[HN Gopher] Ancient shell shows days were half-hour shorter 70M ...
___________________________________________________________________
Ancient shell shows days were half-hour shorter 70M years ago
Author : sohkamyung
Score : 254 points
Date : 2020-03-11 03:24 UTC (1 days ago)
(HTM) web link (news.agu.org)
(TXT) w3m dump (news.agu.org)
| unexaminedlife wrote:
| I read a few years ago that the Earth's rotation is slowing at a
| rate of ~1 ms per century.
| sigmaprimus wrote:
| If 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 wrote:
| Centripetal acceleration at the equator is R o^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 wrote:
| Thanks 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)
| [deleted]
| eesmith wrote:
| 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 wrote:
| did NOT expect this to be that interesting for me.
| Pusha_Drugz wrote:
| same. 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 wrote:
| Interestingly, 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 wrote:
| I thought that was an accepted fact. It's been a while
| since I read about dinosaurs : )
| mrfusion wrote:
| My 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.
| mkl wrote:
| Most dinosaurs were herbivores, so I don't think that
| works.
| 8bitsrule wrote:
| In 'our' era, the megafauna of North America were pretty
| big as well (up until 13ka). Giant sloths, wolves,
| cats... and they survived an ice age. Not so well-known
| either.
| peter303 wrote:
| I 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-jupit...
| raxxorrax wrote:
| Interesting link. I wonder how that actually went down, since
| Jupiter and the other gas giants have relatively circular
| orbits. Is it just the gravity exchange of all the planets
| together? Quite difficult maneuver to just adjust your orbit
| height I would think.
| Balgair wrote:
| It's called the Nice Model:
|
| https://en.wikipedia.org/wiki/Nice_model
|
| Edit: Basically, the gas giants started really close into the
| sun but later moved outwards. We see a lot of other systems
| with gas giants close into their suns right now. In the Nice
| model, the larger gas giants had their orbits move slowly
| outwards, causing havoc in our system. This is possibly when
| the Moon was formed. It's still a lively debate, as we have
| not yet found Planet X yet.
|
| TLDR: Fund NASA more.
| saagarjha wrote:
| Interaction with gas in the protoplanetary disk, apparently.
| pdonis wrote:
| _> I disagree with assumption that year length has stayed
| constant the past 70 million years._
|
| It's not an assumption, it's what our best current data tells
| us. We have abundant evidence of the length of the Earth's day
| changing--this article is certainly not news, we have known for
| decades that the Earth's rotation has been gradually slowing
| over the past few billion years. We have no evidence of the
| length of the Earth's year changing significantly, and
| calculations agree with that (see below).
|
| _> 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._
|
| The size of the tidal bulge on the Earth due to the Sun is
| about a third of that due to the Moon. But that is not at all
| the same as the slowing of the Earth's _year_ due to the Sun 's
| tides being about a third of the slowing of the Earth's _day_
| due to the Moon 's tides. The situations are very, very
| different.
|
| In the case of the Earth-Moon system, tidal friction causes
| angular momentum to be transferred from the Earth to the Moon.
| This slows the Earth's spin and increases the radius of the
| Moon's orbit.
|
| In the case of the Earth-Sun system, tidal friction can't
| transfer angular momentum to the angular momentum of the
| Earth's orbit about the Sun, because the Earth is not orbiting
| itself; the mechanism that transfers angular momentum from the
| Earth's spin to the Moon's orbit about the Earth simply does
| not apply to the Earth's orbit about the Sun.
|
| In fact, while the Sun's tidal friction does make the Earth's
| spin slow down a little more than it would due to the Moon
| alone, the result of this is to make the radius of the Moon's
| orbit increase a little more than it would if the Earth-Moon
| system were alone in space. In other words, the Sun's tidal
| friction simply augments the Moon's tidal friction in driving
| the same mechanism, transferring angular momentum from the
| Earth's spin to the Moon's orbit about the Earth. There is no
| transfer of angular momentum from the Earth's spin to the
| _Earth 's_ orbit about the Sun.
