The Gregorian calendar is the most widely used calendar system in the world today. It was introduced in 1582 by Pope Gregory XIII to replace the Julian calendar, which had been in use since 45 BCE. The Julian calendar had a leap year every 4 years without exception, which meant that it was gradually getting out of sync with the solar year. The Gregorian calendar was designed to correct for this by skipping leap years in years divisible by 100 but not by 400.
The reason for this rule is that the length of a year is not exactly 365.25 days, but rather 365.2425 days. This means that if we added an extra day to the calendar every 4 years, we would be adding slightly too much time, and the calendar would eventually get out of sync with the solar year. To correct for this, the Gregorian calendar skips leap years in years that are divisible by 100 but not by 400. This makes the length of the calendar year slightly shorter than the true solar year, but over the course of many centuries, the difference accumulates to only a few days.
Skipping leap years in years divisible by 100 but not by 400 is a relatively recent development in the history of the calendar. The Gregorian calendar was adopted gradually by different countries over several centuries, and it was not until the 20th century that it became the most widely used calendar in the world.
The year 1700 was not a leap year because it is divisible by 100 but not by 400. Similarly, the years 1800 and 1900 were not leap years for the same reason. On the other hand, the year 2000 was a leap year because it is divisible by both 100 and 400. This means that the year 2000 was an exception to the rule that years divisible by 100 but not by 400 are not leap years.
The reason for this exception is to keep the calendar in sync with the solar year as closely as possible. If the year 2000 had not been a leap year, the calendar would have been slightly out of sync with the solar year, and over time this would have caused the seasons to gradually shift earlier in the calendar year.
While the Gregorian calendar is the most widely used calendar system today, there are other calendar systems that are used in various parts of the world. For example, the Islamic calendar is a lunar calendar, which means that it is based on the cycles of the moon rather than the cycles of the sun. The Jewish calendar is a lunisolar calendar, which means that it is based on both the cycles of the moon and the cycles of the sun.
In conclusion, the leap years that are skipped in the Gregorian calendar are those that fall on years divisible by 100 but not by 400. This rule was introduced to keep the calendar in sync with the solar year as closely as possible, and it has been successful in doing so for over 400 years. The Gregorian calendar is recognized as the international standard for civil purposes, and it is used by most countries around the world for the purpose of determining dates and scheduling events. However, there are other calendar systems that are used in various parts of the world, and each system has its own unique features and advantages.