February 29th is a very special day because it only appears on the calendar once every four years or so but why is there a Leap day? well, of course, it all comes down to astronomy.
A year isn’t actually 365 days. it’s just under 365 and a quarter but what exactly is a day?
You learned in school that a day is a time it takes for the earth to rotate once on its axis but with respect to what in order to define something’s motion it always must be measured relative to something else.
Let’s say we measured the Earth’s rotation with respect to the Sun so we start counting as soon as the Sun is highest in the sky and then stop counting when it reaches that point a second time.
This 24-hour period is called a solar day and it’s what we base our clocks offer but what if we measured the Earth’s rotation with respect to some distant stars instead.
Interestingly this day which we call a satirical day only takes 23 hours 56 minutes and 4 seconds but why the difference.
Well for demonstration purposes let’s leave the earth for a moment whoa okay now we’re here there is a massive arrow sticking out of the earth right now how beautiful for the earth to rotate a full 360 degrees and end up with the arrow pointing back at those distant stars it takes one several days but during that time the earth moves a little bit in its orbit around the Sun so the arrow is no longer pointing in that direction.
For that to happen the earth needs to rotate a little bit more which takes about four minutes
so a solar day is about four minutes longer than a satirical day and consequently stars rise four minutes earlier every night
So now we can define the year that we use for our calendars.
A tropical year or the time it takes to go through all the seasons is 365 point two for two solar days that means every four years we have just under one extra day.
We simply add another one at the end of February but by doing so we added too much time and now we’re a little ahead so every 100 years we don’t have a leap year to make up for us overcompensating.
Then that means we’re behind again so every 400 years we add another day in to fix that mistake and meanwhile all the other leap years are happening and if we want to get even weirder a several years is 20 minutes longer than a tropical year.
That’s because the Earth’s access processes meaning it spins like a top and if we use this for our calendars eventually all the seasons would be shifted around a few months and unless you live in the southern hemispheres snow in August is not okay.
Anyway when in doubt just remember that every four years it’ll probably be a leap year so just enjoy the extra day.