| pfdietz wrote:
| The 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 wrote:
| It 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 wrote:
| _> 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.
| saagarjha wrote:
| You sure about the 3.8 billion number? That seems far too
| recent for any gradual, but drastic changes in orbits to have
| been possible.
| njarboe wrote:
| There is evidence from the Moon that is was heavily cratered
| from about 4.1 to 3.8 billion years ago. What caused this is
| debated, but one leading theory is the Jupiter and Saturn got
| into a 2:1 resonance at that time and highly disturbed
| everything in the Solar System (well, except the Sun)[1]. It
| is even called the Late Heavy Bombardment, highlighting the
| unexpected recent age of the event.
|
| [1]https://en.wikipedia.org/wiki/Late_Heavy_Bombardment
| Sharlin wrote:
| Recent? It was only ~700 million years after the formation of
| the Solar system.
| otikik wrote:
| It was ... a different epoch
| ncmncm wrote:
| Still 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.
| olivermarks wrote:
| '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 wrote:
| It's the lunar tide.
|
| https://bowie.gsfc.nasa.gov/ggfc/tides/intro.html
| hinkley wrote:
| Didn'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 wrote:
| Yes 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 wrote:
| My 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 wrote:
| Oops! My bad!
| redog wrote:
| So is it because the earths' density is moving away from it's
| center? Like when you're spinning and you tighten up?
| njarboe wrote:
| Nope. 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.
| SeanFerree wrote:
| Very cool!
| peterburkimsher wrote:
| Falsehoods programmers believe about time: "3. Years have 365
| days." Dinosaurs had 372 days per year!
|
| https://infiniteundo.com/post/25326999628/falsehoods-program...
| wbl wrote:
| If you can find a dinosaur calendar it will be implemented.
| stoicShell wrote:
| I'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._
| lkrubner wrote:
| 4,500,000,000 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?
| yoyar wrote:
| Are they referring to bash or zsh?
| draw_down wrote:
| Come on
| CamouflagedKiwi wrote:
| If they're ancient shells, surely it must refer to at least the
| original Bourne shell, not these Johnny-come-lately
| replacements.
| sebastianconcpt wrote:
| _The 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_
| sumosudo wrote:
| No wonder this work day feels so damn long!
| hnick wrote:
| In another 70M years my body clock will be just about right.
| forinti wrote:
| 31*12=372
|
| So we would have had a nice regular calendar back then.
| Rhinobird wrote:
| 28*13(+1)=365 (or +2 on leap years)
| hoseja wrote:
| 1 second = 9192631770 unperturbed ground-state hyperfine
| transitions of cesium-133 atom (preferably outside of solar
| system gravitation well)
|
| Calendars are silly. "Neat" ones even more so.
| _jal wrote:
| Nah, humans are just missing about a twelfth of a finger.
| admax88q wrote:
| Brb, just going to check my pocket cesium-133 so I can
| calculate how many days till my dentist appointment.
| austincheney wrote:
| This is probably explained by tidal breaking, which requires
| things like leap seconds.
|
| https://en.wikipedia.org/wiki/Tidal_acceleration
| arethuza wrote:
| So at 2.3 ms/century * 70 million years - so 1610000ms = 1610s
| = ~27 minutes
| austincheney wrote:
| Tidal 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.
| rishav_sharan wrote:
| Mathematically challenged person here. Does this means that after
| 3.3B years, Earth will lose its rotation completely?
| ncmncm wrote:
| No. 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 wrote:
| Venus 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 wrote:
| The 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 wrote:
| Once 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.
| Sharlin wrote:
| It would eventually become tidally locked with the moon, with
| the day becoming equal in length to the lunar month. But the
| sun is going to turn into a red giant before that can happen.
