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SCIENCE 573
arranged in order ofheaviness and lightness. At the Creadon
these four elements, together with light and man's soul, were
made by God ex nibilo; all other phenomena in the corporeal
world were combinations. From Pliny Bede got a much more
detailed knowledge ofthe Greek understanding ofthe diurnal
and annual movements of the heavenly bodies than had been
available to Isidore. He held that the firmament ofstars revolved
round the earth, and that within the firmament the planets
circled in a system of epicycles. He gave clear accounts of the
phases of the moon and of eclipses.
The problem of the calendar had been brought to" Nortlv
umbria along with Christianity by the monks oflona, but long
before that time methods of computing the date of Easter had
formed part of the school science of computes, which provided
the finger exercises of early medieval science. The main problem
connected with the Christian calendar arose from the fact that
it was a combination ofthe Roman Julian calendar, based on
the annual movement of the earth relative to the^sun, and the
Hebrew calendar, based on the monthly phases of the moon.
The year and its divisions into months, weeks, and days be>
longed to the Julian solar calendar; but Easter was determined
in the same way as the Hebrew Passover by the phases
of the
moon, and its date in the Julian year varied, within definite
limits, from one year to the next. In order to calculate the date
ofEaster it was necessary to combine the length ofthe solar year
with that ofthe lunar month. The basic difficulty in these cal
culations was that the lengths ofthe solar year, the lunar month,
and the day are incommensurable. No number of days can
make an exact number oflunar months or solar years, and no
number of lunar months can make an exact number of solar
years. So, in order to relate the phases ofthe moon accurately to
the solar year in terms of whole days, it is necessary in con'
structing a calendar to make use of a system of ad hoc adjust^
ments, following some definite cycle.
From as early as the second century different dates ofEaster,
resulting from different methods of making the calculations,
had given rise to controversy and become a chronic problem for