Contemporary vellum. Vellum a nit wrinkled and soiled. Title-page trimmed close at top margin, not affecting text. Pages toned. Overall a very good copy.

"Computus ecclesiasticus was not directed to the subject of calendar reform but to the calculation of the dates of the Christian festivals, chiefly Easter. As implied by the title, Clavius explains two ways of performing the calculations, one using the fingers and another by means of the extensive set of tables included in the work. The text contains no diagrams of the finger methods." (The Erwin Tomash Library on the History of Computing).

"Christopher Clavius was a German Jesuit astronomer who helped Pope Gregory XIII to introduce what is now called the Gregorian calendar... It is perhaps for his work on the reform of the calendar that Clavius is most widely known. The Julian leap-year rule, introduced by Julius Caesar in 46 BC, created 3 leap years too many in every period of 385 years. As a result, the actual occurrence of the equinoxes and solstices slowly moved away from their calendar dates. The date of the spring equinox determines the date of Easter so the church began to press for reform. The Council of 1511, then again the Trent Council of 1563, urged the Pope to act. Pope Gregory XIII consulted his mathematical experts, of whom Clavius was the most senior. Clavius proposed that Wednesday, 4 October 1582 (Julian) should be followed by Thursday, 15 October, 1582 (Gregorian). He proposed that leap years occur in years exactly divisible by four, except that years ending in 00 must be divisible by 400 to be leap years. This rule is still used today and is so accurate that no further reform of the calendar will be necessary for many centuries. In fact it requires 3500 years before an error of one day is reached." (School of Mathematics and Statistics University of St Andrews, Scotland).

"As the foremost mathematician of the Jesuit order, Clavius wrote a number of textbooks, all of which went through numerous editions during his life. These include his version of Euclid's Elements, his commentary on the Sphere of Sacrobosco, and books on algebra, the astrolabe, and practical arithmetic and geometry. Clavius was the senior mathemtician on the commission for the reform of the calendar that led, in 1582, to the institution of the Gregorian calendar. Because of his prodigious output of mathematical works, he was called 'the Euclid of the sixteenth century.' Through his teaching and textbooks, and also through several mathematical curricula drafted by him, Clavius shaped mathematical education in the Jesuit order all over the world." (The Galileo Project)

STC 624004.

HBS # 68561 $1,500