Global Positioning System Reference
In-Depth Information
FIGURE 2.5.
Two early Islamic
measurements of the size of
the earth. (a) Al-Mamun sent
surveyors to
A
and
B
, from
which the height of Polaris
differed by 2
\
, and measured
the distance from
A
to
B
.
(b) Al-Biruni required only
one observer, at the top of a
mountain. By measuring the
dip angle
a
to the horizon,
and knowing the height,
h
, of
the mountain, he determined
the earth's radius,
R
. (From
the geometry we see that
R
2h/tan
2
a
).
sources are not consistent). The actual mean value of earth's radius is 6,371
km, so Al-Mamun's estimate was extremely good.
6
Light from Afar
After the Greeks came the Romans. Rome advanced Western civilization in
many ways, but science was not one of them. And the history of science in
general, and of geodesy in particular, got worse before it got better. The
Dark Ages followed the collapse of the Roman Empire, and much of the in-
tellectual life of Europe was put on hold. Even the works of earlier, more en-
lightened times were forgotten or swept away. From the time of the Greeks
until the medieval period, little advance was made in European geodesy.
7
6. Much has been written about these three (and other) measurements of the Earth's
radius. See for example Buttiner and Wallin (1999, p. 97), Evans (1998, chap. 1), Fischer
(1975), Harley and Woodward (1987, chap. 8), and Lindberg (1978. chap. 1).
7. An indication of how far European science fell with the eclipse of ancient Greece
comes from another of their philosophers, Aristarchus, who lived in the third century BCE.
