Geoscience Reference
In-Depth Information
Fig. 11
Two of Cassini III's observers make stellar observations with an octant mounted in an
observing shelter. One lies in a reclining chair to sight on a star while the other reads an angular
scale with a microscope. It seems a little unlikely that they could have managed with such a
small hole in the roof through which to view the sky. César-François Cassini de Thury (1744)
to cook, and a bed for sleep as necessary. During all this Picard was supported by
servants who traveled with him, and about whom we know little or nothing.
One trigonometrical station in particular played a continuous role in the history
of the Paris Observatory and the surveys. It was erected next to a windmill (the
Moulin de la Galette) on the hill of Montmartre, directly north of the center line of
the Observatory building. The pillar originally served to calibrate the instruments
of the Observatory where it was visible from (trees and houses now interfere with
the line of sight). In French, this is called a
mire
10
and appropriately the Rue de la
Mire is nearby. Picard placed the first
mire
here, a wooden pole to serve this func-
tion. Its position to the north of the center of the Observatory building was estab-
lished by a survey in 1673, and a substantial wooden post was put at the same place
in 1675. However in 1683 Cassini and La Hire determined that its position was two
feet west of the meridian, an error of azimuth of about 0.5 arc minutes, or 1/120 of
a degree. This post was further superseded by a stone
mire
.
To measure the angles between landmarks, Picard used a seventeenth century
equivalent of a theodolite, a large quarter circle (quadrant) divided into angular
measure that could be read to about 1 arc minute. It was 1 meter in size and
mounted horizontally, and a small telescope with cross hairs (Picard pioneered the
use of spider webs) was pivoted at the center in order to sight landscape features.
10
Une mire
is a target for calibration (the same word is used in modern times to mean a “test card”
for TV transmissions).
