10 ms·
Ancient shell shows days were half-hour shorter 70M years ago
- sigmaprimus 7y agoIf the Earth was spinning faster would that mean that dinosaurs weighed less back then? I know their mass would be the same but wouldn't a faster spinning earth counter gravity similar to how satellites maintain a stable orbit?
- eesmith 7y agoCentripetal acceleration at the equator is R ω^2 = (radius of the earth) * (2 * pi / 24 hours)^2 = 0.0337 m/s^2 . Acceleration due to gravity is 9.8 m/s^2, so you weigh 0.3% less in Singapore than at the North Pole. (There are other factors, like the Earth's bulge, which I won't consider.) This small enough that people don't notice it. (Presumably dinosaurs wouldn't either.) Plus, most people don't live on the equator, and there's a cos(latitude)^2 factor which reduces the centripetal acceleration. At 45 degree latitude the acceleration is 1/2 that of the equator. Speed up the Earth's rotation to 23.5 hours and it's 0.0352 m/s^2. The difference is 0.0015 m/s^2 , which is quite small compared to the normal force of gravity. Thus, it isn't really important for most things.
- sigmaprimus 7y agoThanks for the response, and for doing the math. After reading your response it I agree that it wouldn't have much more effect than an increase in elevation. I thought it might explain how the dinosaurs were able to grow so big without collapsing under their own weight(which probably can be explained too)
- eesmith 7y agohttps://www.thoughtco.com/why-were-dinosaurs-so-big-1092128 https://www.thoughtco.com/why-were-dinosaurs-so-big-1092128 suggests the reasons aren't well known for why some dinosaurs got so large.
- SacredCat 7y agodid NOT expect this to be that interesting for me.
- Pusha_Drugz 7y agosame. I did not think I would even think about this, and now I don't know what I should to with this information, gotta share this with someone
- fastball 7y agoInterestingly, that article doesn't mention the theory that I think is most promising – that there was potentially a higher concentration of oxygen in the air when the dinosaurs were around, making it easier to get enough oxygen even if you were much larger in size.
- me_me_me 7y agoI thought that was an accepted fact. It's been a while since I read about dinosaurs : )
- mrfusion 7y agoMy theory is that insects got larger because of the higher oxygen. And then everything else all the way up the food chain had to get bigger too.
- deleted 7y ago[deleted]
- peterburkimsher 7y agoFalsehoods programmers believe about time: "3. Years have 365 days." Dinosaurs had 372 days per year! https://infiniteundo.com/post/25326999628/falsehoods-programmers-believe-about-time https://infiniteundo.com/post/25326999628/falsehoods-program...
- wbl 7y agoIf you can find a dinosaur calendar it will be implemented.
- mprovost 7y agoAt first glance I thought this was referring to a Bourne or Korn shell.
- tclancy 7y agoFsh would be the only one extant back then.
- sumosudo 7y agoNo wonder this work day feels so damn long!
- hnick 7y agoIn another 70M years my body clock will be just about right.
- forinti 7y ago31*12=372 So we would have had a nice regular calendar back then.
- austincheney 7y agoThis is probably explained by tidal breaking, which requires things like leap seconds. https://en.wikipedia.org/wiki/Tidal_acceleration https://en.wikipedia.org/wiki/Tidal_acceleration
- arethuza 7y agoSo at 2.3 ms/century * 70 million years - so 1610000ms = 1610s = ~27 minutes
- austincheney 7y agoTidal breaking is also a deceleration/acceleration rather than a velocity, which means the rate of change grows as the speed changes. This is because the moon's gravity is uniformly pulling on Earth and stretching the day in proportion to the uniform gravitational force on the rotational velocity. While the moon's gravitational force will remain constant over time, and thus the pull on Earth's rotation, and the proportion of gravitational force to Earth's rotational velocity will also remain unchanged over time Earths rotational velocity is changing as a result, which is a compounding effect. That means leap seconds will need to be inserted at ever increasing frequency over time until the Earth becomes tidally locked. Tidally locked means the Earth stops rotation so that the same side always faces the sun.
- antidaily 7y agoCalled it.
- ericfrederich 7y agoI can see how counting the rings can tell you that there were 372 rotations per revolution vs today's 365 rotations per revolution. What I don't get is how you correlate that to shorter days. Wasn't the Earth's revolution on a different period back then too? If you assume the revolution period was same back then as it is now... sure, half an hour difference I get it. Or are we assuming that the rotation is changing faster than the revolution?
