Dear Audrey,

I use a calendar to keep up with my work as a science cat. I also love calendar apps that count down to big events—like my birthday. People have always tracked time for work and holidays.

I talked about this with my friend Nikolaus Overtoom. He’s a professor of ancient history at Washington State University.

He told me we use the Gregorian calendar today. That’s a revised version of the Julian calendar. The Romans invented the Julian calendar.

But there were calendars before that. Ancient people all over the world had calendars—including a detailed calendar made by the ancient Maya.

“Early people looked to the heavens to understand the movement of planets and stars,” Overtoom said. “They used that information to help structure their societies. They needed to know when to plant crops or move their herds.”

The Julian and Gregorian calendars are solar calendars. A solar calendar tracks the position of the sun as Earth orbits all the way around it. That’s called a solar year.

It takes Earth 365 days, 5 hours, 59 minutes and 16 seconds to go all the way around the sun. That’s a challenge for making calendars because it’s not a whole number.

The calendar the Romans used before the Julian calendar only had 355 days. Since it didn’t line up with the solar year, the seasons shifted a little every year. That made it hard to plan things. Like when to plant crops or have religious festivals.

So, the Romans added temporary months to the calendar to catch up. That was the high priest’s job. Sometimes they added extra months for corrupt reasons. Like so their friends could stay in power longer.

Pre-Julian calendar, rebuilt from pieces found in Rome; photo by Bauglir


“By the 1st century BCE, the Romans realized the calendar was not working,” Overtoom said. “It was almost to the point it was a disaster.”

Then Julius Caesar came to power. He told astronomers to fix the calendar to line up with the solar year. The result was the Julian calendar. It was 365.25 days long. It was close to matching the solar year—but not quite. It was still 11 minutes off. So, the calendar gained one day every 128 years.

About 1,500 years later, Pope Gregory XIII decided to fix it.

“Astronomy had come a long way,” Overtoom said. “They were able to pinpoint that the solar calendar is 365.24 days. They adjusted the calendar, so it doesn’t slip over time.”

The calendar has 365 days most years. Every four years, we have one extra day—February 29. Years with 366 days are called leap years.

Most countries use the Gregorian calendar now. That makes it easier for people to work together. But there are other calendars out there.

The Romans also gave us the months of the year. Have you noticed that some months seem out of order? The prefix “oct” means eight—like the eight arms of an octopus. But October is the 10th month. That’s because the pre-Julian calendar only had 10 months. Back then, October was the 8th month. Eventually, they added two more months at the beginning of the calendar and pushed October back.

You could say that some months were real roamin’ numerals.


Dr. Universe