The modern calendar could use some small changes, such as making sure we don’t have to keep changing calendars each year (see below). But the real problem with the modern calendar isn’t the “human factor”; it’s nature. As our Earth orbits around the Sun, it wobbles like a spinning top in a process called precession. Because scientists can measure the planet’s movements more accurately now than in the past, they know that the wobble is increasing. This is because the tides caused by the pull of the Sun and Moon are slowing the Earth’s spin. And like a top, as the spinning slows, the wobble increases and the length of the year decreases.

What does this mean for our calendar? It is already known that our calendar and the length of a year were only off by 24 seconds (0.00028 days) in 1582—a very small discrepancy that will eventually be noticed. But when you add in the slowing down of the Earth’s rotation, it will make the year even shorter. In fact, since 1582, the year has decreased from 365.24222 days to 365.24219 days, or an actual decline of about 2.5 seconds.

The 2012 world calendar. Each year, the calendar is slightly different because there are 365 days in a year, which is not evenly divisible by the seven-day week. Also, every four years—leap years—there is an extra day in February.