Our calendar years measure the time it takes the earth to orbit the sun, which we round to 365 days. But, it takes the earth 365.24219 days to orbit the sun. Annoyingly, that’s around a quarter of a day - or 5.8 hours - longer than our calendar years. So, every four years, we add the left-over hours together, and make a day - the leap day on February 29.
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00:00 A leap year occurs every four years.
00:03 During these years, a 29th day is added
00:06 to the year's shortest month, February.
00:09 But what is a leap year and why does it happen?
00:12 That all comes down to the rotation of the Earth
00:15 and its orbit around the sun.
00:18 Our calendar years measure the time it takes the Earth
00:21 to orbit the sun, which we round to 365 days.
00:26 But it actually takes Earth 365.24219 days.
00:31 Annoyingly, that's around a quarter of a day
00:37 or 5.8 hours longer than our calendar years.
00:42 So in 45 BC, Emperor Julius Caesar devised a plan.
00:48 That quarter of a day adds up to a full day every four years
00:55 when it's then added to the calendar as February 29th,
00:59 or the leap day.
01:00 Without the extra day every four years,
01:03 the gap between the calendar year and the seasonal year
01:07 would gradually increase over time.
01:09 Over a century, for example,
01:11 the calendar would be off by 25 days,
01:14 which would untether the months from the seasons,
01:17 causing all sorts of problems
01:18 for the world's agricultural production.
01:21 But adding a leap day to the calendar
01:24 isn't a perfect system.
01:26 We don't actually have a leap year every four years
01:29 because every hundred years, it needs to be skipped.
01:33 Why?
01:33 Because adding a leap year every four years
01:36 overcorrects the calendar.
01:38 The Earth's orbit is 365.24219 days, remember?
01:43 But we round that to 365 days in a usual year,
01:49 and 366 in a leap year.
01:53 But in order to do that,
01:54 we've rounded the additional orbital time to a quarter,
01:58 and that rounded difference does add up over time.
02:02 Of course, removing that day every century
02:04 does then push the imbalance the other way,
02:07 meaning that every 400 years,
02:09 the otherwise skipped leap day is re-added to the calendar.
02:13 It's pretty confusing stuff,
02:15 but thanks to Pope Gregory XIII,
02:17 we have some mathematical rules
02:19 we can follow to know when a leap year will fall.
02:23 This is the century rule.
02:25 Years with the added leap year are divisible by four.
02:29 Leap days are not added to years
02:31 that are divisible by 100.
02:34 If that year is divisible by 400,
02:36 then a leap year is added after all.
02:40 That means it was a leap year in 1600 and in 2000,
02:44 but not in 1700, 1800, and 1900.
02:48 See, it's that simple.
02:50 If you're still struggling to remember
02:52 when a leap year falls,
02:54 just remember most of the time,
02:56 the Summer Olympic Games are held in a leap year,
02:59 so that's something to look forward to.
03:01 (upbeat music)
03:04 [BLANK_AUDIO]