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Why are there 366 days in a leap year? Does the Earth move slower every four years? – Aarush, age 8, Milpitas, California
You may be used to hearing that it takes 365 days for the Earth to make a complete lap, but that journey lasts about 365 and a quarter days. Leap years help keep the 12-month calendar in sync with the Earth’s movement around the Sun.
After four years, those remaining hours add up to a full day. In a leap year, we add this extra day to February, making it 29 days long instead of the usual 28 days.
The idea of an annual catch up goes back to ancient Rome, where people had a 355 day calendar instead of 365 because it was based on the cycles and phases of the Moon. They noticed that their calendar was getting out of sync with the seasons, so they started adding an extra month, called Mercedonius, every two years to make up for the missing days.
In 45 BCE, the Roman emperor Julius Caesar introduced a solar calendar, based on one developed in Egypt. Every four years, February got an extra day to keep the calendar in sync with the Earth’s journey around the Sun. In honor of Caesar, this system is still called the Julian calendar.
But that wasn’t the last tweak. As time went on, people realized that the journey around the Earth was not exactly 365.25 days – it took 365.24219 days, which is about 11 minutes less. So a small correction was better than the need to add a full day every four years.
In 1582, Pope Gregory XIII signed an order that made a small adjustment. There would still be a leap year every four years, except in “century” years – years divisible by 100, such as 1700 or 2100 – unless they were also divisible by 400. It might sound like a puzzle, but this adjustment made the calendar even more accurate – and from that point on, it became known as the Gregorian calendar.
What if we didn’t have leap years?
If the calendar didn’t make that small correction every four years, it would gradually fall out of alignment with the seasons. Over hundreds of years, this could cause the solstice and equinox to be at different times than expected. Winter weather may develop as the calendar showed as summer, and farmers may be confused about when to sow their seeds.
Other calendars around the world have their own ways of keeping time. The Jewish calendar, which is ruled by both the Moon and the Sun, is like a big puzzle with a 19-year cycle. Occasionally, he adds a leap month to ensure that a special celebration happens at the right time.
The Islamic calendar is more unique. It follows the phases of the moon and does not add extra days. Since a lunar year is only about 355 days long, important dates on the Islamic calendar move 10 to 11 days earlier each year on the solar calendar.
For example, Ramadan, the Islamic month of fasting, falls in the ninth month of the Islamic calendar. In 2024, it will take place from March 11 to April 9; in 2025, it will take place from March 1-29; and in 2026, it will be celebrated from February 18 to March 19.
Learning from the planets
Astrology emerged as a way to make sense of our daily lives, connecting the events around us with celestial phenomena. The concept of leap years exemplifies how, from a very young age, people found order in seemingly chaotic conditions.
Simple, unsophisticated but effective tools, based on the creative ideas of ancient astronomers and visionaries, who provided the first glimpses of an understanding of the nature within us. Some ancient methods, such as astrology and lists of astronomical objects, remain even today, reflecting the timeless essence of our approach to understanding nature.
People who do research in physics and astronomy, the field I study, are naturally curious about the workings of the universe and our origins. This work is exciting, and also extremely humbling; it always shows that our lives are only a second in the grand scheme of things in the vast expanse of space and time – even in recent years when we add that extra day.
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This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and analysis to help you make sense of our complex world.
It was written by: Bhagya Subrayan, Purdue University.
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Bhagya Subrayan does not work for, consult with, share in, or be funded by any company or organization that would benefit from this article, and does not disclose any relevant connections beyond their academic appointment.
