
Ancient shell shows days were half-hour shorter 70M years ago - sohkamyung
https://news.agu.org/press-release/ancient-shell-shows-days-were-half-hour-shorter-70-million-years-ago/
======
cryptonector
> 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
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
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.

------
peter303
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...](https://www.nasa.gov/topics/solarsystem/features/young-jupiter.html)

~~~
pfdietz
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,

~~~
pdonis
_> 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.

~~~
pfdietz
I have responded to that response and pointed out it cannot be correct.

~~~
pdonis
See my response in that other subthread.

~~~
pfdietz
It was a cloud of obfuscation where you never admitted your statement was
wrong and violates conservation of angular momentum.

------
ericfrederich
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?

~~~
stan_rogers
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.

~~~
londons_explore
> 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?

~~~
jbay808
Wouldn't tidal drag make years shorter? Longer years means a more distant,
high energy orbit.

~~~
TimSchumann
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
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.

------
peterburkimsher
Falsehoods programmers believe about time: "3\. Years have 365 days."
Dinosaurs had 372 days per year!

[https://infiniteundo.com/post/25326999628/falsehoods-
program...](https://infiniteundo.com/post/25326999628/falsehoods-programmers-
believe-about-time)

~~~
wbl
If you can find a dinosaur calendar it will be implemented.

------
sebastianconcpt
_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_

------
rishav_sharan
Mathematically challenged person here. Does this means that after 3.3B years,
Earth will lose its rotation completely?

~~~
ben_bai
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
Earth’s tidal locking is more a result of the Moon than interactions with the
Sun, and it’s not really linear.

------
forinti
31*12=372

So we would have had a nice regular calendar back then.

~~~
Rhinobird
28*13(+1)=365 (or +2 on leap years)

~~~
hoseja
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
Nah, humans are just missing about a twelfth of a finger.

------
sigmaprimus
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
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.

Thus, it isn't really important for most things.

~~~
sigmaprimus
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)

~~~
eesmith
[https://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.

~~~
fastball
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
I thought that was an accepted fact. It's been a while since I read about
dinosaurs : )

------
lkrubner
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?

~~~
divbzero
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.

[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.15.0501...](https://www.annualreviews.org/doi/10.1146/annurev.ea.15.050187.001415)

------
philliphaydon
> 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?

~~~
tenant
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?

~~~
pjc50
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
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](https://qntm.org/destroy)

------
olivermarks
'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
It's the lunar tide.

[https://bowie.gsfc.nasa.gov/ggfc/tides/intro.html](https://bowie.gsfc.nasa.gov/ggfc/tides/intro.html)

~~~
hinkley
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
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
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
Oops! My bad!

------
yoyar
Are they referring to bash or zsh?

~~~
CamouflagedKiwi
If they're ancient shells, surely it must refer to at least the original
Bourne shell, not these Johnny-come-lately replacements.

------
austincheney
This 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
So at 2.3 ms/century * 70 million years - so 1610000ms = 1610s = ~27 minutes

~~~
austincheney
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.

------
pengaru
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?

------
redog
So is it because the earths' density is moving away from it's center? Like
when you're spinning and you tighten up?

~~~
njarboe
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.

------
stoicShell
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._

------
pfdietz
It's amazing what they can discover looking at 1970s sh sources.

------
kazinator
> _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
the shell is not aware of date.c

------
nebulous1
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.

~~~
JorgeGT
Yep, and now I wonder which could be the oldest open shell connection in the
world, maybe some plant or industrial process?

------
markus_zhang
Took me half a second to realize it's not a *nix shell...

------
sumosudo
No wonder this work day feels so damn long!

~~~
hnick
In another 70M years my body clock will be just about right.

------
otikik
It was ... a different epoch

~~~
ncmncm
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.

------
antidaily
Called it.

------
dboreham
Somehow I was thinking of the shell in v6 written in pdp-11 assembler.

------
billpg
Did the clocks built by dinosaurs include leap seconds?

------
behringer
Don't humans operate best on a 23 hour schedule? I wonder if that is an
evolutionary hold over from shorter days.

~~~
saagarjha
It's estimated that human circadian rhythms are just over 24 hours.

------
mprovost
At first glance I thought this was referring to a Bourne or Korn shell.

~~~
tclancy
Fsh would be the only one extant back then.

------
danschumann
I've got a programmer joke for you:

AncientShell#> echo day.length

