Leap year

2016 is a leap year. I wondered why . . .

The calendar’s got an extra day
This year, ’cos it’s a leap one.
I Googled what the reason was
And found it’s quite a deep one:

The Solar System doesn’t work,
Or so it would appear,
With seven days in every week
And twelve months in each year.

The Earth takes longer than a year*
To travel round the Sun,
And that’s why, every now and then,
The error must be undone.

Julius Caesar did his best –
A simple rule, for sure:
“Add a Leap Day if the year’s
Divisible by four”.

But things were  much more subtle.
As the years got out of sync,
Pope Gregory piped up and said:
“The answer is, I think,

“To keep old Caesar’s bright idea,
But add a bit more to it.
So here, in 1582,
Is Pope Greg’s way to do it:

“Just test the year: if it divides
By a hundred, then it’s not one,
Unless you can divide it by
Four hundred – then you’ve got one!”

* According to NASA’s ‘Earth Fact Sheet’, it currently takes about 365.256 days for the Earth’s axis to return to the same alignment relative to the sun (the ‘tropical’ orbital period). The period ‘seen’ by the  stars (the ‘sidereal’ period) is a teeny tad longer.

[Images: researchmaniacs.com (calendar), researchgate.net (orbiting Earth)]
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