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Earth Science

An Average Earth Day Used To Be Less Than 19 Hours Long (theguardian.com) 113

Posted by msmash from the more-you-know dept.
Scientists have determined that some 1.4 billion years ago, an Earth day -- that is, a full rotation around its axis -- took 18 hours and 41 minutes, rather than the familiar 24 hours. The Guardian reports: According to fresh calculations, a day on Earth was a full five hours and fifteen minutes shorter a billion or so years ago, well before complex life spread around the planet. Scientists used a combination of astronomical theory and geochemical signatures buried in ancient rocks to show that 1.4bn years ago the Earth turned a full revolution on its axis every 18 hours and 41 minutes. The number means that, on average, the length of the day on Earth has grown by approximately one 74 thousandth of a second per year since Precambrian times, a trend that is expected to continue for millions, if not billions, of years more.
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An Average Earth Day Used To Be Less Than 19 Hours Long More

An Average Earth Day Used To Be Less Than 19 Hours Long

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  • I'll get more time to do things eventually (Score:4, Funny)

    by ranton ( 36917 ) on Wednesday June 06, 2018 @01:22PM (#56737668)

    So I just have to wait a few hundred million years for those extra hours each day I have been wanting? Sweet.

    • So I just have to wait a few hundred million years for those extra hours each day I have been wanting? Sweet.

      When days are 8 hours longer than they are today, you're just going to have to put with 16hr long work days.

      • So I just have to wait a few hundred million years for those extra hours each day I have been wanting? Sweet.

        When days are 8 hours longer than they are today, you're just going to have to put with 16hr long work days.

        So I can cut back to 16h/day? Whew, can't wait!

      • Re: (Score:2)

        by dcw3 ( 649211 )

        Sweet, we'll be able to do away with Daylight Saving Time then!

  • It has been known for a long time that due to moon's effects on tidal bulges, the Earth's rotation has been slowing as the moon moves further away.

    • Re: (Score:2)

      by nucrash ( 549705 )

      How does the expansion of the Sun factor into this?

      • How does the expansion of the Sun factor into this?

        If the total mass of the sun isn't increasing, and the distance from the center of the sun isn't increasing; I wouldn't expect it does make a difference... at least not whilst it isn't expanded enough to engulf us.

      • Re:Moon (Score:5, Interesting)

        by MightyYar ( 622222 ) on Wednesday June 06, 2018 @01:51PM (#56737904)

        As long as the sun remains approximately the same mass, it should have no significant effect on the gravity felt by Earth at this distance.

        Of course the sun IS losing mass, both by converting mass to energy in nuclear reactions and through boiling off particles into the solar wind.

        And the Earth also experiences an influence on spin from the sun. If the sun survived long enough, Earth would become tidally locked to the sun - with a single rotation lasting an Earth year and the same side of the planet always facing the sun... the same way the same side of the moon always faces Earth. Earth is far enough away from the sun that the sun will die before this occurs. Mercury is close to the sun and is tidally locked. Venus is closer but has a bizarre backwards slow spin, the cause of I don't think is well understood.

        • Re: (Score:3, Informative)

          by Hognoxious ( 631665 )

          Mercury is close to the sun and is tidally locked.

          Not quite. A 3:2 resonance, actually.

          Are you posting from the 1960s?

          • Honestly the textbook that I learned from in the 80s was probably written in the early 60s, so yeah :)

            Point is, Earth won't be tidally locked with the sun.

            • Re: (Score:1)

              by Gr8Apes ( 679165 )
              Not before it is consumed by the red giant the sun will become prior to collapsing.

              • Correct. Unless we yank it out of the way. :)

              • Re: (Score:2)

                by Ramze ( 640788 )

                There is a possibility that the red giant sun might not go out farther than Venus's orbit, thus merely turning Earth into a molten wasteland on the side tidally locked with the sun. There's also the possibility that even if the red giant sun's size extends to Earth's orbit, Earth may have moved to a more distant orbit from the loss of the sun's mass through fusion and solar wind.... but, again -- molten wasteland on tidally locked side.... at least until the sun becomes a white dwarf star.

                Here's hoping w

                • Re: (Score:2)

                  by dcw3 ( 649211 )

                  ...at least until the sun becomes a white dwarf star.

                  What does Peter Dinklage have to do with the sun?

  • Resist our Sloth Overlords!

    Make sure you run counter to the Earth's rotation, so that it speeds up!

  • I'm pretty sure that the earth isn't going to be spinning anymore once it's been engulfed by the expanding sun. It's not going to be doing anything anything.
    • Just what I was thinking. Seems to me I read that by 4 billion years from now the sun will be a red giant the size of the earth's orbit.

      Musk and NASA better speed up getting the human race to Mars as well as other inhabitable planets.

      • Just what I was thinking. Seems to me I read that by 4 billion years from now the sun will be a red giant the size of the earth's orbit.

        Fantastic. That's approximately when I expect to be able to finally retire.

      • Re: (Score:2)

        by mark-t ( 151149 )
        We will be long gone by then... there's no rush.

