Simpler Leap Day explanation

By Phil Plait | February 29, 2012 10:42 am

[Note: Don’t forget: I’ll be on The Late Late Show with Craig Ferguson tonight!]

OK, so if you’re still scratching your head over my earlier mathified Leap Day explanation, then here’s a simpler one by Minute Physics that nonetheless hits all the high points:

Between my math and those animations, you’ve got it all now, right?

I sometimes wonder if, in the far future when we can terraform planets, we won’t adjust every planet’s day length to divide evenly into its year. That might be easier than adjusting the calendar!

CATEGORIZED UNDER: Astronomy, Cool stuff, Time Sink
MORE ABOUT: Leap Day, Leap Year

Comments (22)

  1. Peter Hornby

    Hey, wouldn’t it be easier to change the planet’s year by nudging its orbit just a tad :-)

  2. jason

    Its fun to read sci-fi stories about how other colonized planets go through gyrations to keep the clock the same as on earth even when the day is no longer suited.

  3. Gary Ansorge

    1. Peter Hornby

    Sure, but to shorten the year by six hours, we’d have to move earth closer to the sun,,,which would make global warming a tad worse.

    ,,,or we’d have to move it a lot further away, to lengthen the year by 18 hours,,,which just might be enough to compensate for global warming,,,hey, I’ll get right on that,,,(now, where the heck did I put that gravity generator,,,)

    SO, Phil, when you say Wed night, I presume you really mean Thursday MORNING(hey, it airs at 0030 am,,,)(I know,,, I’m a picker of nits,,,)

    Gary 7

  4. Jeff

    having passed into , er, senior-dom, not senior in high school or college, really gets me philosophical about the earth year, and I say that because it went so darn fast. And earth being an inner planet, the year really is a quick little nothing of a time unit. Maybe i had a misperception of a year because (a) obviously we live our financial/work lives around annual events (b) when 10 years old, 1 year is 10% of life, when 100, 1 year is only 1% of life. If I come back, I’ll ask god to bring me to a planet where the year is 3,600 days instead of 360 days and somehow just the same pressure/temp as earth.

  5. Gary Ansorge

    4. Jeff

    No problem. We’ll just incarnate you on an earth sized moon, in a 3600 day orbit of a gas giant in its stars habitable zone. Of course, THEN you’ll have to deal with the epicycles (as the gas giant completes ITS revolution about the star).

    ,,,or we could just ramp up earths rotational period to make our days about 2.6 hours long,,,

    Gary 7

  6. Jeff

    Gary, sounds pretty good to me.

  7. That is one neat video and much better for the general public than a bunch of math. I’ll have to link to it on my blog, since even my explanation is a bit involved.

  8. Jeff

    I have a theory, maybe someone knows more: Feb. 28/29 used to be the end of year? I actually never thought about that for some reason, but today I’m thinking about it. Because it seems like obviously it is the only “weirdo” length month because it must have been at end of year and was leftover days? And I just realized that a sky viewing for class I scheduled a year ago was on this leap day of all days. I did that for position of Mars/Jupiter/Venus super lineup tonight, but it just fell on leap day. “synchronity” is that what the ATM(is there a difference between ATM and pseudoscience?) people call it?

  9. martisoare

    @Jeff 11:42

    The timing of “new year” has varied a lot over time and cultures. Springtime has been popular. Romanians still celebrate a new year’s festival in the beginning of March. Most Romanians will tell you that their custom began with the Dacians, though Wikipedia suggests that the Thracians and the Romans had similar customs too.

  10. kevbo


    Yessiree. March was the first month (which explains why October has that octo-eightish prefix). I can’t recall all the details, but a fine geeky book, “Marking Time” goes into all sorts of details into how our calendar evolved, including copious descriptions about exactly is a year, day, second…

    A good read… if you have the…time…(see what I did there?)

  11. Infinite123Lifer

    Just some jibber-jabber on my part here, no leaps, no faith, no atomic seconds or discussions of what is arbitrary, just some jibber-jabber.