- jofer 7y agoThat's the key part. We actually have good reasons (conservation of momentum) to believe that the revolution period does not change significantly over that timescale. There's nothing that should significantly change the momentum of the orbit around the Sun by that amount/time. Contrast that to the Earth/Moon's rotational period, which we expect to slow over time due to energy consumed by tides "sloshing around".
- TimSchumann 7y ago> Contrast that to the Earth/Moon's rotational period, which we expect to slow over time due to energy consumed by tides "sloshing around". Not consumed, literally flung out into space via the moon. It’s speeding up, we’re slowing down. No violation of thermodynamics required.
- jofer 7y agoI simplified. Some of it actually is consumed by friction (and therefore heating), too. You're correct that most is transferred. However, the angular rate of the Moon's rotation is staying the same over time (it's tidally locked at one rotation per revolution). It's not exactly speeding up. Instead it's getting further away, which increases the moment of inertia and therefore transfers momentum.
- fhars 7y agoThat‘s what “speeding up” in a 1/r potential means, you move to a higher energy orbit, which happens to be longer and slower. Classical physics can be weird, too.
- markus_zhang 7y agoTook me half a second to realize it's not a *nix shell...
- nebulous1 7y agoAfter the first half of the sentence I thought that somebody had found an old shell connection that hadn't been closed in 40 years.
- philliphaydon 7y ago> Earth turned faster at the end of the time of the dinosaurs than it does today, rotating 372 times a year, compared to the current 365, according to a new study of fossil mollusk shells from the late Cretaceous. This means a day lasted only 23 and a half hours, according to the new study in AGU’s journal Paleoceanography and Paleoclimatology. I'm curious, how do they decide that the earth spun faster on it's axis rather than the earth taking longer to orbit the sun?
- 4gotunameagain 7y agoBecause the solar system has reached the current stability a long time ago. If any of the planets had such fluctuations on their orbits so close in the past(relatively), we wouldn't be here
- tenant 7y agoI'd like to know too how they can confidently state that Earth's orbit does not change. Would it mean we'd spiral into the sun or out of orbit if it did?
- pjc50 7y agoFrom Newtonian mechanics, the orbit can only change by applying a force from somewhere, and a big move would require a lot of energy. And there's no evidence of such an event in the geologic pas; if it was triggered by an impact, it would be far larger than the one which killed the dinosaurs.
- dr_zoidberg 7y agoAddendum: far larger than the one that created the moon.
- wool_gather 7y agoThere's a reasonably thorough discussion of this topic, from an...interesting...perspective, in this essay: "How to Destroy the Earth" https://qntm.org/destroy https://qntm.org/destroy
- rishav_sharan 7y agoMathematically challenged person here. Does this means that after 3.3B years, Earth will lose its rotation completely?
- Robotbeat 7y agoIt may be engulfed by the Sun by that time, so... yeah?
- ncmncm 7y agoNo. The rate of decrease is not constant. But Venus and Mercury have lost their rotation, not having a massive moon to keep them rotating, against solar tides slowing them. Mars has kept its rotation by its distance from the sun. Its tiny moons help only a little.
- jws 7y agoVenus still rotates. It rotates on it's axis very slowly, every 243 earth days, and goes around the sun every 224 earth days. Its direction of rotation is opposite ours, so when combined with the orbit you get a sunrise every 117 earth days. Mercury is weird. It is tidally locked, but not like our moon. Mercury rotates around its axis three times for every two orbits. (Which I just learned while writing this comment because as a child my books told me it had a permanent sunward side.)
- saagarjha 7y agoThe discovery of Mercury’s 3:2 orbital resonance is apparently quite recent, coming from the 60s or so. Apparently this was because of a coincidence in Mercury’s synodic period with Earth being twice its rotation period, making it look like it had the same face towards the Sun all the time.
- ncmncm 7y agoOnce rotation gets slow enough, the orbital period becomes an important factor in further change. On its face, it is surprising that rotational direction can change, but rotational momentum is conserved not by individual bodies, but by the whole, interacting system, subject also to conservation of energy. So, momentum and energy trade around between bodies in complicated ways.
- otikik 7y agoIt was ... a different epoch
- ncmncm 7y agoStill the same eon, though. Life is old. The time between stegosaurus's day and T. rex's is longer that between T. rex's and today.
- redog 7y agoSo is it because the earths' density is moving away from it's center? Like when you're spinning and you tighten up?
- njarboe 7y agoNope. It because of angular momentum transfer between the Earth and the Moon due to tides. The Moon moves farther away and the Earth slows rotation.
- pfdietz 7y agoIt's amazing what they can discover looking at 1970s sh sources.
