1 The Birth of Science - MSRI

266 10. Lost Science
tion between bodies. In this extension, too (which is apparently already
attested in the passage of Seneca examined in Section 10.5), the theory of
tides probably played an important role.
Eratosthenes, as far as we know from Strabo, attributed the tides to the
action of the moon alone. 119 If others discovered the influence of the sun
on the tides, they must perforce have suspected an interaction between
the sun and the earth, at which point the idea of “universal” gravitation
would become natural. In any case the sun-earth interaction would have
been enough to lead to the idea that interests us here, that of “dynamical
heliocentrism”.
From another passage of Strabo we know that Seleucus, studying tides
in the “Erithrean Sea” (which probably means today’s Arabian Sea 120 )
went beyond an understanding of daily and monthly cycles: he found
a correlation between the diurnal inequality (roughly, the difference between
high tides twelve hours apart) and the time of year. In particular,
he found that spring tides show maximum diurnal inequality around the
solstice, and minimum during the equinoxes, thus giving rise to a yearly
cycle. 122
Did Seleucus give a theoretical explanation for this correlation? This
seems likely in view of the following considerations.
First, the theoretical explanation is not very difficult. If one attributes
tides to the effects of the moon and sun, admits that each of these two
effects is maximal when the body is at the zenith or the nadir, and takes page 333
account of the high water interval characteristic of each location, 123 it is
not difficult to conclude that tides in temperate regions should have the
behavior described by Seleucus, if the earth were a perfect solid sphere
surrounded by a layer of water. Suppose, for example, that we have full
119 See note 117 above. The passage says that Eratosthenes related tides with the position of the
moon in the sky, rather than with the phases of the moon (recall that spring tides — the tides of
highest amplitude in a monthly cycle — occur near the full and new moons, while neap tides — of
lowest amplitude — occur at the first and third quarters). But the relationship between tides and
phases of the moon, which probably had been known empirically from very ancient times, was
articulated before Eratosthenes by Pytheas ([DG], 383, §3 = [Roseman], p. 102) and perhaps by an
earlier Massaliote, Euthymenes ([DG], 634 = Pseudo-Galen, De historia philosopha, 88); given that
the Mediterranean is almost tideless, it is not by chance that the Greeks became interested in tides
primarily when they started sailing the Atlantic and Indian Oceans.
120 The expression was also used for today’s Red Sea, but since Strabo sometimes says that Seleucus
came from the Erithrean Sea (Geography, III, v, 9) and sometimes from Babylonia (Geography, I,
i, 8-9; XVI, i, 6), it seems that in this case the Arabian Sea was meant.
122 Strabo, Geography, III, v, 9.
123 The high water interval is the lag between the moon’s (upper or lower) transit over the local
meridian and the next high tide. Because of the solar tidal component, this interval varies with the
phase of the moon, but its average over a lunar cycle at a given time of year basically depends only
on geography, being greater for locations separated from the open ocean by obstacles.
Revision: 1.11 Date: 2003/01/06 02:20:46