    “I sometimes wonder if, in the far future when we can terraform planets, we won’t adjust every planet’s day length to divide evenly into its year.”

    Sounds like a little competition against nature or could I say a little competition against the evolving of a planet on the universal scale? I mean, if we are lucky enough to be alive and skilled enough to terraform and calculating enough to achieve such a feat…would it even matter when we are going by stardates? :) (i possibly gave away my lack of education right here, but being ignorant of so much I would not know now would I?)

    Since we are nature, would we just be bucking ourselves trying to perfect all and everything? Nothing wrong with improving upon oneself or ones environment IMO. But to “adjust every planet’s day length to divide evenly into its year.”, isn’t that a little OCD Phil? :) Perhaps not, perhaps so. All in all it is the type of intrigue which strikes the ramped imagination every so often, not a big deal, but interesting to me nonetheless.

    But, sure, why not control every last thing down to its very last component (since control appears to be increasingly upon the humans hand and mind) until we or something we create thus controls all outcomes everywhere…and then what? Then seriously what? It seems something has already taken on the role of this duty and it is called by many many names. I prefer to call it Nature, and it likes entropy and structure, they appear to go hand in hand. Nature will always seek to attain its ultimate goal, whatever that is?. I prefer to accept my insignificance as significantly subjective. I also believe in making improvements as well, given the fact that I am here to make those improvements.

    If you have time to wonder about these things, (like I do) then we are truly living at an extraordinary time. Well, if you don’t count the distant future as having a friend named Data and what we thinks it could become. Perhaps someday planets will be forged in the workplace of a species of the Universe but in the nanoseconds it takes to wonder about such things, simultaneously me thinks, the effort to undo the “artificial” on a scale such as that is equally at work.

    Imagining past and through all of possible time leaves me. . . wondering what I have missed, and keeping an eye or two poised upon the world around me. Imagination is quite possibly Natures greatest achievement. The ability of Nature to recognize itself and wonder about itself. How else can it be explained rationally? (don’t answer that) Although recognized probably needs to be defined…oh well, jibber jabber it was.

    Sincerely recognizing oneself as significantly not sure about what he is trying to say.
    Why would that stop me from posting though :)

  12. #7 Jeff:
    Yes; as is well known, March was the first month of the Roman calendar, so February was the last. Their calendar originally had alternate months of 31 and 30 days, except for February, which had 29 ( not 28! ). Think about it; ( 6 x 31 ) + ( 6 x 30 ) = 366, which is one day too many, so they had to remove a day somewhere – so it was logical to remove it from the end of the year, making February the shortest month.
    When they introduced leap years, it was equally logical to add the extra day at the end of the year – February again. So February then had 29 days, or 30 in a leap year.
    The numbers of days in some months were later swapped around, for reasons to do with Roman politics and naming months in honour of emperors – resulting in February having one less day, and the apparently haphazard sequence which we are stuck with today!
    In Phil’s previous post, he links to an article which explains this; it’s linked at the phrase “Caesarian shenanigans”.

  13. Jeff

    Neil, thank you, that is an excellent summary.

    Leave it to those egocentric dictators to mess the logical scientific simplicity of something!

  14. Great short clip. :-)

    Just one small thing – at the 1 min 15 ~ 20 mark – they said you can’t see the supermassive black hole Sagittarius A* – then they have it arrowed and seemingly quite visible and stationary! 😉

    So what’s the deal with that? Accretion disk seen in false colour? X-ray /Gamma ray image?


    * Not really a footnote asterisk there but only, confusingly, part of our Milky Way’s Supermassive Central Black Hole’s proper name such as it is. 😉

  15. I’ll have to include it in my “What is Astronomy?” presentation now, but this clip just made me realize that Orion will become a Summer constellation in a few tens of thousands of years as the precession of the equinoxes proceeds. December & January will still be cold, but we’ll be looking at Sagittarius then!

  16. John Paradox

    I have to admit I thought of this when I read the longer BA explanation, but had to head to work….

