5 minute read
Understanding the leap year
As we step into the new year, many of us are making detailed plans to reach our goals. Whether it is learning something new, overcoming a personal challenge, or saving money, we usually have a well-thought-out plan for 365 days. But sometimes, there is a surprise – an extra day is added to our calendar. This seemingly exceptional event has both historical and astronomical significance. It has long intrigued people and has raised questions about the nature of time, ultimately leading to the creation of the calendar as we know it today, where each fourth year has 366 days.
A leap year is the result of two principal time cycles on Earth. One is the time Earth takes to make a full spin on its axis, which is exactly one full day, and the second is the time it takes to orbit around the sun, which is a year. Earth revolves around the sun in 365.2522 days, which is known as the astronomical year. But since there are only 365 days in a calendar year, what happens to the remaining quarter? Well, this is where the leap year comes in and saves the day. Every year we set aside the quarter day until the fourth year, when we have enough quarters to make up a full day, which is then added to February. And with that, we have regained synchronisation… or maybe not quite.
It was the Egyptians who first figured that a year is 365.2522 days by watching the stars, especially Sirius, which would noticeably drift off track. Many years later, astronomers under the Roman emperor, Julius Caesar, also figured out the drift and created the Julian calendar, incorporating the leap year system. This calendar approximated the leap year at 0.25 days by adding a day every four years. This reform aimed to synchronise the calendar with Earth’s orbit around the sun, ensuring a proper alignment of seasons. However, the Julian calendar slightly overestimated the solar year. After rounding off to the quarter day, there are still 0.0022 days in a year to account for – a seemingly minute difference. However, over long enough periods it adds up, pushing the whole system out of sync by 3 days within 400 years and leading to a gradual misalignment between the calendar and the astronomical year.
By the 16th century, this discrepancy had become noticeable and so the Gregorian calendar, introduced by and named after Pope Gregory XIII, came into existence. To address the imbalance, several new rules were introduced. The first is that a year is considered a leap year if it is divisible by 4. But, and this is what distinguishes the Gregorian from the Julian calendar, there are two exceptions: number one is that a leap year cannot fall on a year that is divisible by 100; however, number two stipulates that if a leap year is divisible by a 100, it should also be divisible by 400 for a leap day to be added. That is why there was a leap year in 1600 and 2000 but not in 1700, 1800 and 1900. This adjustment resolved the accumulated error, ensuring that the average length of a calendar year stays closely aligned with the actual length of the astronomical year.
Still, in the long run, we will have to adjust our calendar once more. The present Gregorian calendar would be nearly ideal if Earth’s rotation rate, the orientation of its axial tilt, and its orbital speed around the sun were all constant. But in actual fact, the rotation rate increases somewhat each time there is an earthquake – an effect which, however, is outweighed by Earth’s gravitational pull from the sun and moon, which slows it down. And while that slowing down rate is not noticeable to us who are living today, if we go back in time, right before the Cambrian explosion of life (that is some 540 million years ago), a day was a little shorter than 22 hours – meaning the axial spin has slowed down significantly over the past 500 million years or so. And at the time of Earth’s formation some 4500 million years ago, a day was actually only about seven hours long! As the days get longer, the calendar years shorten, because Earth spins less often on its axis during its journey around the sun. So, at some point down the line, we will need to cut back on the leap days and eventually eliminate them altogether. In a few million years, the average length of a day will have gained the 56 seconds it currently falls short of the full 24 hours, so no leap year will be necessary. And still further on, each day will be longer than 24 hours by that amount, and our remote descendants will have to introduce a leap year which requires the subtraction instead of addition of a day to keep the calendar in line with the astronomical year.
The concept of leap years has extensive implications in other fields of science besides astronomy, as well as in culture. Astronomically, leap years ensure the synchronisation between our calendar and Earth’s orbit, facilitating accurate astronomical calculations and observations. In science, accurate measurement of time is crucial in fields such as physics and technology, while in culture, the leap year has had wide-ranging effects on societies, their traditions, superstitions and folklore, but also on economic cycles.
The introduction of leap years indicates humanity’s continuous attempts to comprehend and coexist with nature. In the development of our calendar system, we tried to accurately track time, navigate the seasons and synchronise our activities with the rhythms of the sky. The current calendar – including the leap year – is the result of that diligent study over thousands of years, reminding us of the complex interactions between our lives and the cosmos.
