Geoscience Reference
In-Depth Information
contact) and the satellite starts to fade. When its receding side enters into the
shadow the satellite disappears (second contact). This is the most decisive moment
to time, the disappearance of the
last speck
. As the satellite exits the shadow the
forward side of the satellite emerges first (third contact). Although this moment
(
first speck
) is as well defined as the second contact, it may occur at an unexpected
time, given the uncertainty of the predictions, and catch the observer by surprise;
hence this contact is the less accurate to time than the second contact. The moment
when the satellite has fully emerged from shadow is the fourth contact. The first
and the fourth contacts are not so decisive as the second and third, because they
show to the eye as the beginning and end of a slow progression of brightness, rather
than as abrupt appearance and disappearance.
Ideas
Kepler's Laws
First Law
: The planets move in elliptical orbits.
Second Law
: They move such that the line drawn from the Sun to a planet sweeps out equal
areas of the orbit in equal times.
Third Law
: The distance and period of each planet are related to all the others in what is
known as the harmonic law, that the square of the period of a planet is proportional to the
cube of its distance from the Sun.
It is possible by eye to time an eclipse of the closest, fastest moving satellite, Io,
as it passes into Jupiter's shadow to a few seconds, as measured on a clock at each
observatory (van Helden 1996). The same eclipse observed simultaneously in two
locations at different apparent times yielded the observers' longitude difference to
an accuracy of better than 1 arc minute, the equivalent of a couple of kilometers.
During his stay in Hven, Picard worked with the Danish astronomer Ole Rømer
(1644-1710) returning to Paris with him. In Paris, Rømer observed the satellites of
Jupiter and noticed that the eclipses of its satellites occurred earlier than scheduled
when Jupiter was close to the Earth and behind schedule when Jupiter was furthest
away. Following this observation in 1676 he deduced that light travels at a finite
speed and measured it very accurately and this discovery helped improve the pre-
dictions of the eclipses. This was important because it was not usually possible to
compare observations of an eclipse from one place with observations of the same
eclipse from another; it was more feasible to compare observations with a predic-
tion. When the observed eclipse was timed relative to the tables of predictions, the
longitude difference could be in error by several minutes of time, perhaps a degree
of longitude, due to the inaccuracy of the predictions. Curiously Cassini did not
take up the advantage of extra accuracy that Rømer's work had given him, and it
was left to astronomers at the rival observatory at Greenwich to fully to exploit
Rømer's discovery.
Observing Jupiter's satellites as a means to determine one's position was feasible
from land but was not a practical way to determine time in order to navigate a ship.
In order to view the eclipses of the satellites, a telescope was necessary. Telescopes
of the time were long and thin (since opticians could make only small lenses), and
the lenses could only produce good images if they were weak with shallow curvature;
such weak lenses throw an image at a long distance. On the heaving deck of a ship
the telescopes wavered in the hands of the observer, therefore the observing navigator