| ben_bai wrote:
| I think you have it backwards. Days are getting longer. So
| assuming tidal lock with Sun in the future which means 1 year =
| 1 day (1 revolution per year). So how long until 1 day becomes
| 365 days long. Assuming the 30min per 70M years is linear, this
| would take 1.2 Trillion years.
|
| Somebody double check this please. ;) Also, the Sun will only
| last for another 4B years.
| saagarjha wrote:
| Earth's tidal locking is more a result of the Moon than
| interactions with the Sun, and it's not really linear.
| thdespou wrote:
| Only???
| user982 wrote:
| "The alternations of night and day grew slower and slower, and
| so did the passage of the sun across the sky, until they seemed
| to stretch through centuries. At last a steady twilight brooded
| over the earth, a twilight only broken now and then when a
| comet glared across the darkling sky. The band of light that
| had indicated the sun had long since disappeared; for the sun
| had ceased to set--it simply rose and fell in the west, and
| grew ever broader and more red. All trace of the moon had
| vanished. The circling of the stars, growing slower and slower,
| had given place to creeping points of light. At last, some time
| before I stopped, the sun, red and very large, halted
| motionless upon the horizon, a vast dome glowing with a dull
| heat, and now and then suffering a momentary extinction. At one
| time it had for a little while glowed more brilliantly again,
| but it speedily reverted to its sullen red-heat. I perceived by
| this slowing down of its rising and setting that the work of
| the tidal drag was done. The earth had come to rest with one
| face to the sun, even as in our own time the moon faces the
| earth."
| nitrogen wrote:
| Where is that quotation from?
| philiplu wrote:
| H. G. Wells, "The Time Machine"
| Robotbeat wrote:
| It may be engulfed by the Sun by that time, so... yeah?
| pengaru wrote:
| This 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?
| danschumann wrote:
| I've got a programmer joke for you:
|
| AncientShell#> echo day.length
| fag_tits wrote:
| HOW DARE YOU!
| antidaily wrote:
| Called it.
| ericfrederich wrote:
| I 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 wrote:
| That'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 wrote:
| > 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 wrote:
| I 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 wrote:
| That'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.
| jofer wrote:
| Yes, absolutely, but must people will misunderstand
| "speeding up" as increasing rate of rotation, which is
| why it's useful to clarify.
| stan_rogers wrote:
| There is no force comparable to lunar tidal forces affecting
| the period of revolution. The Earth's slower rotation is
| coupled to the Moon's recession. The closest thing we've got
| with our orbit is resonance with Jupiter, and that's an awfully
| long way away.
| TimSchumann wrote:
| Yeah, I was trying to come up with a way to address the
| parent comment's first slight inaccuracy -- that the
| revolution period (year timescale) of our planet is being
| slowly changed by some other force. It's essentially not.
|
| Or, stated another way, anything that could account for that
| large of time variation in our year over that short of a time
| period... imagine ocean tides but with the Earth's mantle
| instead. Then we're not here to have this debate.
|
| For all practical purposes, the rotational period of the
| earth around the sun can be considered a constant.
| londons_explore wrote:
| > There is no force comparable to lunar tidal forces
| affecting the period of revolution.
|
| But there are... The earth orbiting causes tiny tides on the
| sun. They might only be a few millimeters, but they're non-
| zero. Over time, tidal drag will tend to make years longer.
|
| Anyone have the time and skill to do a ballpark guess the
| magnitude of this effect?
| stan_rogers wrote:
| That would explain a change in the rotational speed of the
| sun. Our orbit would then be affected by the tidal bulge on
| the sun leading us, and millimetres (or less) over the
| distance involved isn't going to do it, at least not to any
| degree comparable to the tidal interactions between the
| Earth and Moon. Also, we're not the only body that would
| have significant tidal effects on the mass distribution of
| the sun - we're not even at the top. Venus would have a
| larger effect pushing us one way; Jupiter a larger effect
| pulling us the other way. We're a bit of fluff, a dust
| mote. We're as likely to lose kinetic energy as to gain it,
| making the year shorter and bringing us closer.
