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Astronomy How many days are there in one year?
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Astronomy
How many days are there in one year?
A year is the average time that it takes for the Earth to complete one orbit around the sun. For accurate measurements, we must consider a “mean tropical year”. This is the average time that the Sun takes to return to the same position as seen from the Earth.
A mean tropical year lasts 365.24219879 days, equal to 365 days, 5 hours, 48 minutes and 49.98 seconds. Most years are counted as 365 days. Several methods have been devised to account for the extra part day.
Julius Caesar introduced the “Julian calendar” to the Romans Empire in 46 BC. It had a 366-day leap year every four years, which assumes exactly 365.25 days in a year – an error of 0.0078 days per year.
In 1582, Pope Gregory XIII introduced the “Gregorian calendar” in several countries in Europe and this system is widely used today. This also has leap years (366 days) every four years with the exception of years divisible by 100 but not 400 (i.e. 97 leap years per 400 years). By this time, there was a 10-day difference between the tropical year and the calendar year. Pope Gregory decreed that the day after October 4, 1582 would be October 15, 1582, causing riots among people who thought that they had lost 10 days of their lives and had been cheated of 10 days rent by their landlords.
The Gregorian calendar is not perfect. It assumes 365.2425 days per year – an error of 0.0003 days per year. This means that 10,000 years from now we will need to lose three days!
See also Eric Weisstein's World of Science and Wikepedia
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