1 The Birth of Science - MSRI

10.7 The Idea of Gravity from Aristotle to Hipparchus 267
moon at the northern summer solstice; ignoring the high water interval,
we then have a highest spring tide at point A on the Tropic of Cancer
where it is noon, and at the antipodal point B on the Tropic of Capricorn,
where it is midnight. The next high tide on the same spot is rather less
severe, since point A has moved to A ′ and no longer has the moon and
the sun on the zenith-nadir axis, but aslant (Figure 10.3, left). Thus the
moon
B
A ′
solstice
A
sun
moon
C ′
A ′
equinox
FIGURE 10.3. Configuration of spring tides.
diurnal inequality is large. On the other hand, an equinoctial spring tide
(Figure 10.3, right) has diurnal inequality close to zero, as can be seen by
comparing the same two points A and A ′ on the Tropic of Cancer (or, for
that matter, any two points of same latitude where it is respectively noon
and midnight). In this situation the highest spring tide is at points C and
C ′ on the equator. On other days of the year we have an intermediate
situation.
The interesting thing is that in the Arabian Sea the actual behavior of
tides (which may in general depart far from what might be expected on
the basis of simple models) agrees both with Seleucus’ description and
with the theoretical scheme just discussed, as was recognized in 1898 by
G. H. Darwin, one of the founders of modern tidal theory. 124
Secondly, certain testimonia from Pliny imply that at least part of the
preceding ideas were present in Hellenistic science. Just before talking
about the moon’s attraction, Pliny states that the moon and the sun are
the causes of tides, 125 and he may have Seleucus in mind when he mentions
the diurnal inequality and the fact that it vanishes only during the
equinox. 126 It is especially meaningful that Pliny discusses the lag with
which actual tides occur 127 (the high water interval), a notion that only
makes sense in connection with a theoretical explanation.
124 [Darwin: Tides], p. 76.
125 Pliny, Naturalis historia, II, 212.
126 Pliny, Naturalis historia, II, 213.
127 Pliny, Naturalis historia, II, 216.
Revision: 1.11 Date: 2003/01/06 02:20:46
A
C
sun