Dr Stephen Woodcock loves Easter, but not because he's a big chocolate egg fan (although he is).

Dr Woodcock is a mathematician. Mathematicians love cracking open chaos to find the pattern within.

And Easter has one of the most interesting patterns of all.

Easter has no fixed date. It slides across the calendar, propelled by two huge cycles: the cycle of the moon, and the cycle of the calendar.

Together, they create a third cycle, which Dr Woodcock terms the "Easter cycle". If you lived for 5.7 million years, you would see it through just once.