It’s Another Leap Year!

Mind Bending | January 1, 2020

Barlow SkyWatch by Alan J Peche, Director, Barlow Planetarium

It is important to always remember – astronomers’ first duty is time.

  • The Day: the time it takes Earth to rotate once
  • The Month: the time it takes for the Moon to make one orbit around the Earth
  • The Year: the time it takes Earth to orbit around the Sun

Today we know the Earth takes exactly 365.2422 days to complete its trip around the Sun.

The current form of our calendar dates back 4,000 years.

However, the early calendars only had 10 months. This fact can still be seen in our calendar; December, our 12th month, has the root “dec” for ten.

The 12-month calendar can be traced back to Roman King Numa Pompilius, around 1700 B.C. Although, at that time they thought the year was only 355 days long. The calendar was so flawed it lost a month every 3 years. It lost 6 months in less than 20 years. That sounds very interesting until you consider the real-life ramifications; the hottest part of the year would occur in “winter”, while the coldest part in “summer”!

By definition, a leap year is “a calendar year of 366 days, that occurs in years whose number is divisible by 4 EXCEPT for century years whose number is not divisible by 400.”

Thanks to modern measurements we know that there are 146,097 days in 400 years. That is just 365.2422 X 400 (round up to the nearest day). Since our current leap year schedule occurs over 400 years, let’s see what happens when the incorrect numbers are used and compare them to the 146,097 days.

In the earliest calendar that used 355 days – 400 years would have had only 142,000 days – 4,097 days (11+ years) to short!

Julius Caesar in 46 BC started the Julian Calendar based on a year that had 365.25 days –
much better (but not perfect). In the Julian Calendar 400 years would have
146,100 days – 300 years of 365 days (109,500 days) plus 100 years (1 every 4 years)
of 366 days (36,600 days) for a total of 146,100 days – 3 days too long!

In the 16th century, Pope Gregory XIII’s Vatican Observatory astronomers determined the year to accurately be 365.2422 days AND realized the calendar was already behind 10 days since the Julian calendar started. In 1582, Thursday, October 4 was followed by Friday, October 15 – erasing 10 days from the calendar.

Using the leap year rule on the Gregorian calendar from 1601-2000, there were 303 (all years not divisible by 4 and the 3 century years of 1,700, 1,800 and 1,900) of 365 days for a total of 110,595 days. There were 97 years (all years divisible by 4 except the century years) of 366 days for a total of 35,502 days.

110,595 + 35,502 = 146,097 days.

Leap Year Facts

  • With the start of the Julian Calendar, the 5th month named Quintilinus was renamed to July to honor Julius Caesar. It was a month that already had 31 days.
  • Augustus Caesar wanted a month named for himself as well, he picked the 6th month to be renamed August. Since it didn’t have 31 days, a day was taken from February – the last month before the year restarted in March with the start of Spring.
  • In order to correct the errors of the calendar, the first year of the Julian Calendar was called “The Year of Confusion” and had nearly 450 days!
  • To remember which months have 31 days, remember July and August have 31 days thanks to the Caesars. Odd months before July – January, March and May – have 31 days. Even months after August – October and December – have 31 days.
  • Since the Gregorian Calendar was named after a Catholic Pope, it took a while for everyone to accept it. Turkey was the last country to adopt the calendar in 1927.

Each month, the Barlow Planetarium, Wisconsin’s first major planetarium, provides interesting science, technology, engineering and mathematics (STEM) topics for students of all ages.

TO GET A CURRENT SKYCHART: Visit the Skymaps link under the “Astronomy Resources” link at

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