Part 1 of Daniel's Messianic Prophecy
For ancient civilizations, the moon was the celestial body that determined time. A year was composed of twelve lunar months. The length of a lunar month was determined by observing the phases of the moon. An old month ended and a new month began at the beginning of a new moon when a thin crescent curving towards the north appeared on the western horizon at sunset.
Modern astronomers define a lunar month as the time it takes for the moon to revolve around the earth with reference to the sun. One revolution is completed in 29 days, 12 hours, 44 minutes, and 2.7 seconds. When expressed as a decimal fraction, the length of a lunar month is 29.531 days. Consequently, a lunar year spans 354.372 days.
Ancient priests determined the appropriate time for festivals not only by observing the moon but also by monitoring the crops. If the crops weren’t mature enough, the incoming year would be postponed by one month. That manoeuvre added an extra month to the outgoing year, resulting in an anomalous 13 month year. It was an arbitrary act that wasn’t performed with any regularity.
The Babylonians went a step further. Not only did they consider the moon in their determination of time but also the sun and the seasons. In the sixth century BC, they discovered a 19 year cycle in which 7 years had an extra month. In other words, a 19 year Babylonian cycle contained seven 13 month years. That cycle matched up almost perfectly with 19 solar years.
In 130 BC, Hipparchus determined the length of a solar year or the time it took for the earth to revolve around the sun. The time that elapsed in one complete revolution was 365.25 days according to the Greek astronomer.
The Julian calendar was introduced by Julius Caesar in 46 BC. It was based on the solar year. To offset the rounding down of the decimal fraction, a leap day was added to the Julian calendar every four years.
Our modern Gregorian calendar was introduced by Pope Gregory the 13th in 1582. He discovered that the duration of a solar year was 365.2425 days. That's 10.8 minutes less than the 365.25 days used as the basis for the Julian calendar.
Gregory's calendar provides for a leap day every fourth, even year. However, to offset the 10.8 minute difference, century years are leap years only if they are exactly divisible by 400. In other words, only one in every four century years is a leap year. With that arrangement, the difference between the Gregorian calendar and solar time amounts to only one day every 3,500 years. By comparison, the difference between the Julian calendar and solar time amounts to one day every 131 years.