Friday, March 18, 2011

Dumb things about the calendar

The Letters section of this morning's SMH had one of those really dumb letters claiming some feature of the calendar was really rare and we should "savour it".

The letter was really short so I'll quote it in full.

By way of sustained research and learning I have discovered that July this year has five Fridays, five Saturdays and five Sundays. This happens every 823 years so we should savour it.

In reality July has three Fridays, Saturdays and Sundays any time that the 1st of July is a Friday. That happens on average once every seven years.

We can be more precise about when the next one occurs as the only variable is leap years. The 1st of a month progresses one day each year (as 365 = 52*7 +1) except in leap years.

The exact numbers are (starting where we are), when you are two years before a leap year, the 1st of July will next be on a Friday in 11 years, when you are 1 year before it is 5 years, when you are in a leap year or three years before it is 6 years. The average of these is 7 as we'd expect.

Where do these nonsensical ideas come from?

(Note: The above analysis does fall down on centuries which are not leap years unless the number of centuries is divisible by 4 as 2000 was.)

Novae Meridianae Demetae Dexter delenda est

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