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From: (JamesOberg)
Subject: Re: Nodal Regression?
Date: 21 Feb 1998 22:07:41 GMT

<<They are unknown to me.  What do they mean?>>

Nodal regression refers to the shift of the plane of an orbit under the
gravitational force of Earth's (or any planet's) equatorial bulge. For low
orbit satellites, it can be as much as 6 to 8 degrees per day westward (for
example, at inclinations of 52 degrees and 28 degrees respectively).

For inclinations higher than 90 deg the shift is eastwards, so an orbit of say
96 degrees at about 300 miles high will have its plane shift about one degree
eastward per day, which can be set to counteract the daily shift of the Sun wrt
the celestial sphere (360 degrees every 365.26 days, natch!). This leaves the
orbital plane constant wrt the Earth-Sun line, it's called sun-synchronous, and
it's dandy for Earth surface observation missions.

wrt= with respect to

The regression rate depends on altitude (the higher, the lower the rate) and
inclination (the higher, the lower the rate). There are equations for this if
anyone wants to get really technical.

Even Earth's moon's plane shifts in space due to various perturbations, mostly
Earth's off-spherical bulge. It takes 19 years ("the saros") for the lunar line
of nodes to shift 360 degrees -- that's why  the dates of eclipses move through
the calendar every year by about 18-20 days on average.

It's a neat concept, understanding it opens up a LOT of insights to orbital

Jorge and I used to do this for a living. You guys paid. We're much obliged.

From: Henry Spencer <>
Subject: Re: Nodal Regression?
Date: Sun, 22 Feb 1998 00:46:59 GMT

In article <01bd3ee2$b5149d40$>,
Rob Brown <> wrote:
>>The words "nodal regression" are unknown to them.
>They are unknown to me.  What do they mean?

To a first approximation, the plane of an orbit remains fixed in space as
the satellite goes around the orbit and the Earth spins underneath.
However, that's only a first approximation.  In particular, because Earth
has an equatorial bulge, it does *not* act like a point mass when looked
at more closely.  The biggest effect of this is that an orbit precesses,
with its plane slowly rotating around the Earth's axis.  For a typical
orbit, the point in space where (for example) the satellite crosses
Earth's equator going southward creeps westward a fraction of a degree per
orbit.  (The Earth is also spinning eastward underneath that point.)

The points where an orbit crosses the equator are known as its nodes,
and the precession is also called nodal regression.

The rate of precession varies with altitude and inclination and other
things, and it complicates life seriously if you want to keep two objects
in the same orbit or bring them into the same orbit.  For example, the
quickie explanations of rendezvous all discuss how a spaceship in a lower
orbit, going around the Earth in less time, "catches up" with a station in
a higher orbit.  What they don't mention is that the planes of those two
orbits are precessing at slightly different rates, so the games you can
play are sharply limited:  you have to launch into a slightly different
plane than that of the station, and then time the rendezvous maneuvers so
that the spaceship moves up to the right altitude at just the time when
its orbital plane has precessed to match that of the station.
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         |

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