New Year Special: Why We Have 365 Days in a Year, 24 Hours in a Day, and the Strange Length of Months

As we welcome the new year, many of us reflect on time—celebrating the end of one year and the start of another. But why exactly does a year have 365 days, a day 24 hours, and months that vary in length? It’s a fascinating story that spans astronomy, ancient history, and human decisions.

The reason a year is 365 days long has to do with Earth’s orbit around the Sun. It takes approximately 365.25 days for the Earth to complete one full orbit, or revolution, around the Sun. This period is known as a “tropical year.” However, because we can’t measure time in fractions of a day for our calendar system, we round it down to 365 days. The extra quarter day is accounted for every four years with a leap year, when we add an extra day to the calendar.

(NASA Science)

A day, on the other hand, is 24 hours long because that’s how long it takes for the Earth to rotate once on its axis. Interestingly, the concept of a 24-hour day comes from the ancient Egyptians, who divided the day into 12 hours of daylight and 12 hours of night, based on their observations of the movement of the Sun and stars. This system stuck, and even though many cultures had different ways of measuring time, the 24-hour day became standard.

The months, however, have a more complex history. The length of months is rooted in ancient Rome. The Roman calendar originally had 10 months, with a year that totaled 304 days. Eventually, two months—January and February—were added to align the calendar with the lunar cycle and the seasons. However, this still didn’t perfectly match the solar year, so adjustments were made. Emperor Julius Caesar introduced the Julian calendar in 45 BCE, with 12 months and a leap year to account for the discrepancy. Augustus Caesar later made further tweaks, resulting in the calendar we roughly follow today. The uneven number of days in months—some with 30, some with 31—is a relic of these early adjustments, and the need to fit 12 months into a solar year.

So, as we celebrate the start of a new year, it's worth remembering that our modern calendar is the result of a fascinating mix of astronomy and ancient history.