Hello there, I'm an expert in the field of calendar systems and timekeeping. Let's dive into the intricacies of why the year 100 is not considered a leap year in the Gregorian calendar.
The Gregorian calendar, which is the calendar system most widely used today, was introduced by Pope Gregory XIII in 1582 as a reform to the Julian calendar. The Julian calendar, established by Julius Caesar in 45 BCE, had a leap year every four years without exception. This simple rule, while easy to remember, led to a discrepancy as it resulted in the year being about 11 minutes longer than the actual solar year. Over time, this discrepancy accumulated, leading to a misalignment between the calendar year and the astronomical year, which is determined by the time it takes for the Earth to orbit the Sun.

To correct this discrepancy, the Gregorian calendar introduced a refinement to the leap year rule. According to the Gregorian calendar, a year that is divisible by 4 is generally a leap year. However, if that year is also divisible by 100, it is not a leap year, unless it is also divisible by 400. This additional rule helps to account for the slight overcompensation made by leap years in the Julian calendar.

The year 100, for instance, is divisible by 4, but it is also divisible by 100. According to the Gregorian rule, it would not be a leap year because it is not divisible by 400. The year 400, on the other hand, is divisible by both 100 and 400, making it a leap year despite being divisible by 100.

This system more accurately reflects the length of the solar year, which is approximately 365.2425 days. By omitting three leap years every 400 years (those years that are divisible by 100 but not by 400), the Gregorian calendar aligns more closely with the astronomical year.

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To eliminate this error, the Gregorian calendar stipulates that a year that is evenly divisible by 100 (for example, 1900) is a leap year only if it is also evenly divisible by 400. This is because they are evenly divisible by 100 but not by 400. This is because they are evenly divisible by both 100 and 400.Apr 17, 2018read more >>