Why does a leap year have 366 days?

The conversationWhy does a leap year have 366 days? Does the Earth move slower every four years? – Aarush, age 8, Milpitas, California

You may be used to hearing that it takes the Earth 365 days to complete one full lap, but that journey actually takes about 365 and a quarter days. Leap years help keep the 12-month calendar in line with the Earth’s movement around the sun.

After four years, the remaining hours make up a whole day. In a leap year, we add this extra day to the month of February, making it 29 days long instead of the usual 28.

The idea of ​​an annual catch-up dates back to ancient Rome, where people had a calendar with 355 days instead of 365 because it was based on cycles and phases of the moon. They noticed that their calendar was no longer in sync with the seasons, so they began adding an extra month every two years, which they called Mercedonius, to make up for the missing days.

In the year 45 BCE, Roman Emperor Julius Caesar introduced a solar calendar, based on one developed in Egypt. Every four years, February was 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 that wasn’t the last adjustment. As time went on, people realized that Earth’s journey was not exactly 365.25 days; in reality it took 365.24219 days, which is about 11 minutes less. So adding a full day every four years was actually a little more correction than necessary.

In 1582, Pope Gregory XIII signed an order that made a minor adjustment. There would still be a leap year every four years, except in ‘eternal’ years – years that are divisible by 100, such as 1700 or 2100 – unless they were also divisible by 400. It may sound a bit like a puzzle, but this one adjustment made the calendar even more accurate – and from then on it was known as the Gregorian calendar.

What if we didn’t have leap years?

If the calendar did not make that small correction every four years, it would gradually move out of step with the seasons. Over the centuries, this could lead to the solstices and equinoxes occurring at different times than expected. Winter weather could develop into what the calendar indicated as summer, and farmers could become confused about when to plant their seeds.

Without leap years, our calendar would gradually become separated from the seasons.

Other calendars around the world have their own ways of keeping time. The Jewish calendar, which is regulated by both the Moon and the Sun, is like a large puzzle with a 19-year cycle. Every now and then a leap month is added to ensure that special celebrations occur at just the right time.

The Islamic calendar is even more unique. It follows the phases of the moon and does not add extra days. Because a lunar year lasts only about 355 days, important dates on the Islamic calendar shift 10 to 11 days earlier on the solar calendar each year.

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 this will take place from March 1 to 29; and in 2026 it will be celebrated from February 18 to March 19.

Learning from the planets

Astronomy emerged as a way to give meaning to our daily lives, by linking the events around us to celestial phenomena. The concept of leap years illustrates how people from an early age found order in circumstances that seemed chaotic.

Simple, unsophisticated but effective tools, born from 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, persist even today, revealing the timeless essence of our quest to understand nature.

part of the ceiling of the tomb of Senenmut, a high court official in Egypt, was destroyed sometime around 1479–1458 B.C.  drawn.  It shows constellations, protective deities and 24 segmented wheels for the hours of the day and months of the year.

The ancient Egyptians were dedicated astronomers. This portion of the ceiling of the tomb of Senenmut, a high court official in Egypt, was drawn sometime around 1479–1458 BCE. It shows constellations, protective deities and 24 segmented wheels for the hours of the day and months of the year.

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 continually shows that in the grand scheme of things, our lives only take up a second in the vast expanse of space and time – even in leap years if we add that extra day.


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Bhagya SubrayanPhD student in physics and astronomy, Purdue University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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