Hello there! As an expert in the field of calendars and timekeeping, I'm here to provide you with a detailed explanation of the calendar year and how many days are in a non-leap year.
The Gregorian calendar, which is the calendar system most widely used today, was introduced by Pope Gregory XIII in 1582 as a refinement to the Julian calendar. The main purpose of this reform was to bring the date for the spring equinox closer to March 21st, which is important for the calculation of Easter. To achieve this, ten days were dropped from the calendar, and a new rule for leap years was established.
In a non-leap year, there are 365 days. This is the standard length of a year in the Gregorian calendar. However, as you mentioned, February 29, known as leap day, is added to the calendar every four years to account for the fact that the Earth's orbit around the Sun is not exactly 365 days. This extra day is necessary because it takes the Earth approximately 365.2425 days to complete one orbit around the Sun. Without this adjustment, our calendar would drift out of sync with the astronomical year, and over time, seasonal events like the equinoxes and solstices would occur on different dates than they do now.
The rule for determining a leap year in the Gregorian calendar is as follows:
1. If the year is evenly divisible by 4, it is a leap year, except:
2. If the year is evenly divisible by 100, it is not a leap year, unless:
3. The year is also evenly divisible by 400, in which case it is a leap year.

This means that the year 2000 was a leap year, as it is divisible by 400, but the year 1900 was not a leap year, even though it is divisible by 100, because it is not divisible by 400.

Now, let's move on to the translation of the answer into Chinese.

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February 29 is a date that usually occurs every four years, and is called leap day. This day is added to the calendar in leap years as a corrective measure, because the Earth does not orbit the sun in precisely 365 days. The Gregorian calendar is a modification of the Julian calendar first used by the Romans.read more >>