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From: Henry Spencer <>
Subject: Re: Why Keep Mir Alive
Date: Wed, 25 Feb 1998 17:16:08 GMT

In article <>,
Jacob McGuire  <> wrote:
>>>Orbital spaceport for what purpose?
>>assembly in LEO of planetary missions or GEO missions.
>  Assembly of planetary missions in LEO is very questionable.

No, actually it's quite reasonable, although it takes careful planning,
and it's not obvious that an existing space station is the right place
to do it, especially one at a high inclination.

>Especially in a high-inclination LEO.  Launching from high-inclination
>orbits incurs a severe mass penalty, and phasing requirements will be a
>bear to meet.

Launching from high-inclination orbits incurs no special penalty; it's
launching *into* high-inclination orbits that is costly.  Of course, this
distinction is unimportant when the hardware starts out on the ground,
since it has to go through both of those processes...

Phasing requirements do take some planning ahead, and possibly some orbit
alterations well ahead of time.  When you've got years to set things up,
it doesn't take a big change to get the orbit plane in the right place at
the right time.  (For those who aren't following this, the issue is as
follows, simplifying slightly.  Your choice of interplanetary trajectory
dictates the departure direction.  You want that direction to be in the
orbit plane at departure time, so that there's some point on the orbit
where the orbital velocity points in that direction, so that the orbital
velocity counts as part of the velocity needed for the interplanetary
trajectory.  Because a low Earth orbit's plane precesses a few degrees per
day westward, this takes careful timing, but you can adjust the precession
rate a little by adjusting the orbital altitude.)
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         |

From: (Henry Spencer)
Subject: Re: Precession
Date: Sat, 27 Jun 1998 00:37:38 GMT

In article <35932fc9.48193817@news>,
Bond, James Bond <> wrote:
>Recent discussion regarding "tilting" of Earth drifted through this
>I know it is an established (?) principle but, has anyone actually
>observed evidence of "precession" (stereographic plates, etc.) or is
>it just part of the "standard model"?

Precession was observed long before anyone could explain it.  You don't
even need a telescope, never mind photographic plates; it was discovered
by Hipparchus circa 125 BC, when he noticed systematic discrepancies
between star positions as he measured them and positions given in a star
catalog from about 150 years earlier.

>Seems like it would be next to impossible to observe from ground base.

Precession moves the equinoxes just about an arcminute per year.  This may
not sound like much, but really good naked-eye instruments like Tycho
Brahe's could measure star positions to a fraction of an arcminute.  Mind
you, Tycho's instruments were of unprecedented size and quality, and the
average standard of observational astronomy before him was much lower, but
if you're willing to wait a decade or two between observations, it's
impossible to miss the motion.  Hipparchus was seeing a discrepancy of
about two degrees after 150 years, and for that you don't even need
instruments -- just a few reasonably permanent markers to sight on, and
some systematic record-keeping.  Your thumb, held at arm's length, is
about a degree wide.

(By the way, the Hipparcos satellite -- named after guess who -- measured
a very large number of star positions to about 1/1000 of an arc*second*.
Precession is something that all precise astronomical observations have
to allow for nowadays; there's nothing at all uncertain about it.)

>My understanding (limited, as it may be) is that the rotating Earth
>wobbles, much like a top slowing down, and that this wobble causes the
>axis of rotation to describe an arc in the celestial sphere.

Basically correct.  In the case of the top, the necessary perturbing
force is supplied by Earth's gravity.  In the case of the Earth, it's
the gravity of the Moon, and to a lesser extent the Sun, and to a still
lesser extent the other planets, acting on the Earth's equatorial bulge.
It was Newton who first understood this.
Being the last man on the Moon is a |  Henry Spencer
very dubious honor. -- Gene Cernan  |      (aka

From: (Henry Spencer)
Subject: Re: Precession of the ecliptic
Date: Mon, 31 Jul 2000 15:38:19 GMT

In article <>,
Mark  <mlm1023@yahoo.NS> wrote:
>Just as the Earth has a pronounced wobble in it's axis of revolution,
>is it possible that there is similar precession in the plane of the
>ecliptic.  That is, is the solar system "wobbling" from the
>perspective of the rest of the Milky Way?


The precession of Earth's axis is due to lunar and solar gravity acting on
Earth's equatorial bulge.  There is no equivalent nearby body to precess
the solar system (such effects fall off *very* rapidly with distance).

There is a slow precession of the ecliptic, in fact, but that is caused by
perturbations due to the other planets, and there are counterbalancing
precessions of *their* orbital planes.  The "invariable plane" of the
solar system, determined by the angular-momentum vector of the whole
system, is (last I heard) thought to be fixed in space to the limits of

(There have been suggestions that the invariable plane would make a better
reference for solar-system coordinates than the ecliptic, which is *known*
to be precessing... but the position of the invariable plane cannot be
measured directly, and it is not known precisely enough to make a good

>Could this account for some
>of the observed proper motion of stars calculated to this point?

No.  This would produce a systematic pattern of proper motions spread over
the sky, which would have been obvious to astronomers long ago.
Microsoft shouldn't be broken up.       |  Henry Spencer
It should be shut down.  -- Phil Agre   |      (aka

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