It's that time of the year again, Saint Patrick's day. That is St. Patrick, patron of Ireland, Nigeria, Montserrat and engineers

We all remember how Christianity was brought to the island of Ireland, and who provided the inspiration for Downpatrick's glass-and-stone visitor centre. Patrick was of course the inspiration for the name of the town. I can only thank local sensibility for keeping the Enler brown (the picture is the Chicago River in 2005). Unusually, St. Patrick's day also lies in holy week this year, which last occured in 1940, when the feast was celebrated on 3rd April rather than interfere with Palm Sunday. It won't happen again until 2160.  This is a result of the unusually early Easter, which is  calculated from a 5,700,000-year cycle. This year Easter Sunday falls on the 23rd March, the second-earliest possible, the earliest being the 22nd. 0.97% of all Easters within the aforementioned cycle lie on this day, compared to 3.9% on 19th April, the most common day.  The calculation of the date of Easter is in itself a complicated process, and is performed by use of the computus, named such because at the time of its discovery it was the most important function of mathematics.