Earth’s orbit prompts a kink in the calendar

The year 2024 is a leap year, giving February an extra day. This will make Thursday, Feb. 29, a leap day – something that happens once every four years.

But how is this kink in the calendar tied to Earth science?

Benjamin Black, an assistant professor in the Department of Earth and Planetary Sciences in Rutgers–New Brunswick’s School of Arts and Sciences, can explain.

man with light blue oxford button shirt with top button unbuttoned with microscope in background
Benjamin Black, a volcanologist, says creating a calendar to closely match Earth's yearly orbit around the sun has required some fine-tuning over the centuries.
Benjamin Black

Black is an Earth scientist whose research focuses on volcanism and Earth’s past climate. Earth’s long-term climate history is shaped by variations in Earth’s orbit and exchange of gases between the atmosphere and Earth’s interior. He is a 2023 Sloan Research Fellow, a 2024 Kavli Fellow and recipient of a Faculty Early Career Development Program (CAREER) award from the National Science Foundation.

As far as the calendar is concerned, why do we need to add an extra day in February? And what does this have to do with Earth and its orbit?

As we all know, there are 365 days in a calendar year. But it actually takes 365.25 days for the Earth to complete its journey around the sun. So, every 4 years, we need to add an extra day to average out to the actual amount of time it takes for the Earth to orbit the sun. The fact that we add this bonus day in February is a quirk of history. I had to look this part up, and I was surprised to learn that the story goes back to the Romans and to the 16th century Catholic church. Julius Caesar was the one who first instituted the Julian calendar in 45 BC, creating the first leap years. This system was tweaked in the 16th century to create the Gregorian calendar, which accounts for the fact that Earth's orbit does not take precisely 365.25 daysit's just a hair less.

Is the present solution to this calendar-orbit quandary a good one in your mind? Will civilization always need to do this?

I think our current solution works. But actually, Earth's days are gradually getting longer, by a couple of milliseconds per century, as Earth transfers angular momentum to the moon. So, in a few million years, maybe we won't need a leap day anymore!

Q. What do you expect to do with your "extra day" on Thursday, Feb. 29, 2024?

My plan for this leap day is to work on a paper about Earth’s greatest mass extinction. There’s no particular connection to Feb. 29, but it was a major turning point in Earth’s history, without which humans (and the calendars they have created) arguably might not exist.