Geoscience Reference
In-Depth Information
could not easily hold such a telescope steady enough to view the planet continuously
for the many uncertain minutes of the predicted eclipse, see it and time it accurately.
To correct this, some effort was put into making a pendulum arrangement of a
bosun's chair for the observer to sit in, suspended with a system of ropes and pullets
from a pole or mast. The idea was that the observer would stay still while the boat
rocked below him but this was not the experience. Eclipses often happened while the
telescope was off target, and when the eclipse was seen the observer was too
unsteady to decisively see decisively the moments of contact between the satellite and
the shadow so as to time the eclipse accurately.
Even ashore on steady dry land, the telescope was liable to tremble in the slightest
wind. It was, however, possible during calm weather to observe the eclipses from
shore with the telescope supported by strings on static poles. This gave the longi-
tude of the telescope's observing station, and, with the latitude observed from star
transits across the meridian in the sky, the means to map a coast line.
In 1690 as an aid for sailors and explorers, the Academy began publishing the
Connaissance de Temps
, literally the 'Knowledge of Time' and the equivalent of a
Nautical Almanac
. Its annual issues contained predictions of the eclipses of the
satellites of Jupiter, for the purposes of determining the time as measured at Paris,
and instructions on how to do this:
For finding the longitude of any place on Earth it suffices to observe any immer-
sion or emersion. One compares the true time of the observation with the hour and
the minute of the same immersion or emersion calculated for Paris, or observed there
the same day. The difference in time, reduced to degrees minutes and seconds will
be the difference between the meridian of this place and the meridian of Paris.
USING TECHNIQUES that he had developed in Hven, Picard and Cassini mapped
the shore-line of France, determining the latitude and longitude of capes, ports and
other cities. Picard chose Philippe de la Hire to help them both.
People
Philippe de la Hire (1640-1718)
Philippe de la Hire (
Fig. 13
) was the son of a painter and sculptor and this background gave
him geometry training. While attempting a difficult sculpture, he developed a new method
for constructing conic sections. His mathematical interest soon led to a curiosity in astronomy
and physics, and in 1682 he became a member of the Observatory in Paris where he took
part in observing programs of a wide variety. He was very productive and wrote and
lectured extensively but produced no lasting innovations in science
In 1672-4 Picard went to Touraine, Languedoc and Lyon and in 1672 Cassini
traveled to Provence. In 1679 Picard and La Hire traveled to Nantes and Brest to
map western France and in 1680 to Bordeaux. In 1681-2 they worked eastwards
along the northern coast, Picard in Normandy and La Hire near Calais. During this
time, the Paris Meridian had been accurately measured across France from the
northern to the southern coasts, and the length of the axis of the Hexagon had been
determined but the task still remained to draw it on the map. In 1681, Picard pro-
posed a plan to the Academy to extend his triangulation survey to cross the whole
of France. He noted that parts of France that had been accurately surveyed near the