    How could we come to explain and coordinate our ‘time measurements’ with an alien (ET) species? Our Year is dependent on how long it takes our planet to orbit our star, the Month is how long our natural satellite orbits our planet, a Day is one rotation and shorter periods are from that (1/24th is an Hour, 1/60th of that is a Minute, etc.)

    Trying to use, say, ‘timed pings’ to demonstrate a Second could run into problems with Relativity if we’re trying to communicate remotely with someone who’s moving at a good fraction of lightspeed relative to Earth. In fact, trying to set up Universal measurements of any kind could present problems in dealing with Aliens if not face-to-face(?).

    Just some random thoughts.


  17. DisinfoAgent

    “I sometimes wonder if, in the far future when we can terraform planets, we won’t adjust every planet’s day length to divide evenly into its year. That might be easier than adjusting the calendar!”

    More likely, the planet’s year length will be an obscure curiosity known to astronomers but disregarded by everyone else, who will just keep measuring time the way they’ve always done: in Earth units! That way they won’t need to adapt to a new, unfamiliar system, and they’ll be able to talk to people from other planets without having to convert their units all the time. Different time units for different planets makes sense at first glance, but it’s patently impractical and unrealistic once you start to think about it.

    One exception might be the day: I can easily envision people on Mars using Martian days as a reference because they could guide themselves by their own sunrise and sunset. But as far as years, hours, minutes and seconds are concerned, I’m sure they’d keep on using Earth units.

  18. #13 Jeff:
    Actually, it wasn’t the emperors themselves who renamed the months; the Roman Senate did so in their honour. But yes, after the scientists had established a system as simple and logical as possible, the politicians had to stick their oar in and screw it up!

  19. Yesterday, I had to explain how the Gregorian Calendar works to a bunch of friends.

    At some point, someone asked if it was true that years divisible by 4000 (four thousand) would NOT be leap years. I said it’s not the case, but we ended up doing calculations as to what would be ideal – we reached the same conclusion as you: suspending the leap day every 3200 years yields a difference of less than one second (-0.97…), something easily dealt with with a leap second.

    Someone then asked, why isn’t this rule introduced? I explained it wasn’t worth the trouble, as the Earth’s day is getting steadily longer and by the time this rule would be worth considering, the duration of the day would be too different, requiring a major adjustment of the calendar.

    I tried to search for predictions on the duration of the day and the tropical year for a couple millenia in the future, but came up empty.

    Phil, I bet *you* can solve that. Please? :-)

    Thanks in advance.

  20. hhEb09'1

    The current rate of slowing of the earth is about two milliseconds per *century* so in a couple millennia the day will only be longer by about 40 msec

  21. amphiox

    Hey, wouldn’t it be easier to change the planet’s year by nudging its orbit just a tad

    The simplest method available with known technology would be to a gravity tractor to nudge a moderate sized NEO into an earth-crossing orbit of just the right position, so that with each close passage it donates a little bit of orbital energy to earth, thereby increasing earth’s orbital speed, and expanding earth’s orbit.

    But miscalculate the asteroid’s path just a little bit, and BOOM!

    Or perhaps we could install a series of giant rocket engines in a ring around the planetary equator, and time their daily firing pattern just right….

    It might be easier to capture an NEO (or several) into an earth orbit, producing a series of new moons, whose combined gravitational effects will slow down the day instead….

    The current rate of slowing of the earth is about two milliseconds per *century* so in a couple millennia the day will only be longer by about 40 msec

    Some 200 odd million years ago, the day length was just a shade of 22hours, as demonstrated by the ratio daily and yearly growth rings in fossil corals.

    But I don’t think the rate of slowing of earth’s rotation is actually a constant….

  22. amphiox

    Or perhaps we could excavate a series of everest-sized chunks of crustal mass, and lob them up, intact or in pieces, into solar orbit (I don’t think earth orbit alone would work), at just the right series of intervals to change earth’s momentum.

    Or maybe it’d ultimately be easier to try to siphon some gases off the sun to reduce the solar mass….


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