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592 MEDIEVAL ENGLAND
reform, "every computer knows that the beginning of lunation
is in error 3 or 4 days in these times, and every rustic is able
to see this error in the sky'. The recommendations made by
Grosseteste and Bacon, based on determining accurately, from
astronomical evidence, the exact length of the year and the
relation between this and the mean lunar month, were used
in attempts made to revise the calendar in the fourteenth and
fifteenth centuries, but such is institutional and popular con^
servatism that nothing was achieved until the Gregorian reform
of 1582, and this was not accepted in England until 1752.
In astronomy itselfthere was little advance in England during
the fifteenth century. An English contemporary of Grosse/
teste, John Holywood or Sacrobosco, as he was called, work'
ing mainly in Paris, gave an account of the Ptolemaic system
in his De Sphaera, an elementary textbook based on Arabic
sources which remained standard for three centuries; an Eng/lish
translation was made for Prince Henry, son of King James
I, and reading it as a schoolboy is said to have decided John
Hamsteed (1646-1719), first astronomer royal, to devote hinv
self to astronomy. Sacrobosco also wrote a treatise on the
quadrant, an instrument for measuring angular altitudes.
Bacon observed the heavens with instruments and dis'
Roger
cussed astronomical theories at length in various writings; his
emphasis on measurement seems to have helped to build up
both the Parisian school of astronomy at the end of the thiy
teenth century and the school associated with Merton College,
Oxford, in the fourteenth century.
Walter de Merton made his foundation, towards the end of
the thirteenth
century, expressly
for the
training of secular
clergy, so that the learned professions and the civil service
would be
adequately supplied with men of sound education.
The success of the
college was immediate, and in science,
especially in astronomy, mathematics, and medicine, it rapidly
took over in England the leadership that had formerly be'
longed to the Franciscans. Practically every important English
scientist ofthe fourteenth century was at some time in his career
associated with Merton College, many ofthem as fellows.