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                      \title{       Galilean Satellites}
\author{J.H. Lieske\\
California Institute of Technology\\
Jet Propulsion Laboratory\\
Pasadena, California USA}
\date{}


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\maketitle

    Jupiter's Galilean satellites have played an important role in science 
since their discovery in 1610.  Ole Roemer (1644-1710) employed eclipse 
timings in 1676 to investigate whether the speed of light was finite or 
infinite.  More recently, S.J. Peale predicted volcanism on Io in 1979, 
before the Voyager spacecraft encountered Jupiter and dramatically 
photographed volcanic activity on Io.
        The investigation by J.L. Lagrange (1736-1813) won the prize of 
the Royal Academy in 1766.  P.S. Laplace (1749-1827) investigated the 
motion of the inner three satellites and discussed what we today call the 
Laplace libration, where the mean motions of the inner three satellites 
are locked together and follow the equation $n_1 - 3 n_2 + 2 n_3 = 0$.  
French scientists have played very significant roles in the study of 
Jupiter's Galilean satellites.
    One of the major efforts at international scientific cooperation in 
the 17th century involved the Galilean satellites and the attempt to 
determine terrestrial longitudes of observers by employing eclipses of the 
satellites.  French Jesuits were active participants in these efforts, 
especially in the Orient, and some of the early maps of the world were 
drawn based upon Galilean satellite eclipse timings.
    J-B. J. Delambre (1749-1822) collected more than 6000 eclipse 
observations prior to 1800 and his collection was considered the best in 
the world.  It disappeared about the time of founding of the Bureau des 
Longitudes and was thought to be lost forever.  A-G. Pingre (1711-1796) 
had prepared a manuscript on 17th century science for publication at the 
end of the 18th century, but it disappeared at the time of the French 
Revolution.  The manuscript contained a large collection of Galilean 
satellite eclipse observations because of the great interest in 
determining terrestial longitudes. C.G. Bigourdan (1851-1932) ultimately 
published Pingre's re-discovered manuscript in 1901.  The extensive 
collection of observations of the Galilean satellites by J-N. Delisle 
(1688-1768) was re-discovered in 1980 and thus the lost Delambre 
collection has been recovered.
    While eclipse observations are of interest in their historical 
setting, they are equally relevant in today's science.  The observations 
by Roemer are being employed in ephemerides which guide the spacecraft 
Galileo in its forthcoming encounter with the Jovian system later this 
year.  Typical eclipse timing observations are accurate to about 800 km.
    Other types of observations play important roles in the study of the 
satellite orbits also.  The extensive photographic observations by D. 
Pascu at the U.S. Naval Observatory over the last quarter-century are  
accurate to 0.10 arcsec per exposure (or 0.05 arcsec per plate, equivalent 
to about 160 km).  Optical Navigation frames made by the Voyager 
spacecraft in 1979 are good to about 45 km. Mutual event photometric 
observations 1973-1991 vary in their accuracy but appear to be about 100 
km, with wide differences for the same event by different observers.  Over 
the past several years CCD photometric observations have been made by the 
USNO Flagstaff and these data appear to be good to about 100 km per normal 
point.
    This research was sponsored by NASA and conducted at the Jet 
Propulsion Laboratory, California Institute of Technology.


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