what's the difference between the leap years of julian and gregorian calendars

The Julian and Gregorian calendars both have systems for accounting for leap years to ensure that the calendar year aligns with the astronomical year, which is approximately 365.2425 days long. However, they handle leap years slightly differently:

Julian Calendar Leap Years:

• The Julian calendar was introduced by Julius Caesar in 45 BCE and added an extra day to the calendar every four years.
• In the Julian calendar, years divisible by 4 are generally considered leap years.

Gregorian Calendar Leap Years:

• The Gregorian calendar, introduced by Pope Gregory XIII in 1582, aimed to correct the error in the Julian calendar, which accumulated over time due to the slight discrepancy between the length of the solar year and the Julian leap year.
• In the Gregorian calendar, a year is a leap year if:
  • It is divisible by 4.
  • It is not divisible by 100, unless it is also divisible by 400.

This means that while most centennial years are not leap years, years like 1600 and 2000 are leap years because they are divisible by 400.

Examples:

• The year 1900 is not a leap year in the Gregorian calendar because it is divisible by 100 but not by 400. However, it would be a leap year in the Julian calendar because it is divisible by 4.
• The year 2000 was a leap year in the Gregorian calendar because it is divisible by both 4 and 400.

The Gregorian system's adjustment more accurately accounts for the solar year's length, reducing the need for further calendar reforms. As a result, the date of the vernal equinox is better aligned with the date used for calculating Easter, which was one of the primary motivations for the Gregorian reform.