'Pole Stars' on the Moon?

Peter Hayes wrote:

The Earth's Pole Star changes over 25,000 years,
Polaris - Vega - [somewhere else I can't remember].

The phenomenon is called "precession".

Does this happen with the moon, "in sympathy" as it were?


Good question! Since the Earth's precession is caused partly by
interactions with the Moon one would expect there to be some sort of
reciprocal effect. I don't know how thoroughly the Moon's rotational
behaviour has been characterized, though; in order to gather the
necessary data I think we'd need an astrometric telescope stationed
there over a period of years.

--
Odysseus

"Peter Hayes" wrote in message
...
John Zinni wrote:

"JimO" wrote in message
.. .
Great. And could you tell me the south polar region in the sky?

Hey Jim

South Pole is about 2 deg away from Delta Doradus, 1 deg away from
Epsilon
Doradus.

Epsilon Doradus is roughly between South Pole and South Ecliptic Pole.

The Earth's Pole Star changes over 25,000 years,
Polaris - Vega - [somewhere else I can't remember].

Does this happen with the moon, "in sympathy" as it were?

It would appear that it does in a very specific way and on a much quicker
time scale than I would have expected.

The Moon is locked into more than just its 1:1 spin-orbit resonance. Its
spin axis is also locked into a relationship with its orbital plane and the
plane of the ecliptic.

The motion of the Moon was summed up in a collection of laws collectively
known as "The Cassini Laws."
see: http://scienceworld.wolfram.com/phys...ssiniLaws.html

They a

1. The rotation rate is synchronous with the mean orbital rate.
2. The spin axis maintains a constant inclination to the ecliptic plane.
3. The spin axis, orbit normal, and ecliptic normal remain coplanar.

Law 1 is a restatement of the 1:1 spin-orbit resonance.

Law 2 states that at any given time the spin axis of the Moon will be
somewhere on a cone centered around a normal to the ecliptic plane.

Law 3 ... Picture a normal to the ecliptic plane and originating from the
same point, a normal to the orbital plane of the Moon. These two vectors
will describe a third plane that is perpendicular to the first two. Law 3
states that the spin axis of the Moon will be in this third plane. The
orbital plane of the Moon presesses with respect to the ecliptic in a 18.6
year cycle, which means that the spin axis of the Moon must also presess
with respect to the ecliptic with this same period. I have not quite wrapped
my head around whether the Moons spin axis presesses with respect to its own
orbital plane around the Earth, but I think it must be fixed.

JRS: In article , seen in
news:uk.sci.astronomy, JimO posted at Thu,
11 Dec 2003 19:09:25 :-

OK, I thought I was Internet-literate and could google with the best of 'em,
but after half an hour of dead-endism, I've given up. Please help!

For an article i'm writing, I need the RA/Dec of the Moon's axis
of rotation, so I can check out whether there are any 'pole stars' of note.

The axis is within about a degree of perpendicular to the ecliptic, if that
helps.

Since the ecliptic is a plane, that defines the axis nearly enough.

A low-resolution chart shows the Ecliptic nearest to our N Pole at about
6h, 67 deg away; so the N Lunar pole must be under 18h 23 deg away, in
Draco (!) near 6543 whatever that is. The S Lunar pole must be under 6h
23 deg from our S pole, in Dorado on the edge of the LMC. No doubt a
better chart would help - but there seem to be no stars nearly as good
as Polaris nearby (less important, given Lunar weather).

It would be of interest to have, on a star chart display, the positions
representing the rotation axes of major "bodies" - Sun, Moon, Earth,
Mars, etc., Solar System, Galaxy - and lines representing their present
motion or future path, if any known. The corresponding equators might
optionally appear.

Programs showing the positions of such at different times must already
"know" the axes.

--
© John Stockton, Surrey, UK. Turnpike v4.00 MIME. ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
some Astro stuff via astro.htm, gravity0.htm; quotes.htm; pascal.htm; &c, &c.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.

"JimO" wrote in
:


OK, I thought I was Internet-literate and could google with the best
of 'em, but after half an hour of dead-endism, I've given up. Please
help!

For an article i'm writing, I need the RA/Dec of the Moon's axis
of rotation, so I can check out whether there are any 'pole stars' of
note.

The axis is within about a degree of perpendicular to the ecliptic, if
that helps.

Jim O

www.jamesoberg.com


The moon's axis is titled (inclination) 1°32'32" (1.5422°) to the ecliptic
pole. However this axis precesses in an 18.6 year cycle tracing a circle
around the ecliptic pole 3°05'04" (3.0844°) in diameter.

You may determine the instantanious position of the lunar pole on this
circle by noting that the noth pole of the moon is always tilted in the
direction 90 degrees from the north node.

Here's an example for today 2003 Dec 19 00:00UT

Mean node node 48.3833 ecliptic longitude
Hence north lunar pole direction
is 48.3833 + 90 = 138.3833 ecliptic longitude
and 90 - 1.5422 = 88.4577 ecliptic latitude

From these we derive:
RA 17h47m56s
Dec +67°33'23" (+67.5563)

Martin Lewicki

North node