It’s leap day, which means lovers of February get to enjoy it just a little bit longer. Fans of 2016 get an extra 24 hours as well, and it’s all due to a bit of astronomy. 

A solar day (from noon to noon) and a solar year (from vernal equinox to vernal equinox) don’t evenly line up. It turns out that a year is 365.2421897 days long. If you had a calendar that was only 365 days, your seasons would gradually drift relative to your months. This was actually the case with the Julian calendar. For this reason the Gregorian calendar was adopted in the 1500s and 1700s.

The Gregorian calendar is similar to the Julian, but occasionally adds an extra day in February from time to time. The extra day is added in every year divisible by 4 with the exception of years divisible by 100 but not 400. So 2016 is a leap year, as was 2000, but 1900 wasn’t and 2100 won’t be. This makes an average year 365.2425, which is still a bit off from the actual year. However the drift is now only 1 day every 7,700 years, rather than the 1 day every 128 years that the Julian calendar had.

There have been proposes to tweak the calendar even further to improve its accuracy. William Herschel, for example, proposed that the year 4000 shouldn’t be a leap year, which would be an even better match to the actual year. Over even longer timescales a modified calendar isn’t particularly useful. That’s because the rotation of the Earth is gradually slowing down, so the days are getting longer, and the number of days in a year is gradually decreasing. Any correction spanning millennia would become obsolete before it could be used.

For now the Gregorian calendar will do, so every 4 years we get an extra day to celebrate our trip around the Sun.

The post Give Or Take A Day appeared first on One Universe at a Time.

It’s often said that Isaac Newton was born on December 25, 1642. That’s true in the fact that Newton’s birth is indicated as that date in historical records, but it is more accurate to note that Newton was born on January 4, 1643. That’s his birthday in our modern Gregorian calendar. At the time of Newton’s birth, England used the older Julian calendar, hence the difference in dates.

The Julian calendar was introduced by Julius Caesar in (what is now) 46 BC. The calendar had 365 days, with an extra day every four years (or leap year). This meant that the average length of a Julian year was 365.25 days. That sounds a lot like the way we do things now, so what makes our modern Gregorian calendar different? The Gregorian year is also 365 days long, with a leap year every four years, but with some exceptions. If a leap year is divisible by 100, (1800, 1900, etc) then there is no extra day. However if the century year is divisible by 400 (1600, 2000, 2400) then there is an extra day. As a result, the average Gregorian year is 365.2425 days long.

This extra complication is necessary because an actual solar year (from vernal equinox to vernal equinox) is 365.2421897 days long. The Julian calendar is a good approximation, but the calendar ran fast by about 3 days every 400 years. Again, not a big deal, but by the time the 1600s rolled around it was about 10 days out of sync with the solar year. The Catholic Church adopted the Gregorian calendar in 1582, and it was quickly adopted by Catholic countries of the day. Protestant countries took longer to adopt the calendar, with England finally adopting it in 1752.

I should point out that when the Julian calendar was adopted, it was known to be slightly off. A century before the Julian calendar was adopted, Hipparchus had calculated the solar year to within 6 minutes of the modern value. But the calendar was easy, and close enough to work well for centuries.

The post Newtonmas appeared first on One Universe at a Time.