        Barring extinction by some event before then, I have little doubt we will be a fully interstellar species before the next turn of the millennium.

        • We will be long gone by then... there's no rush.

          Barring extinction by some event before then, I have little doubt we will be a fully interstellar species before the next turn of the millennium.

          It would be a pretty depressing thing if we weren't interplanetary by 2118. Interstellar? That's a different order of magnitude and impossible to guess when we might even come close to that.

          • Re: (Score:2)

            by mark-t ( 151149 )
            I was saying interstellar, not merely interplanetary. Obviously we will be interplanetary much sooner.

          • Barring extinction by some event before then, I have little doubt we will be a fully interstellar species before the next turn of the millennium.

            It would be a pretty depressing thing if we weren't interplanetary by 2118. Interstellar? That's a different order of magnitude and impossible to guess when we might even come close to that.

            Umm, "before the turn of the millennium" means "before 3000AD", not before 2118....

        • Panicked for a moment - the first time I thought it said 4 million.

    • Actually the latest thinking is the the orbit of Earth might expand enough to avoid the incineration Mercury and Venus get. Then the only question is how long it takes the Earth's orbit to decay into the "black dwarf" that the sun will cool into. that timescale is unbelievably huge.

  • 1.4 bn years of that time, or years from our time?

    • Re: (Score:1)

      by Anonymous Coward

      Just because the days were shorter doesn't mean the years were any different; there'd just be more days per trip around the sun.

  • Well it is a BS "holiday". (Score:3)

    by Kenja ( 541830 ) on Wednesday June 06, 2018 @01:30PM (#56737750)
    I mean, do we really need a full 24 hours to act environmentally aware if we don't do it the rest of the year?

  • I think my employer has figured out how to stretch a 24 hr day into 30 hrs on a regular basis

  • Yes, but there would have been 469.188 days per year, unless the earth's rotation around the sun has also been slowing in which case there would have been fewer. Regardless, that would suck because employers still only gave 15-20 days off and required 8 hour workdays. Maybe the extra ~104 days were all weekend days?

    • Actually, the length of the year is getting longer. The reason for this is that the sun is slowly losing mass - both from directly converting it into energy and from the solar wind. As the sun loses mass, the Earth's orbit moves away from the sun, increasing the length of the year.

      The effect from this is small though, even by the standards of how the length of the day is changed. Even though the sun burns millions of tons of fuel a second, it's also truly massive, so even billions of years from now when

  • When the rotation speed was higher

  • I can't remember where I read or heard this, but the moon and Earth's days would eventually be the same length and they'd lock into each other.

    • you can look that one up, about 50 billion years, and the latest calculations (educated guesses with numbers) are that the earth won't be vaporized by the sun when it becomes a red dwarf but instead Earth's orbit will expand enough to save it

  • Doing the math on a slowing of 1/74,000th of a second (per year) and 1.4bn years, comes out to a rotation speed of 18.75 hours per revolution 1.4bn years ago. Amazingly close to the 18 hrs and 41 min claimed in the article. If the extrapolation is this close, why bother with the "astronomical theory, geochemical signatures, and modeling"?...just for confirmation?

    • Doing the math on a slowing of 1/74,000th of a second (per year) and 1.4bn years, comes out to a rotation speed of 18.75 hours per revolution 1.4bn years ago.

      It comes out even closer if you start from a sidereal day of 23 hours 56 minutes.

      I suspect they came up with the approximation of the rate of slowing from the evidence, and extrapolated that backwards, so it's not surprising it's so "accurate."

      • They've known about the slowing of the day (and had a pretty good estimation of it) since the middle of the twentieth century. With the advent of atomic clocks the effect of the slowing day had to be considered, and since then it has been closely monitored. Leap seconds [wikipedia.org] are an invention developed to bridge the gap that has already grown between solar time and the day as defined in 1960. The important thing to note is that the figures given are averages, the day itself does not slow by a constant rate as the
  • Comment removed based on user account deletion

  • Slashdot doesn't even know how long a day is... (Score:5, Informative)

    by wonkey_monkey ( 2592601 ) on Wednesday June 06, 2018 @02:14PM (#56738052) Homepage

    Scientists have determined that some 1.4 billion years ago, an Earth day -- that is, a full rotation around its axis -- took 18 hours and 41 minutes, rather than the familiar 24 hours.

    If you're going to go so far as to specify "a full rotation around its axis" - a sidereal day - then you should know that that does not currently take 24 hours. It takes 23 hours and 56 minutes (and 4 seconds).

    The article gets it right when it says:

    According to fresh calculations, a day on Earth was a full five hours and fifteen minutes shorter

  • Summary Ignores the Subject of the Article (Score:5, Informative)

    by careysub ( 976506 ) on Wednesday June 06, 2018 @02:23PM (#56738096)

    The article (which you can download with Sci-Hub) is not about the length of Earth's day, although it does produce a new and more accurate estimate of it at early epochs on Earth. The paper is really about the Milankovitch Cycle that controls climate on a ~22,000 year time scale which be evident if TFS bothered to include the paper's title Proterozoic Milankovitch cycles and the history of the solar system.

