However, this was actually a slight overcorrection to the problem. In the year 46 BC, Julius Cesar proposed the new Julian Calendar which would add an additional day to the shortest month of the year (February) every four years in an attempt to allow for a predictable correction to the issue of the quarter day drift.

As they were always at war, the calendar and the solar year quickly got out of sync. However, the Romans believed that the correction during war time was bad luck. The Romans knew about this drift and at that time the Pontifex Maximus (the chief high priest) would correct the calendar by occasionally adding an extra day to minimise the drift. This is not ideal as agricultural and religious events would not be in sync with the earth’s position around the sun year after year. Therefore, if we assume that a year is 365 solar days long, the timing in our calendar and the position of the sun (and therefore the seasons) drift apart by a quarter of a day each year. In fact, the solar year was 365 and one quarter solar days long. Early astronomers found that a solar year was not exactly divisible by a whole number of solar days. The solar (or tropical) year was defined as the time it takes for the sun to return to the same position in the sky after a complete cycle of the four seasons. A solar day was defined as the time it takes for the sun to return to the highest point in the sky on two consecutive days. This would allow them to plant crops and harvest them at the correct times with respect to the different seasons.Īn easy way of measuring time back then was by tracking the position of the sun in the sky. However, after the invention of agriculture, it became important for farmers to keep track of each day in the year. But how are leap years calculated and who worked it out? The answers lie in astronomy and early agriculture with a little help from Julius Caesar and Pope Gregory XIII, explains Dr James McCormac, an expert in astrophysics from the University of Warwick’s Physics Department.īefore the invention of agriculture people only really needed to know roughly what season it was and what season was coming up next. 151.You may know that every four years February gets an extra day and we have what’s called a “leap year”.

And Weidler, in doing honour to the astronomer for his candour and caution, seems to follow Pliny. et Sosigenes ipse tribus commentationibus, quanquam diligentior esset ceteris, non cessavit tamen addubitare, ipse semet corrigendo.' According to our view of this passage the tres commentationes are of the three occasions on which, during the time of Augustus, an intercalation had to be omitted : Pliny seems to make each of them a separate interference of Sosigenes (whom he may seem to keep alive at Rome for the purpose) for the correction of his period. 18.25) mentions the Augustan correction, most probably, as if it had been a correction of the theory of the calendar, arising out of the further investigations of Sosigenes himself : his words are ' ea ipsa ratio postea comperto errore correcta est, ita ut duodecim annis continuis non intercalaretur. Riccioli and others represent that he remained at Rome until the time of Augustus, and aided in the final establishment of the calendar according to the intention of Julius.īut it must be clear that if Sosigenes had remained at Rome, the Augustan correction never could have been needed : the leap-year would never have been made a triennial intercalation under the eye of the astronomer himself.

2.8) is interpreted by Weidler as implying that Sosigenes maintained the motion of Mercury round the sun. The most definite of them is that of Simplicius, who says he wrote on astronomy.Ī sentence of Pliny ( Plin.

With the exception of certain allusions to him by name, which simply confirm the fact that he was considered a skilful astronomer, nothing can be found concerning him. 46), is called an Egyptian, but may be supposed to have been an Alexandrian Greek. （ Σωσιγένης), the peripatetic, the astronomer employed by Julius Caesar to superintend the correction of the calendar (B.

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