| jbay808 wrote:
| Wouldn't tidal drag make years shorter? Longer years means
| a more distant, high energy orbit.
| TimSchumann wrote:
| The sidereal year is what we're talking about here, the
| time it takes the earth to orbit the sun and come back to
| the same position. What's changing isn't that, it's the
| sidereal day.
|
| Earth spins slower > Moon Speeds Up Less Rotations per
| Orbit > More Hours per Day
|
| Number of days is changing because the day is going from
| 23.X hours to 24.X hours due to the Earth rotating
| slower. Hence, same length year if you measure it in
| absolute time, just less days in relative time.
| jbay808 wrote:
| Assuming that revolution means sidereal year, this
| discussion thread is speculating about how minute the
| changes in the sidereal year would be, and in what
| direction they would have been. I don't think there's
| confusion about the fact that the orders of magnitude
| larger change as discussed in the article is in the
| sidereal day, except perhaps for GGGGP's comment that
| started the thread.
|
| >>>>> Wasn't the Earth's revolution on a different period
| back then too?
|
| >>>> There is no force comparable to lunar tidal forces
| affecting the period of revolution
|
| >>> But there are... The earth orbiting causes tiny tides
| on the sun [...] Over time, tidal drag will tend to make
| years longer.
|
| >> Wouldn't tidal drag make years shorter?
|
| > What's changing isn't that, it's the sidereal day.
| pfdietz wrote:
| It's amazing what they can discover looking at 1970s sh sources.
| mcntsh wrote:
| Hacker News is reddit now
| CyanBird wrote:
| Sad but true
| kick wrote:
| _Please don 't submit comments saying that HN is turning into
| Reddit. It's a semi-noob illusion, as old as the hills._
|
| https://news.ycombinator.com/newsguidelines.html
|
| Example from 12 years ago, from an account still commenting
| today:
|
| https://news.ycombinator.com/item?id=66057
|
| One bad post does not a trend make.
| mcntsh wrote:
| Look at all of the non-tech-or-science click bait headlines
| and one-liner joke comments.
| ewfwfewefewfwef wrote:
| One of those hidden gold comments that make me read comments in
| the first place.
| ascar wrote:
| Maybe 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 wrote:
| > 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 wrote:
| Makes 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 wrote:
| You'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.
| hombre_fatal wrote:
| Seeing shell in the title and then going "haha computers have
| shells too, such comedy" is a pretty low bar for "hidden
| gold".
|
| I prefer we leave such knee-slappingly advanced humor on
| r/funny where it can be truly appreciated.
| everybodyknows wrote:
| One of Paul Graham's essays cites the "dumb joke" as the #1
| pollutant of discussion threads. Might be "What I've
| Learned from Hacker News", February 2009, though alas
| Firefox won't load the article due to untrusted encryption.
|
| https://paulgraham.com/hackernews.html
| learn_more wrote:
| works without encryption:
|
| http://paulgraham.com/hackernews.html
| billpg wrote:
| Did the clocks built by dinosaurs include leap seconds?
| behringer wrote:
| Don't humans operate best on a 23 hour schedule? I wonder if that
| is an evolutionary hold over from shorter days.
| saagarjha wrote:
| It's estimated that human circadian rhythms are just over 24
| hours.
| mprovost wrote:
| At first glance I thought this was referring to a Bourne or Korn
| shell.
| tclancy wrote:
| Fsh would be the only one extant back then.
| dboreham wrote:
| Somehow I was thinking of the shell in v6 written in pdp-11
| assembler.
| markus_zhang wrote:
| Took me half a second to realize it's not a *nix shell...
| cryptonector wrote:
| > 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
| saagarjha wrote:
| From 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 wrote:
| It 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 wrote:
| On 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.
