You probably already know that 2020 is a leap year and thus tomorrow, Saturday, February 29 is a leap day. You may even know why there’s an extra day in the calendar this year. But, do you really know why? (OK, you probably do – it’s hard to surprise a group of smart people). Are you ready, Quiz Kids? Let’s find out how much you know!

True or false? *One year ago, right now as you read this, the Earth was in exactly the same place in its orbit around the Sun as it is now.* The correct answer requires an explanation, but here’s a hint: don’t lean on the calendar year for support – there are exactly 365 days in the calendar year, or sometimes 366 days.

Astronomers have long known that the Earth takes approximately 365.25 days to make one complete orbit around the Sun. We say approximately, for a couple of reasons. First, to make the math and the explanation simple to understand, and second, to avoid the unnecessary complication of discussing the difference between a sidereal year and a tropical year. [Spoiler Alert: Purely in terms of time, the difference between the two is 20 minutes, 24.5 seconds, with a sidereal year being slightly longer than 365.25 days and a tropical year being slightly shorter than 365.25 days. (We told you that it was an unnecessary complication)].

In an ideal universe, a year would be exactly divisible by the number of days in the year. In other words, there would be no ‘0.25’, or one-quarter (six hours) of a day appended to ‘365.25 days in a year’. It would be 365 days per year, exactly.

But the universe is not ideal, and the Earth’s orbit cannot be exactly divided by a whole number of days, with no fraction of a day left over. If it could be, that would be an incredible coincidence!

That extra quarter of a day (0.25) means that a year is not evenly divisible by Earth’s rotational period of 24 hours (i.e., a single day). So if you’re reading this, for example, at 6AM, you’ll have to wait approximately another six hours – until about noon – before the Earth is in exactly the same place in it’s orbit around the Sun as it was one year ago this morning at 6AM.

True or false? *A day (the time it takes for the Earth to make one full turn about its axis) is 24 hours.* Everyone knows the answer to this one, right!? False: an Earth day is exactly 23 hours, 56 minutes, and 4.1 seconds. That’s 3 minutes, 55.9 seconds short of a full 24 hours. Hmmm, the universe is starting to look mighty messy.

So now we have Earth days that are not a full 24 hours, and an Earth year that’s a full quarter of a day more than 365 days; messy, messy, messy. The non-ideal universe played havoc with the Julian calendar, which was losing time and therefore ill-equipped to deal with the disparities of time and orbital mechanics. To deal with these realities, in 1582, Pope Gregory replaced the Julian calendar with what we now know and love as the Gregorian calendar.

If we stayed on the old Julian calendar, you would have a discrepancy of one day, roughly every 128 years. With the Gregorian system now we have a discrepancy of one day in something like 3,500 years.

Geoff Chester, US Naval Observatory

True or false? *Every four years, an extra day (February 29) is added to the calendar. *This is true, but with extremely important exceptions! Adding a leap day once every four years in the course of 132 years overcorrects by nearly a full day. To rectify this, three out of every four ‘century years’ (e.g., the years 1700, 1800, and 1900) do not observe a leap day, but the fourth ‘century year’ (e.g., the year 2000) does have a February 29.

In other words, if the year is divisible by 4, it is a leap year, unless it is divisible by 100 (the ‘century years’), but if the century year is divisible by 400, it is a leap year. In concrete terms, the year 2020 is a leap year (divisible by 4), while the year 1900 and 2100 are not leap years (divisible by 100, but not 400), and the years 1600, 2000, and 2400 are leap years (divisible by 400).

That’s how humans deal with keeping a reasonably accurate calendar in a far-from-ideal universe. Got it?

Question of the night: Were you, or do you know anyone who was, born on February 29?