1. Romulus (ten lunar months)
2. Numa Pompilius (twelve lunar months)
3. Julian – Julius Caesar, 46BC (the Solar Year)
4. Gregorian Calendar
* This one is complicated but it stems from the fact that although there are 24 hours in a day, the Earth actually takes 23 hours 56 minutes 4 seconds to rotate on its axis and 365 days 5 hours 48 minutes 46 seconds to orbit the Sun. This leads to leap years every 4 years but also to the seasons shifting.
The original Roman calendar had 10 months and was 304 days long. Numa Pompilius added January and February extending the year to 355 days. He also also ordered a new month called Mercedinus to be included on alternate years adding another 22 or 23 days to it.
Julius Caesar ordered a thorough reform in 46BC, and the new calendar was named after him. The Gregorian Calendar was the brainchild of Pope Gregory XIII; his papal bull of February 24, 1582 decreed that October 4 that year would be followed by October 15. This change did not reach Britain until 1752 when following an Act of Parliament in 1750, September would lose 11 days, jumping from September 2 to September 14. Additionally, the first day of the year would move from March 25 to January 1. Needless to say, these changes caused much confusion, and confusion continues with the use of both Arab and Jewish calendars.
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