| uoaei wrote:
| Theory says that the moon is sapping energy from the Earth's
| rotation because it's moving away from the Earth -- angular
| momentum decreases as moment of inertia increases, all else
| being equal. I suspect the friction between water and Earth's
| crust also plays a role, since the tides are drawn to and fro
| by the moon.
| Sharlin wrote:
| Tidal drag is not an additional cause, it's _exactly_ the
| mechanism of how momentum is transferred from Earth to the
| moon. However as far as I know, oceans play a relatively
| minor part, with the deformation of Earth 's crust and mantle
| having the greatest effect.
| uoaei wrote:
| Isn't there a difference between "tidal forces" in the
| gravitational sense and the forces induced by the momentum
| of the fluid? Or are they the same?
| Sharlin wrote:
| Tidal force is the force that causes an object to stretch
| in a nonuniform gravitational field. If the object is
| rotating relative to the field, tidal forces induce
| dynamic stresses on the object, heating it up and slowing
| down its rotation (which means something else in the
| system has to speed up because momentum is conserved).
| kazinator wrote:
| > _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 wrote:
| the shell is not aware of date.c
| nebulous1 wrote:
| After the first half of the sentence I thought that somebody had
| found an old shell connection that hadn't been closed in 40
| years.
| kbrisso wrote:
| I was confused too. :)
| JorgeGT wrote:
| Yep, and now I wonder which could be the oldest open shell
| connection in the world, maybe some plant or industrial
| process?
| philliphaydon wrote:
| > 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 wrote:
| Because 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 wrote:
| I'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?
| cjensen wrote:
| Rotational speed of Earth is 1,000 mph at the equator where
| it is moving fastest. Other parts of the mass are moving
| slower.
|
| The entire Earth orbits the Sun at 67,000 mph -- around 67X
| faster. And note that it is the _entire_ mass of the Earth
| moving at that speed, not just the equator.
|
| Changing the rotation speed by 1% is a whole lot easier than
| changing the orbital speed by 1%.
| saagarjha wrote:
| Changing rotational speed or revolutional speed matters
| more on tidal forces and other sources of drag, which for
| the latter are much less.
| pjc50 wrote:
| From 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.
| wool_gather wrote:
| There's a reasonably thorough discussion of this topic,
| from an...interesting...perspective, in this essay: _" How
| to Destroy the Earth"_ https://qntm.org/destroy
| dr_zoidberg wrote:
| Addendum: far larger than the one that created the moon.
| contravariant wrote:
| Without any outside input of energy we probably can't really
| escape the Sun's gravity well.
|
| Oddly enough it's pretty tricky to steer the Earth into the
| sun as well, but we should be losing minute amounts of energy
| that will eventually put the Earth closer to the Sun. This
| probably won't happen before the Sun explodes though.
| dexen wrote:
| _> how do they decide that the earth spun faster on it's axis
| rather than the earth taking longer to orbit the sun?_
|
| It's both effects really. Celestial bodies' orbits do undergo
| decay and also their rotation undergoes decay. The question is
| which happened to which degree, and I think the rotational slow
| down is the dominant effect in Earth's case.
|
| Orbits decay due to various forms of drag. The long-term rate
| of decay of orbits in the Solar system is relatively well
| established.
|
| Rotation also is slowed down due to drag, but in our case
| there's another major force: the tidal influence from the Moon.
| Earth's Moon is a relatively large companion (at 1.23%[1] by
| mass). Both bodies influence each other tidally, and that
| influence saps away rotational energy and also Moon's orbital
| energy; the Moon already got tidally locked to Earth. Aside of
| that there's a (smaller) tidal influence from the Sun, which
| again saps Earth's rotational energy.
|
| --
|
| [1]
| https://www.wolframalpha.com/input/?i=mass+of+the+moon+%2F+m...
| excursionist wrote:
| Is the Earth's magnetic field affected by its rotation speed?
| saagarjha wrote:
| Since the magnetic field is tied to rotation of the
| planet's core, I would expect it to slowly weaken as it
| caught up with the lack of rotation of the surface.
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