- ewfwfewefewfwef 7y agoOne of those hidden gold comments that make me read comments in the first place.
- ascar 7y agoMaybe that's just me, but I read hackernews comments for the usually insightful additional information and related discussions that are often more interesting than the article itself. GP comment provides no useful information nor sparks interesting discussion related to the topic. It's better suited for reddit than hn.
- arbitrage 7y ago> GP comment provides no useful information nor sparks interesting discussion I disagree. Turns out, that there is a LOT of collected specimens that nobody ever looked at closely. We've seen this a lot in terms of biology, archaeology, anthropology, astronomy, even mathematics. Something some found ages ago, and said, "Huh, that's interesting ..." then logged it away, just shows that there is much more science to be done.
- cgriswald 7y agoMakes sense. If I’m collecting data to look for exoplanet transits, I might go “wow, weird” if I get some strange light curve, but then not really follow up on it since I’m busy looking for exoplanets and only have so much time with the scope.
- 7777fps 7y agoYou've been wooshed, the comment was a joke about computer shells like "sh". It's the top comment for me, I'm not sure if people are upvoting because they got the joke or upvoting because they missed the joke.
- peter303 7y agoI disagree with assumption that year length has stayed constant the past 70 million years. The Earth experiences tidal friction from the Sun just like the Moon and Earth affect each other. I am guessing the year lengthening effect slower than month & day lengthening because Suns tidal force is a third of Moons. In addition Earth year may have changed during the Great Solar System Reconfiguration Event when Jupiter and the other gaseous planets hypothesized to have migrated outwards from orbits closed to the Sun. This may have happened 3.8 billion years ago causing increase of craters on the terrestrial planets at that time. https://www.nasa.gov/topics/solarsystem/features/young-jupiter.html https://www.nasa.gov/topics/solarsystem/features/young-jupit...
- pfdietz 7y agoThe energy required to change the length of the year is very large. Solar tides could transfer some energy from the Earth's rotation to the Earth's revolution, but even tapping all that energy (leaving the Earth tidally locked to the Sun) would not change the year length very much,
- raxxorrax 7y agoIt is basically defined by distance to the sun since it has ~99.8 of all mass of the solar system, so the change of Jupiters orbit would have a large effect (edit: for Jupiter).
- pdonis 7y ago> Solar tides could transfer some energy from the Earth's rotation to the Earth's revolution No, they can't. See my other response just upthread.
- billpg 7y agoDid the clocks built by dinosaurs include leap seconds?
- kazinator 7y ago> Ancient shell shows days were half-hour shorter 70M years ago. Nope! That was just a typo bug in date.c: int seconds = days * 84600; See the transposed digits? That makes it exactly 1800 seconds shorter than 86400, or half an hour.
- 867-5309 7y agothe shell is not aware of date.c
- sebastianconcpt 7y agoThe new method focused a laser on small bits of shell, making holes 10 micrometers in diameter, or about as wide as a red blood cell. Trace elements in these tiny samples reveal information about the temperature and chemistry of the water at the time the shell formed. The analysis provided accurate measurements of the width and number of daily growth rings as well as seasonal patterns. The researchers used seasonal variations in the fossilized shell to identify years. The new study found the composition of the shell changed more over the course of a day than over seasons, or with the cycles of ocean tides. The fine-scale resolution of the daily layers shows the shell grew much faster during the day than at night
- stoicShell 7y agoI've always known I should have been born some 420-490 millions years in the future¹. Now it's science fact! [1]: A good day for me is more like 27-28 hours, on average. Don't ask. Yes, I'll donate my hypothalamus to science.
- olivermarks 7y ago'Earth turned faster at the end of the time of the dinosaurs than it does today, rotating 372 times a year, compared to the current 365, according to a new study of fossil mollusk shells from the late Cretaceous'. I can't begin to imagine the forces that made this happen!
- Balgair 7y agoIt's the lunar tide. https://bowie.gsfc.nasa.gov/ggfc/tides/intro.html https://bowie.gsfc.nasa.gov/ggfc/tides/intro.html
- hinkley 7y agoDidn’t we already know this, for quite a long time? I feel like I learned about this 20 years ago and I’m confused as to why it’s news today.
