What are the rules for calculating leap years? Why is 2000 a leap year when 1900 wasn’t? NOOKS AND CRANNIES

What are the rules for calculating leap years? Why is 2000 a leap year when 1900 wasn’t?

Craig Keeley, London

A year is a leap year if it is evenly divisible by four (4), unless it is a century year (eg. 1900, 2000, etc) when it is a leap year only if the year is evenly divisible by 400.

Chris McKenna, Swansea

In answer to the implied question of why we need such complicated rules, the ratio between the length of the year and that of the mean solar day is approximately 365.2422 to 1. (There is no reason why it should be a whole number.) The Gregorian calendar approximates the fractional part of this to 97/400 and the rules are a convenient way of providing 97 leap years in a four hundred year cycle. Omitting multiples of 4000 copes with most of the (very small) residual error in the Gregorian calendar.

The fraction 0.2422 could be more closely approximated as 163/673, but the rules required to fit 163 leap years into every 673 years would be highly inconvenient to apply.

Pelham Barton, Birmingham

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