TOMORROW, Saturday, February 29, 2020, is an intercalary day, better known as a leap day.

That means 2020 is 366 days long, but why? Let me explain why we need leap years.

Imagine you were alive thousands of years ago. No internet, no calendars, no watches. No way of keeping track of time. But you would notice the passage of time, as day turned into night and back into day again. Measuring the time between two sunrises, middays or sunsets gives us a day.

You would also notice the changing of the seasons, as winter blossomed into spring, which warmed into summer and then turned into autumn, which greyed into another winter.

Measuring the time between two midsummers or two midwinters gives us a year.

And if you counted the days between two midsummers or two midwinters you find that the year is 365 days long. You might then design a calendar with 365 days in it, maybe broken down into more manageable chunks.

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All very straightforward, you might think. But as the days and years pass, you’ll notice something odd happening. The dates for midsummer and midwinter that you’ve marked in your calendar won’t line up with what’s happening with the seasons outside. The seasons will drift, happening about one day later every four years.

It would take some time for this drift to become really noticeable. Every 28 years the drift would be about a week, so midsummer would happen a week later than you expected. If you passed on your calendar to future generations they would notice that after 750 years midsummer would actually happen when your calendar said it was midwinter. Which would be a little embarrassing.

Luckily for future generations, our stargazing ancestors spotted this drift and knew we needed to correct for it. They didn’t know why it happened, but nowadays we do. The time it takes the Earth to orbit the Sun (from which we get the year) isn’t a whole number of days (the time it takes the Earth to spin about its axis once).

One year is close to (but still not exactly) 365 and one quarter days long, which is why the seasons drift at the rate they do: the four quarter days add up to one full day every four years.

So, all we need to do is add an extra day into our calendar every four years, and everything is fine. We call this extra day a leap day, and the year it happens in a leap year.

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To keep things easy to remember, if a year is divisible by four it’s a leap year with 366 days in it; if not it’s just a normal year with 365 days. That’s why this year, 2020, is a leap year, and why February is 29 days long.

Sounds simple enough, right? Actually, no.

When we measure really precisely the time between identical points in two successive seasons we find that the year is actually 365.24219 days long, or 11 minutes shorter than 365 ¼ days. Which is a pain.

This means that, even if we correct for the extra quarter day each year, the seasons will drift a little. The drift will be much smaller than the one we’ve just corrected for, but it’s still noticeable over long enough timescales.

After 23,000 years or so your midsummer festival would be happening in a blizzard.

So we include another correction in our calendar. Years that are divisible by 100 but not by 400 are not leap years, even though

they’re divisible by four. That means 2000 was a leap year, but 2100, 2200, and 2300 won’t be.

Many people think of leap days as an “extra” day, but of course they aren’t. In our busy lives, our nine-to-fives, our seven day weeks, tomorrow is just a Saturday like any other.

And finally, why “leap”? No one is really sure. Between the same date (1 January, say) in two normal years the day of the week will go up one day every year. So 1 Jan 2019 fell on a Tuesday, while 1 Jan 2020 fell on a Wednesday.

Whenever a leap year happens, the days of the week “leap over” a day, so 1 January 2021 will fall on a Friday, leaping over Thursday.

Maybe that’s why they’re called leap days. Whatever the reason, enjoy your “extra” day.