> 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.
> 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.
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.
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.
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.
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.
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).
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.
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,
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).
> 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 from the Earth's spin 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.
"In the case of the Earth-Sun system, tidal friction can't transfer angular momentum from the Earth's spin 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."
This argument cannot be correct. Suppose the sun were a point mass. It would still have tides at Earth, just like the real Sun does, but the interaction would not be able to cause any change to its spin. So, by conservation of angular momentum, if the Earth loses rotational angular momentum, that MUST be transferred to orbital angular momentum.
The mechanism of course still exists. The Earth and Moon orbit each other; the Sun and Earth orbit each other. The bulge on the Earth induced by the Moon accelerates the Moon, and the bulge on the Earth induced by the Sun accelerates the Sun.
> The bulge on the Earth induced by the Moon accelerates the Moon
More precisely, it exerts a torque on the Moon, which is equal and opposite to the torque exerted by the Moon on the bulge. So does the bulge on the Earth induced by the Sun.
> the bulge on the Earth induced by the Sun accelerates the Sun.
This statement is correct in principle (or more precisely, the statement that the bulge on the Earth induced by the Sun exerts a torque on the Sun), and you are right that I left it out of my previous post.
Also, as I noted above, the bulge on the Earth induced by the Sun also exerts a torque on the Moon. And, for that matter, the bulge on the Earth induced by the Moon also exerts a torque on the Sun. I believe the net torque on the Sun is about 20% of the net torque on the Moon.
However, that does not mean the effect on the Earth's orbit is 20% of the effect on the Moon's orbit. The relative magnitudes of the effects depend on how large the torque is compared to the relevant orbital angular momentum. Since we are working in an Earth-centered frame, that means we need to compare the Moon's orbital angular momentum around the Earth with the Sun's orbital angular momentum around the Earth. If you do the math, you find that the latter is about 300 billion times larger than the former. Multiply that by another factor of 5 (for the 20% ratio of torques) and the effect on the Earth's year is about 1.5 trillion times smaller than the effect on the Moon's month.
> the bulge on the Earth induced by the Sun exerts a torque on the Sun
There's a further point here, though. The torque exerted on the Moon by the bulge is constant over time, because the position of the Moon relative to the bulge is constant (the bulge "leads" the Moon by a constant angle which is the result of a balance between the torque on the bulge exerted by the Moon and the pull on the bulge exerted by the Earth's spin). But the position of the Sun relative to the bulge is not constant: it goes through a complete cycle over the course of a month. So it seems to me that, at least to first order, the net torque of the bulge on the Sun over the course of a month should cancel out, so there will be no net exchange of angular momentum between the Earth's spin and the Sun-Earth orbit.
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.
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.
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.
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?
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".
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.
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.
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.
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.
> 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?
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.
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.
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.
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
"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."
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.
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.)
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.
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.
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.
Centripetal 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.
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)
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.
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.
Assuming 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.
> 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?
>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.
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.
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
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?
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.
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.
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
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%.
'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!
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"
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.
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?
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.
> 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.
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.
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.
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