Geoscience Reference
In-Depth Information
of land, to identify legal ownership, and to assess taxes. An accurately measured
north-south line can serve as the basis of an accurate map. Such a line is called a
meridian
2
; the site of the “Incredible Picnic” was the meridian that runs through the
astronomical observatory in Paris.
In the seventeenth century the need for accurate maps had become clear. In the
sixteenth century, ships had been navigated by experienced mariners using “ruttiers”
- written sailing instructions that described currents, winds, hidden hazards, land-
marks, depths, anchorages, port facilities, and the nature of the sea-bed (Fernández-
Armesto 2006). Charts were used as adjuncts but they were unreliable, in part
because there were unexplored gaps and also because the representations of the
geographic features of the shoreline were inaccurately placed. It was only as the
accuracy of charts improved that navigators began to use them as the main naviga-
tional aid. Their accuracy increased as the result of improved technology, including
more accurate measuring instruments, telescopic sights that made it easier to measure
small angles, etc. In the seventeenth century, the Paris Meridian became the starting
point to create accurate charts of France (not only the shoreline but also inland
areas) and, for French sailors, to create accurate charts of the world. It was the origin
of the French system of longitude, just as the meridian of Greenwich was the origin of
the longitude system used in Great Britain. The Paris Meridian lies 2° 20′ of longi-
tude east of the Greenwich Meridian.
The orientation of the French meridian north-south was based on the position of
the stars. Most stars and the Sun rise up from the eastern horizon and pass across
the sky, reaching their highest elevation before setting in the west (
Fig. 3
). Their
position at their highest elevation, when they “culminate,”
3
marks south
4
- north is
opposite this direction, east and west are at right angles. This is the basic way that
astronomers determine the four cardinal points.
In practice and equivalently, south can be found by measuring the position of a star
while it is rising and the position when it is at the same altitude while setting, taking
the midpoint (
Fig. 3
). For example, south is the midpoint between the positions of a
star or the Sun on the horizon when it rises and when it sets (with allowance for the
motion of the Sun between these times). The direction of the star or Sun measured as
an angle from north is called its “azimuth”; this is an Arab word that came into the
2
In the English language the word
meridian
means the line running north-south through the poles,
however determined and for whatever purpose. The French language distinguishes between
le
méridien
and
la méridienne
, the former being the abstract line of points of the same longitude and
the latter being a real line.
Une méridienne
can serve as a baseline for geographic measurements,
as in this topic, or it may be the visible line of a sundial showing noon local time.
Une méridienne
is oriented along
un méridien
.
3
Culminate
in astronomy or astrology means of a star “to reach its highest point in the sky” (from
the astronomical meaning is derived the general meaning of the word “to reach an acme of
development”).
4
The position of a star at its highest elevation lies in the south if you are in the northern hemi-
sphere, as in France. In the southern hemisphere it would be in the north.
