4469. LATITUDE AND LONGITUDE, Jupiter's Eclipses.—
To get the longitude at
sea by observation of the eclipses of Jupiter's
satellites, two desiderata are wanting: 1st, a
practicable way of keeping the planet and satellite
in the field of a glass magnifying sufficiently
to show the satellites; 2nd, a time-piece which
will give the instant of time with sufficient accuracy
to be useful. The bringing the planet
and satellite to the horizon does not sensibly
facilitate the observation, because the planet in
his ascending and descending course is at such
heights as admit the direct observation with entire
convenience. On the other hand, so much
light is lost by the double reflection as to dim
the objects and lessen the precision with which
the moment of ingress and egress may be
marked. This double reflection also introduces
a new source of error from the inaccuracy of
the instrument; 3rd, the desideratum of a time-piece
which, notwithstanding the motion of the
ship, shall keep time during a whole voyage
with sufficient accuracy for these observations,
has not yet been supplied—
To Captain Grove. Washington ed. v, 374.
(W.
1808)
