Geoscience Reference
In-Depth Information
Fig. 3
Like all stars, Jupiter rises in the east and sets in the west. Seen from the northern
hemisphere it “culminates”, at its highest point in the sky, when it is due south. The direction
of south is in practice found by measuring the position of a star at a given altitude as it first
rises and then sets and taking the midpoint of these two directions
English language from the Islamic world, a culture that preserved and developed the
classical knowledge of astronomy while Europe was in the Dark Ages.
In the other direction, the circumpolar stars, like the North Pole Star, define
north; Polaris is not exactly at the North Celestial Pole, but circles around it and
other circumpolar stars do the same. Astronomers measure the position of Polaris
and other circumpolar stars at their western and eastern-most extensions from the
Pole and determine the point midway between, which is the Celestial Pole, the
direction of north. In the seventeenth and eighteenth centuries, the Paris astrono-
mers mapped the geographic line to the north and the south of the Paris Observatory,
progressively extending outwards through its grounds. This created the meridian
and the French zero of longitude.
To relate places to the east and west of this meridian, the astronomers made
simultaneous measurements of the positions of the same stars. As seen from a place
to the east, a star comes to culmination earlier than when seen from a place to the
west. The time difference represents the longitude difference between the two
places (Sobel 1999), but how do you measure time difference? In modern times, the
answer to this question is easy - you look at a clock. In earlier times before the
invention of the chronometer, however, this question was more difficult to answer
because seventeenth century artificial clocks were too inaccurate.
In contrast there are natural clocks, and in 1610 Galileo discovered that the planet
Jupiter had satellites that revolve around it with accurate periods. The satellites are
repeatedly eclipsed as they pass behind Jupiter and into its shadow. The satellites
define a natural clock with the eclipses as the ticks. The timing of the eclipses was
