Re: Astronomers,amateur or otherwise.
- From: Quadibloc <jsavard@xxxxxxxxx>
- Date: Sun, 30 Mar 2008 11:57:40 -0700 (PDT)
On Mar 30, 9:53 am, oriel36 <kelleher.ger...@xxxxxxxxx> wrote:
Axial rotation generates rotational orientation/tilt and does nothing
else so you cannot say that Uranus tilts through a full 360 degrees
with respect to the Sun over an annual orbit ,what you conclude is
that as a location turns through 360 degrees with respect to the
central Sun as part of its orbital motion and leave axial rotation
and orientation seperate.Then you go on to appreciate why there are
enormous hemispherical variations in daylight/darkness on Uranus
while the Earth it is less so,the principles are the same for both.
I agree that the principles are the same for both.
I agree (or assert in disagreement to you, I'm not entirely sure
which) that when the Moon orbits the Earth in about 29 1/2 days,
always keeping the same face to the Earth, astronomers do not call
this "The Moon has no axial rotation", but instead "The Moon has a
period of axial rotation of 29 1/2 days, the same as the period of its
orbital motion around the Earth".
From a naive standpoint, one could say that since the Moon's situationis the natural outcome of tidal friction, and friction slows things
down on Earth, we should call the Moon's situation one of no rotation.
But the idea used by astronomers is to refer all the motions in the
Solar System to an outside perspective, as though the Solar System
were a tabletop orrery, and we measured motions compared to the walls
of the room the table was in. The "fixed stars" supply this
perspective.
Because the convention is to use what you have criticized as
"astrological geometry", we think of the Earth's axis as always
pointing to Polaris - and the axis of Uranus as always pointing to
some other star. And in both cases, these pole stars are to one side
of the perpendicular to the plane of the ecliptic drawn from the
center of the Sun - so we have northern hemisphere (spring and) summer
when the planet is on the opposite side of the Sun from the planet's
pole star, and northern hemisphere (autumn and) winter when it is on
the same side.
Rotation brings night and day. The rotational axis doesn't change or
move - well, not much. Nutation and precession are small and slow
changes which don't affect the year-to-year routine motions of the
planets. Orbital motion brings the year.
And, strictly speaking, night and day come not from rotation alone,
but from the difference between rotation and orbital motion.
This is what makes perfect sense to us, and whatever different way you
want us to look at it seems only to make things more complicated.
John Savard
.
- References:
- Astronomers,amateur or otherwise.
- From: oriel36
- Re: Astronomers,amateur or otherwise.
- From: ukastronomy
- Re: Astronomers,amateur or otherwise.
- From: Chris L Peterson
- Re: Astronomers,amateur or otherwise.
- From: oriel36
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