- njarboe 7y agoYes we did. This is a press-release (It's right in the URL). Unfortunately AGU has gone down hill recently in its attempt to engage a wider audience and now you are getting the click bait "Ancient shell shows days were half-hour shorter 70 million years ago" instead of a more sober one about what the study is really about. The journal paper's title is "Subdaily‐Scale Chemical Variability in a Torreites Sanchezi Rudist Shell: Implications for Rudist Paleobiology and the Cretaceous Day‐Night Cycle". So maybe at title for the scientifically interested general public could be, "70 million year old Rudist shells improve length of day estimates during the late Cretaceous"
- olivermarks 7y agoMy comment was more a romantic one about the forces in play rather than a literal question. I'm also in awe of our solstice declination moments.
- Balgair 7y ago
- behringer 7y agoDon't humans operate best on a 23 hour schedule? I wonder if that is an evolutionary hold over from shorter days.
- saagarjha 7y agoIt's estimated that human circadian rhythms are just over 24 hours.
- dboreham 7y agoSomehow I was thinking of the shell in v6 written in pdp-11 assembler.
- cryptonector 7y ago> The length of a year has been constant over Earth’s history, because Earth’s orbit around the Sun does not change. That is decidedly not true. At the very least the eccentricity of the Earth's orbit around the Sun is known to change[0]: "The major component of these variations occurs with a period of 413,000 years (eccentricity variation of ±0.012)". Moreover, I seem to recall reading that over the 4.5 billion year scale the distance of various planets to the Sun has varied as well, though I don't have a reference for that right now. [0] https://en.wikipedia.org/wiki/Milankovitch_cycles https://en.wikipedia.org/wiki/Milankovitch_cycles
- saagarjha 7y agoFrom your link: > The orbital period (the length of a sidereal year) is also invariant, because according to Kepler's third law, it is determined by the semi-major axis. Which makes sense, because the semi-major axis depends on the energy of the orbit, and there's not really much that would be altering that in the short timeframe of a Milankovitch cycle.
- simcop2387 7y agoIt should be mostly invariant, but tugs and changes from other bodies in the system (Jupiter and Saturn particularly) can change the energy of the orbit. It's one of things that makes n-Body solutions to orbital mechanics nearly impossible to make. I doubt it's change too significantly for something as massive as earth though, but the collision with the mars sized body that created the moon early on in Earth's history definitely would have been able to change the orbital energy.
- cryptonector 7y agoOn the scales of the Milankovitch cycles, yes. But on galactic time scales, IIUC, no, because the gas giants' gravity does affect the orbits of each other and the smaller planets.
- ChrisLomont 7y ago>because according to Kepler's third law, it .... Kepler's law is an approximation, and it most certainly does not hold over large time scales. Heck, it fails on short time frames too - one of the early successes of relativity was to explain the precession of Mercury's perihelion from Kepler style theory to observation. Heck, even restricting to classical physics, Kepler's law fails as soon as you have three bodies, since the derivation is only for a two body problem. A third (or more) body makes his laws fail. Our solar system has well over thousands of bodies all interacting. So this is only an argument for the invariance of the (incomplete) model of Kepler, but fails in real life. We have plenty of better models (i.e., that agree better with observation) that are not invariant.
- fag_tits 7y agoHOW DARE YOU!
- SeanFerree 7y agoVery cool!
- danschumann 7y agoI've got a programmer joke for you: AncientShell#> echo day.length
- yoyar 7y agoAre they referring to bash or zsh?
- draw_down 7y agoCome on
- CamouflagedKiwi 7y agoIf they're ancient shells, surely it must refer to at least the original Bourne shell, not these Johnny-come-lately replacements.
- pengaru 7y agoThis makes me wonder if life expectancy in terms of years would be proportionally shorter if our days were say 48hrs instead of 24. Would slowing the earth's rotation have the side effect of extending our lifespan?
- lkrubner 7y ago4,500,000,000 divided by 70,000,000 equals 62. If the Earth slowed 30 minutes 62 times, uh, the days would be less than zero. Any reason why the slowing of the day would be more dramatic these last 70 million years?
- deleted 7y ago[deleted]
- divbzero 7y agoAssuming a constant force of friction the slowing of Earth’s rotation would be linear ~ t and the lengthening of our days would be the inverse ~ 1/t so we cannot simply divide 4,500,000,000 by 70,000,000 to estimate the original length of a day. Still, extrapolating back to Earth’s early years would yield a rather short day. Sources from Wikipedia [1] [2] estimate a 5 hour day after the Theia Impact that created the Moon. [1]: https://en.wikipedia.org/wiki/Earth's_rotation#Origin https://en.wikipedia.org/wiki/Earth's_rotation#Origin [2]: https://www.annualreviews.org/doi/10.1146/annurev.ea.15.050187.001415 https://www.annualreviews.org/doi/10.1146/annurev.ea.15.0501...
- deleted 7y ago[deleted]