better than 20 milliarcseconds (mas)
. Placing accuracy requirements
upon a stellar reference frame has implications for how accurately we must
then know, among other things, the positions of the planets and other solar
system objects. The sequence of connections that joins the two seemingly
disparate accuracies (that of the stars and that of the planets) requires a dis-
cussion of dynamical and astronomical reference frames, which I will get
to in a moment.
The second broad requirement category is the astronomical need for ac-
curate planetary positions. Besides the intrinsic interest of astronomers in
planetary, asteroidal, and cometary positions, knowledge of these positions
over time -- called an ephemeris -- fundamentally affects many areas of
solar system and even stellar astronomy:
1. To the general public, perhaps the most apparent astronomical need
for precise planetary positions is in spacecraft navigation.
2. Solar system celestial mechanics depends greatly on accurate posi-
tions. Theories of planetary and satellite motions live or die according
to how well their predictions agree with observational knowledge of
positions. These theories are the means by which we develop our
most fundamental understanding of the many complicated dynamical
processes and interactions in the solar system.
3. Another area where accurate knowledge of planetary positions is cru-
cial is stellar occulations. If a planet is passing in front of a star, and
we can predict where on the Earth's surface this event is visible, then
we may learn several things, including density and composition of the
planetary atmosphere, certain facts about the atmosphere of the oc-
culted star, and, of course, better knowledge of either the star's posi-
tion, the planet's position, or both. Further, if the occulting body is an
asteroid, we can even determine the projected shape of the asteroid.
MURISON: MODELING PLANETARY MOTIONS
2 of 20
One arc second is equal to one 3600th of a degree. One milliarcsecond equals one one-
thousandth of an arc second. To provide some context, consider that 1 arcsecond corre-
sponds to 1/16 of an inch around the perimeter of the 1000-foot radius Observatory Cir-
cle here at USNO. How much is 50 microarcseconds [the nominal accuracy of the pro-
posed space astrometry mission FAME (
)]? That is the
angle subtended by the width of a typical strand of human hair as seen from a distance
of 65 miles.