

This business of tracing the arc of a shadow as an accurate
representation of axial and orbital motion has to cease.The
determination of local noon due to the rotation of the Earth occurs on
the same line with each rotation (no such things as an analemma).the
variation in lengths of the shadow at that moment represent the orbital
orientation of the Earth and certainly not the 'tilt' of the Earth.

There is really no big deal with determining that the return of a
shadow to local noon has nothing to do with the arc of a shadow but
rather the pace of the shadow back to noon representing the total
length of a day.The change in this pace is due to Keplerian orbital
geometry and the changing rate of orbital orientation.

Pity I cannot discuss this with somebody as courageous as you.

.

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