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Why are there 366 days in a leap year? Is the world moving slower every four years? – Aarush, 8 years old, Milpitas, California
You may have heard that it takes 365 days to complete one trip around the world, but this journey actually takes about 365 and a half days. Leap years help the 12-month calendar align with the Earth’s motion around the Sun.
Four years later, the remaining hours add up to one day. In a leap year, we add this extra day to February, making it 29 days instead of the normal 28 days.
The idea of an annual track dates back to ancient Rome, where people had a calendar of 355 days instead of 365 because it was based on the cycles and phases of the Moon. They realized that their calendars were out of sync with the seasons and began adding an extra month, which they called Mercedonius, every two years to make up for the missing days.
In 45 BC, Roman emperor Julius Caesar instituted a solar calendar based on the solar calendar developed in Egypt. Every four years, February is given an extra day to keep the calendar in line with the Earth’s journey around the Sun. In honor of Caesar, this system is still known as the Julian calendar.
But this was not the last change. As time went on, people realized that the Earth’s journey was not exactly 365.25 days, but actually took 365.24219 days, which is about 11 minutes less. So adding a full day every four years was actually a bit more adjustment than necessary.
In 1582 Pope Gregory XIII signed an order making a minor adjustment. Every four years would be a leap year, except for “century” years (years divisible by 100, such as 1700 or 2100), unless they were also divisible by 400. It may sound like a bit of a puzzle, but this adjustment made the calendar even more accurate and from then on was known as the Gregorian calendar.
What if we didn’t have leap years?
If the calendar did not make this small correction every four years, it would gradually lose harmony with the seasons. Over centuries, this can lead to solstices and equinoxes occurring at different times than expected. Winter weather may develop during what the calendar shows as summer, and farmers may be confused about when to plant their seeds.
Other calendars around the world have their own ways of keeping time. Regulated by both the Moon and the Sun, the Jewish calendar is like a big puzzle with a 19-year cycle. A leap month is occasionally added to ensure special celebrations occur at the right time.
The Islamic calendar is even more unique. It follows the phases of the Moon and does not add extra days. Since a lunar year is only 355 days long, important dates in the Islamic calendar move 10 to 11 days earlier each year than in the solar calendar.
For example, Ramadan, the Islamic month of fasting, falls in the ninth month of the Islamic calendar. In 2024, it will run from March 11 to April 9; In 2025, it will take place between March 1-29; In 2026, it will be celebrated from February 18 to March 19.
learning from planets
Astronomy emerged as a way to make sense of our daily lives by connecting events around us to celestial events. The concept of leap years exemplifies how humans have found order in seemingly chaotic conditions since ancient times.
Simple, basic yet effective tools born from the creative ideas of ancient astronomers and visionaries provided the first insights into understanding the nature that surrounds us. Some ancient methods, such as astrometry and lists of astronomical objects, survive even today and reveal the timeless essence of our quest to understand nature.
People who do research in physics and astronomy, the fields I work in, are inherently curious about the functioning of the universe and our origins. This work is exciting and also incredibly humbling; consistently shows that, in the grand scheme of things, our lives occupy only a fraction of a second in the vast expanse of space and time; even in leap years when we add an extra day.
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This article is republished from The Conversation, an independent, nonprofit news organization providing facts and analysis to help you understand our complex world.
Written by: Bhagya Subrayan, Purdue University.
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Bhagya Subrayan does not work for, consult, own shares in, or receive funding from any company or organization that may benefit from this article, and has disclosed no relevant affiliations other than academic appointments.