To improve upon the over correction made by the Julian calendar, the Gregorian calendar skips three leap days every 400 years. Many countries including the UK still use the Gregorian calendar today. In 1582 he gave us the Gregorian calendar, which modified the Julian Calendar to account for the 11.2-minute drift.
Cue Pope Gregory XIII, the Pontifex Maximus of the Catholic church at the time. As the solar year was not exactly 365.25 days but was in fact slightly less at 365.2422 solar days, the Julian Calendar and the solar year were now drifting apart again, albeit much more slowly, at a rate of 11.2 minutes per year.īy the late 1500’s this small overcorrection in the Julian calendar had accumulated to a drift of 13 days with respect to the solar year. 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. You may know that every four years February gets an extra day and we have what’s called a “leap year”.
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