4. Finally, General Relativity has been tested by observing starlight that
grazes a body -- the Sun, Jupiter, and Earth have all been used thus.
Again, accurate knowledge of planetary positions is essential.
This is not an exhaustive list of the astronomical benefits of accurate
planetary positions, but it gives a flavor of the value of such positional
knowledge and prediction.
Theory, observation, and application are interdependent, as illustrated in
Figure 1. Observations can be interpreted only in the context of our under-
standing -- represented by theoretical models -- of the solar system and
its dynamics. The observations can then be used to correct and update our
theoretical models. The fusion of the two results in planetary (and
satellite) ephemerides as well as a better solar system reference frame.
The combination of planetary system model -- as determined by theory
and refined by observations -- plus stellar position catalogs, gives rise to a
multitude of practical applications in many areas, including astronomy,
geophysics, and military.
1.3. Reference Frames
Although we here on
the Earth's surface might
often prefer to work with
a nearly inertial frame of
reference tied to the dis-
tant stars, we must in-
stead put up with the
various noninertial refer-
ence frames to which we
find ourselves affixed.
Defining and/or connect-
ing them is both observa-
tionally and theoretically
a complex undertaking.
We must tie together a lo-
cal reference frame, at-
MURISON: MODELING PLANETARY MOTIONS
3 of 20
Figure 1 -- Interplay between theory, observation, and