1 The Birth of Science - MSRI

11.6 Terrestrial Motion, Tides and Gravitation 319
how the monthly cycle of spring and neap tides can be explained by the
joint action of the two bodies. 107 The Euripus even contains the observation
that, given that when the sun or the moon are directly above a point on
the tropic of Cancer, the antipodal point is on the tropic of Capricorn, the
two daily tides should be unequal and the diurnal inequality should be
greatest at the solstice and least at the equinox. 108
What makes this all the more fascinating is that de Dominis, far from
writing in support of the theory that he expounds, in fact disproves it on
the grounds that its consequences are false! He was convinced that the
two daily tides are invariably equal. Knowledge of the regime of diurnal
inequality in such distant places as the “Erythrean Sea”, of which Strabo
wrote in connection with Seleucus, 109 had vanished in Europe. We know
from no less an authority than G. H. Darwin that the import of Seleucus’
achievement would have been impossible to appreciate even in the early
nineteenth century. 110 The discussion in the Euripus is thus in a sense complementary
to the Strabo passage, in that it includes a theoretical explanation,
not found in Strabo, for the observable phenomena described by
Strabo but rejected by de Dominis. 111
Thus we have found an additional and important argument in support
of the thesis, introduced in Section 10.7, that a reasonably well-developed
astronomical theory of tides existed in Antiquity. Indeed, if it is already
less than likely that our religious reformer and jurist could have created
an original mathematical theory that correctly explains the annual varia- page 393
tion of diurnal inequality, it beggars belief that he should have done so
while denying that the diurnal inequality exists. The explanation is that
he merely transmitted (not terribly well, it must be said) the elements of
an ancient theory born from observations made in seas he knew nothing
about. And it is not surprising that a high ecclesiastical figure, who
was successively the bishop of Segna (Sinj) and the archbishop of Spalato
(Split), and the scion, to boot, of an illustrious Dalmatian family going
107
The cause of the monthly cycle is recapitulated in note 119, page 266.
108
The yearly cycle of diurnal inequality is explained in Figure 10.3 (page 267) and the surrounding
text.
109
Strabo, Geographia, III, v, 9; see page 266 for discussion.
110
[Darwin: Tides], p. 84: “The meaning of [the passage on Seleucus reported by Strabo] was
obviously unknown to the Dutch commentator Bake — and indeed must necessarily have been
unintelligible to him at the time when he wrote, on account of the then prevailing ignorance of
tidal phenomena in remoter parts of he world”.
111
Naturally de Dominis (like any of his contemporaries who might be interested in physical
geography) had to be familiar with Strabo’s work. What happens is that after reporting Seleucus’
observations, Strabo adds that Posidonius unsuccessfully tried to verify in Cádiz the phenomenon
Seleucus noticed in the “Erythrean Sea”. So probably de Dominis deduced not that the two seas
have different regimes, but that Seleucus should be disregarded.
Revision: 1.11 Date: 2003/01/06 07:48:20