The time it takes for the Earth to orbit the Sun does not correspond to an even number of days. A solar year actually has 365 days, 5 hours, 48 minutes, and 46 seconds, according to calculations by NASA. As a result, every year, the 365-day calendar falls about a quarter of a day behind the solar year. Although this may not seem like a significant difference, over four years, it amounts to approximately a whole day. This is where leap years come in. Without them, the summer season that we typically experience in June in the Northern Hemisphere would occur in December in 700 years.
For example, if we were to eliminate leap years today, in 2028, the calendar would advance by 1 day. While the winter solstice would be on December 21, the new date for winter to begin in the Northern Hemisphere would be December 22 in 2028.
In 2053, the calendar would advance by 7 days. That is, the winter solstice would now be on December 28.
In 2148, the calendar would advance by 30 days, and the winter solstice would be on January 20.
In 2396, the calendar would have advanced by 90 days, and the winter solstice would be on March 21.
In 2777, the calendar would have advanced by 182 days, and the winter solstice would be on June 21, which is when the summer solstice begins in the current calendar.
Let’s understand a bit of history
Many ancient populations, including the Sumerians, Chinese, and Romans, created calendars based on the phases of the moon. While lunar calendars do a good job of tracking months, they are not as effective at monitoring the seasons.
This created some problems in ancient Rome. Around 500 B.C., the Roman republican calendar consisted of 12 lunar months, totaling 355 days—about 10 days shorter than the solar year. To keep the calendar synchronized with the seasons, the Romans added an extra month of 27 or 28 days, called Mercedonius, every few years. However, due to political maneuvering, this was done so inconsistently that this system caused widespread confusion.
Around 46 B.C., the Roman Emperor Julius Caesar proposed a solution: the Julian calendar. This new 12-month calendar would always consist of 365 days, except every four years, when an additional day would be added. This raised the average length of the Julian year to 365.25 days—much closer to the 365.2422 days of a solar year.
But unfortunately, it was still not close enough. The average year in the Julian calendar was still 11 minutes and 14 seconds longer than the solar year, so calendar errors began to accumulate over time.
By the end of the 16th century, the calendar was about 10 days behind, and the Catholic Church was struggling to determine the date of Easter, which was supposed to fall on the Sunday following the first full moon after the spring equinox.
The modern calendar
Pope Gregory XIII realigned the seasons in 1582 by eliminating the extra 10 days. That year, Thursday, October 4, was followed by Friday, October 15. To fix the calendar going forward, he introduced what we now use: the Gregorian calendar. It is essentially the Julian calendar but with a new rule: all years divisible by four must be leap years, except for centennial years, which must be divisible by 400 to be leap years.
Thus, 800, 1200, and 2000 are leap years—but 1700, 1900, and 2100 are not, because although they are divisible by four, they are not divisible by 400.
This raised the average length of the Gregorian year to 365.2425 days, a difference of just 26 seconds compared to a solar year.
While in the Julian calendar there is an accumulated daily shift every 129 years, this only happens every 3,333 years in the modern calendar.
In other words, this is a problem for another millennium.
Source: CNN