by C. E. A. Burnham

The true tale of how an Emperor and later a Pope wrestled with the Bit.

On one of his campaigns Julius Caesar found legions experiencing winter conditions when, according to the calendar, it should have been high summer. The blame for the mishap is laid solely on the calendar which at that time had a year of exactly 365 days. But the time taken by our Earth to describe one circuit of the Sun is more nearly 365 1/4. (Not that Caesar would have expressed it in this way! ) So that the
calendar was slow by about a quarter of a day each year. This discrepancy of about six hours, negligible in the short term, accumulated over a long period, producing errors of months, seasons … Nature’s year has 365 plus a-certain-bit days. Failure to allow for the Bit would disrupt our pattern of life.

Caesar brought into operation a revised calendar – the Julian Calendar – in which an extra day was added every fourth year. This extra day became February 29th. Why February? The Roman year began with March (as did our own year till the mid-eighteenth century). So February was the twelfth month; the extra day was added at the end of the calendar.

Now the period of the Earth’s rotation about the Sun is not precisely 365 1/4 days; more accurately it is 365.2422 days. The all important Bit is 0.2422, and the Julian Bit (0.25) is too large by 0.0078 or so years. Caesar’s calendar created too many leap years; eight too many in a thousand or so years. By 1500 A. D. it was eleven days out of phase and in 1583 A. D. Pope Gregory XIII replaced it by the Gregorian
Calendar.

Recall that we (i.e. Pope Gregory) aim to reduce Caesar’s Bit of 0.25 by 0.0078. This we attempt by making changes to the Julian Calendar: –

i) We decree that the exact centuries – 1700 A.D., 1800 A.D – shall not nowcount as leap years; but,
ii) We retain leap-year status for those centuries which are multiples of 400, i.e. 1600, 2000, 2,400. Caesar’s addition of one every four years produced a Bit of 0.25.

Pope Gregory’s amendment (i) – a removal of one day every hundred years (i.e. 0.01 of a day per year), together with his amendment (ii) (the addition of 1/400th (0.0025) of a day per year, brought the attempt to 0.25 – 0.01 + 0.0025, i.e. 0.2425. Not quite there! We aimed at 0.2422.

But that is where our present Gregorian calendar now stands – a calendar year of 365.2425 days, with an error of the order of 0.0003 – three days in ten thousand years.

1900 A. D was not a leap-year. 2000 A. D will be.

The Gregorian Calendar was not accepted in 1582 by everyone. In England the change was not made until the mid-eighteenth century. The accumulated error of 11 days was rectified by Act of Parliament of 1751:-
” Wednesday 2nd September 1752 shall be followed by Thursday 14th September 1752″

New Year’s Day was established: –
“The Computation of the year now begins on the first day of January.”

Published in Mourholme Magazines Issue 18 1992