“Calendar: Humanity's Epic Struggle To Determine A True And Accurate Year”, by David Ewing Duncan

328 pages, Harper Perennial, ISBN-13: 978-0380793242

Have you ever wondered why every four years an extra day is added to the calendar year? Do you know why ten days were missed forever in the middle ages? Why some years the Holy Week takes place on March and others it takes place on April? What did Julius Caesar, Roger Bacon, and Pope Gregory XIII have in common? Made of a mix of history, science, religion, astronomy, mathematics and politics, David Ewing Duncan's Calendar: Humanity's Epic Struggle to Determine a True and Accurate Year is as entertaining as illustrating; in 15 easy-to-read chapters, David Ewing Duncan narrates to the lay public what an erudite from other times was ever able to discover. Although ostensibly about a very narrow subject, this book tells a much broader story, combining as it does both intellectual and social history with science, with the ultimate issue being “how do we define and measure a year”.

For this is not a simple a scientific question, and the input of religion makes it more difficult still. For example, the most holy of days for Christians is Easter, yet the formula used to determine Easter was based, in part, on the spring equinox. The calendar in use before Pope Gregory was not quite accurate, with the result that Easter in the 16^th Century was being celebrated, according to astronomers, ten days “off”. Science and religion have never been particularly comfortable bedfellows (one only needs to recall Galileo), so any reform was not as simple as it might seem. Duncan tells an excellent story, and what he does best is place in full context the seemingly narrow question of how we set the year. Although seemingly about a narrow subject, this is a wide-ranging and insightful work of history, ably written.