    The main purpose of the study was to use geological data to construct the Milankovitch cycle going back more than a billion years.

    • And how about the needless repetition: All the paragraph does is say three different ways that 18 3/4 long, or 5 1/4 shorter. No other actual information. Sheesh. Almost as bad as reading Reuters (and other) news where the same news article commonly appears 3 or more times in different "sections", with the exact same summary. Nothing like adding a bunch of filler rather than unique content. News these days has ended up in a bad place.

  • In Soviet Earth, life slows down the world.
  • An hour still would have been defined as 1/24th of the day. The hours themselves would have just been shorter when compared to phenomena that were not intrinsically tied to the rotation of the earth. Think of how sundials work and how they influenced the idea of an hour; the shadow on the sundial wouldn't just magically skip over several hours each day.

    • Is an hour 1/24 of a day, or is it 3599-3601 seconds? Because if it's the latter, it has nothing to do with Earth's rotation.

      According to NIST: The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

    • An hour still would have been defined as 1/24th of the day.

      Defined in that way, an hour is not a measure of time: it's a measure of relative motion.

      The modern hour has since be redefined as a proper measure of time, relative to some kind of gnomonic nanofizz emanating from caesium-133.

      (Somehow caesium-133 must be inherently more "timey" than planet earth.)

      Julian Barbour apparently doesn't think that time really exists in deep physics; he seems to believe it's relative motion all the way down.

    • Since 1967, the second has been defined as exactly 9,192,631,770 times the period of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

      https://en.wikipedia.org/wiki/... [wikipedia.org]
      And yeah the discrepancy is made up by adding time at the end of the year....
      https://www.timeanddate.com/time/leapseconds.html?hc_location=ufi
      Aren't seconds just as important as minutes? The whole construct of 24 hours in a day is very recent....

    • Re: (Score:2)

      by jrumney ( 197329 )
      That's nice. Would a day have still been defined as approximately 1/365.25 of a revolution around the sun? We could all pretend we're in Scandinavia every few days, when it is dark all day and light all night.

    • An hour still would have been defined as 1/24th of the day.

      That was the original definition but this would mean that an hour would be changing on a continuous basis as the Earth's rotation is affected by tidal forces etc. This would make it useless for many things in the modern world e.g. GPS. As a result, now an hour is defined as 3,600 seconds and a second is defined in terms of periods of radiation from a particular transition in a caesium atom.

  • ... and since all of the weight is distributed on the outside of the sphere, it slows rotation down. But if dying early isn't going to make people lose weight, I doubt having fewer days in the time they do have will manage it.

  • nothing new here (Score:4, Interesting)

    by u19925 ( 613350 ) on Wednesday June 06, 2018 @05:47PM (#56739404)

    Using the ancient paintings of eclipses and comparing with the predictions using Newton's laws of motions shows some discrepancy. If this is assumed due to shift of moon than conservation of angular momentum implies drifting of moon of 3 cm/yr. This was known for over a century ago. Using this knowledge and doing back of the envelope calculations game me almost same results. So there is nothing new here.

  • Are our years shortening or lengthening? When the Sun goes to Gas giant, will this be resolved? I gotta put it on my calendar. Pretty sure this is against our policy of delivering Less content for more money every year. This just doesn't make financial sense. We'll have to find our way out of this arrangement. --- and last... does this mean there is cosmic torsional friction.. where the vacuum of space is bleeding off speed??? I got it!! Its that damn Dark matter!
  • All that weight moving around on the planet, is slowing it down LOL.
  • An earlier article talked about slowing hurricanes the first thing that came to mind was the slowing rotation while not much it can be quite a change for the climate. The bigger the fluctuation of temp between night and day due to heating for a fraction longer and cooling a fraction longer and newtons law of cooling seems to be an exponential relation, this should have a measurable effect on climate after just a century

    • eh, 1.7 milliseconds per century in length of day....no

      0.17 seconds longer after 10,000 years

      1.7 seconds longer after 100,000 years

  • I attended a presentation regarding the rhythmites relation to the length of the day some 18 years ago. Sure took a long time to verify the hypothesis.
  • If a day was shorter a billion years ago, does that mean a year was also shorter or does that mean there were more days in a year?
    If the year is shorter then a 100,00 years (1 billion years ago) wouldn't be the same duration as a 100,000 years currently.....correct?

    • If a day was shorter a billion years ago, does that mean a year was also shorter or does that mean there were more days in a year?

      The latter. The time for one complete revolution around the sun is (mostly?) independent of the time for one complete rotation. A year might have been shorter because the Earth's orbit was closer to the sun, but it wouldn't be because of the rotation speed.

  • Perfect! In just about 3000 years we'll have those perfect 24-hour days we've been longing for! Leap years will become obsolete.. and after that .. they'll be negative.. like once in every couple of years, February will be 27 days long.. COOL!

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