Re: What astromony can do
- From: Quadibloc <jsavard@xxxxxxxxx>
- Date: Wed, 28 Jan 2009 22:14:24 -0800 (PST)
On Jan 28, 10:43 pm, Sam Wormley <sworml...@xxxxxxxxx> wrote:
Sam Wormley wrote:
oriel36 wrote:
I quite enjoyed reading about the magnification hobby and the
chestbeating financial contest between participants here but
personally I prefer Marty's or Barbara's homely observations of the
night sky and rely on images from Hubble,Keck or other large
telescopes to do my own research.
Beyond magnification,which is a valuable facet of astronomy,there is
work to be done such as a closer link between astronomy (specifically
the motions of the Earth)...
Speaking of motions of the earth...
"The Earth rotates about its polar axis once a day and produces an
rotate clockwise.
Oops....
The Earth rotates about its polar axis once a day and produces an
apparent motion on the night sky about the celestial poles. In the
Northern Hemisphere the north celestial pole is elevated above the
horizon. Facing away from the elevated celestial pole, an observer sees
the Sun, Moon, planets, and stars rise in the east and set in the west.
They reach their highest altitude as they cross the local meridian.
When the observer turns to face the elevated celestial pole, stars
nearest the pole neither rise nor set. They become circumpolar and
cross the meridian each day once above the pole at their highest
altitude and once below the pole at their lowest altitude. In the
Northern Hemisphere, circumpolar stars appear to rotate about the north
celestial pole counterclockwise. In the Southern Hemisphere the effect
is reversed and they appear to rotate clockwise.
"Superimposed on the diurnal rotation is an annual rotation caused by
the Earth's orbiting the Sun. Since the stars are seen by the naked eye
after sunset, the constellations appear to move from east to west, and
to return to the same position after a year. Relative to the Sun, the
stars rise and set roughly four minutes earlier each day. In the course
of a month, the night sky appears to move two hours in right ascension
to the west. Also because of this orbital motion of the Earth, the
circumpolar stars in the Northern Hemisphere appear to rotate once a
year in a counterclockwise direction around the north celestial pole
and in a clockwise direction about the south celestial pole.
"The Moon moves in an orbit inclined to the ecliptic by 5.1 degrees;
the Moon makes one revolution about the sky from west to east in about
a month. During this period the phases on the Moon complete a cycle
from new to full and back to new. The orbit of the Moon is moving
around the ecliptic, so that other aspects of the Moon's position in
the sky, such as its maximum and minimum declination, change from one
month the next. It is important to know when the planets are in the
most favorable position for observation. The outer planets, for
example, are best seen around opposition. They are in their least
favorable position around conjunction.
"The inner planets are different--they are in their most favorable
position near greatest elongation, even though they are not at full
phase. At superior conjunction the phase is around full, but the
planets are difficult to see because they are further from Earth and
usually too close to the Sun. At inferior conjunction the inner planets
are nearest to the Earth, but again they are difficult to see because
their phase is small, and they are too close to the Sun.
"Often the times of phenomena need not have any great precision;
sometimes the nearest hour, day, or even the nearest week are
sufficient for observational purposes. The dates and times, however,
usually depend on the coordinate system. For historical reasons the
conjunctions and oppositions of planets have always been calculated in
geocentric ecliptic coordinates. On the other hand, the conjunctions of
planets with other planets, bright stars, or the Moon have always been
calculated using equatorial coordinates; the phenomena are then
observed more easily with an equatorially mounted telescope. In some
cases the times of phenomena have been defined as the maxima or minima
of the distances from the Sun or the Earth or the elongation from
another body. In such cases, the phenomena are independent of the
coordinate system".
---From the Explanatory Supplement To The Astronomical Almanac (1992)
I was wondering about the apparent typo as well; so it turned out to
be a fairly large elision.
This subsequent part of our original quote:
Superimposed on the diurnal rotation is an annual rotation caused by
the Earth's orbiting the Sun. Since the stars are seen by the naked
eye
after sunset, the constellations appear to move from east to west, and
to return to the same position after a year. Relative to the Sun, the
stars rise and set roughly four minutes earlier each day. In the
course
of a month, the night sky appears to move two hours in right ascension
to the west.
could also be taken as explaining the Earth's motions in the Solar
System in precisely the way Gerald Kelleher does, anticipating his
discovery of a "new motion" of the planets.
Given this, it does make me wonder if I'm being too harsh on his point
of view; maybe it is a legitimate (instead of confusing) way of
describing the Solar System. Certainly the conventional way _can_ be
confusing if not done with care - as the controversy with him points
out.
As it seems to be natural to think of the Moon as _not_ rotating,
rather than rotating in synchrony with its orbit around the Earth, it
does seem natural to speak of axial rotation as having a 24 hour
period. But given the tilt of the Earth's axis, such a 24-hour
rotation is a funny bent kind of rotation.
On the surface of Uranus, so favored by him as an example, it's hard
to see the synodic day as even having much practical meaning; during
the portion of the year when a part of the planet is alternately in
light and darkness, the interval between those periods will be the
sidereal day.
John Savard
.
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