Geoscience Reference
In-Depth Information
the line of sight to the Sun because the ground is usually horizontal and the Sun is
not usually directly overhead. The dappling under trees is thus a collection of ellip-
tical images of the Sun.
As the Sun moves in the sky, rising towards its highest point at midday and then
setting, the marks move on the ground below the trees. (Everybody knows that even
if they are in the shade of a tree when they fall asleep after a picnic, they are in
danger of having their siesta terminated by the light of the Sun as it moves the
shadow of the tree off the sleeper). In the natural case it is hard to follow one par-
ticular image from a given pinhole because other branches intervene and block the
image of the Sun that it produces. A
meridiana,
though, has one hole making one
image, and a lens may be placed over the hole to give the image of the Sun extra
clarity. At midday local time the image of the Sun lies on the meridian of the loca-
tion in question. At midday on midsummer's day, when the Sun reaches its greatest
elevation, its image is cast on the meridian at a point most nearly directly below the
hole. By contrast on midwinter's day when shadows are longest the image is cast
furthest from the point immediately below the hole.
Every day there are two measurements that can be made in a
meridiana
- the
time at which the Sun crosses the meridian line and the position along the meridian
line at which it does so. This constitutes a measurement of the position of the Sun
at a particular moment. The
meridiana
can thus be used to check a theory of motion
of the Sun, which attempts to say where the Sun is at any given time; this was the
purpose of the St. Sulpice
meridiana
.
THE RE-BUILDING of St. Sulpice, including the
meridiana,
was completed by
the Curé of St. Sulpice, Jean-Baptiste-Joseph Languet. For nearly half a century the
building program had stalled and its re-activation was a tribute to Languet's fund-
raising skills; it is said that he never returned from dinner with his wealthy parishioners
without a silver service, and his collection was melted down to make a statue of the
Virgin.
It was Languet who was responsible for the installation of the St. Sulpice
meridiana
as part of the new church. The accuracy of a
meridiana
depends upon the accuracy of
the construction of the instrument, with the meridian line accurately north-south on a
horizontal floor. Languet arranged for the floor of the transept, along which the merid-
ian line is laid out, to be supported by its own pillars, independent of the rest, so that
as the massive walls of the church settled the floor remained horizontal.
Languet's motive for making the
meridiana
was to determine better the dates for
church celebrations, particularly Easter. The Council of Nicaea in 325 AD had the
ambition to unite the Christian community across the world in its celebration of the
events surrounding the crucifixion and therefore Easter was defined as the Sunday
after the first Full Moon after the vernal equinox. To plan the celebrations, though,
it is necessary to forecast this day, but this requires some astronomical knowledge
about the motion of the Sun. If this knowledge is lacking, the predictions will be
incorrect. The
meridiana
provided a means to check the predictions and to identify
corrections to the basic formulae, which had been first developed in antiquity but
as the centuries passed proved to be deficient.
