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574 MEDIEVAL ENGLAND
successive Councils. Various cycles relating the lunar month
to the solar year were tried at different rimes and places, until in
the fourth century a nineteen^year cycle, according to which 1 9
solar years were considered equal to 235 lunar months, came
into general use. But there was still the possibility of differences
in the manner in which this same cycle was used to determine
the date ofEaster, and, even when there was uniformity at the
centre, sheer difficulty of communication could and did result
in such outlying provinces as Africa, Spain, and Ireland celex
brating Easter at different dates from Rome and Alexandria.
Shortly before Bede's birth Northumbria had, at the synod of
Whitby, given up many practices, including the date of cele^
brating Easter, introduced by the Irislvtrained monks oflona,
and had come into uniformity with Rome. But there was still
much confusion, by no means confined to Britain, as to how the
date ofEaster was to be calculated. Bede's main contribution,
expounded in several treatises, beginning with De Temporibus
written in 703 for his pupils at Jarrow, was to reduce the whole
subject to order. Using largely Irish sources, themselves based
upon a good knowledge ofearlier continental writings, he not
only showed how to use the nineteen^year cycle to calculate
Easter tables for the future, but also discussed general problems
oftime/measurement, arithmetical computation, cosmological
and historical chronology, and astronomical and related pheno^
mena. Though often relying on literary sources when he could
have observed with his own eyes as, for example, in his
account ofthe Roman Wall not ten miles from his cell Bede
never copied without understanding. He tried to reduce all
observed occurrences to general laws, and, within the limits of
his knowledge, to build up a consistent picture ofthe universe,
tested against the evidence. His account of the tides in De
Temporum Ratione (chap, xxix), completed in 725 and the
most important of his scientific writings, not only shows the
practical curiosity shared by him and his Northumbrian conv
patriots, but also contains the basic elements ofnatural science.
From his sources Bede learned the fact that the tides follow
the phases ofthe moon and the theory that tides are caused by