
Technically, Earth Does Not Orbit Around the Sun (2014) - gojomo
https://www.realclearscience.com/blog/2014/08/technically_the_earth_does_not_orbit_the_sun.html
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narnianal
The center of gravitation is also just a simplification, or how scientists
call it: "a model". Actually everything gravitates around everything else, as
the article also explains.

Models, or simplifications, aren't bad though. Because what value can you get
out of "everything gravitates around everything else"? Not much. But if you
use a model, you can use the model to explain what happened in the past and
use it to make an approximated assumption about the future. You can use it to
guide decisions, and you can use it to focus on more details in other areas.
E.g. if you don't use the actual trajectory of everything but a model, you can
still calculate where Mars will be when you reach it if you start right now
from point X on Earth. Of course the center of Mars will not be at exactly the
same point, but it will be a good enough approximation that you can start
planning resources required to get there, and it can help you decide if "now"
and point X are good inputs to that travel plan.

Models are a good, helpful tool. They are just not explaining everything, and
that on purpose, and as a human we always need to be aware of that models are
just models and not reality.

~~~
azernik
> Because what value can you get out of "everything gravitates around
> everything else"?

Expensive numerical simulations!

~~~
narnianal
Simulations with numbers are also models. You can make them indefinitely more
complex and still have a model that is only an approximation of reality.

~~~
earenndil
I would say, rather, that models are not approximations of reality, but
_descriptions_ of it.

~~~
shaftway
How about _predictions_?

~~~
earenndil
Nope. You can use them to _make_ predictions, but they are not in themselves
predictions.

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puzzledobserver
I was actually surprised that if all the planets were lined up, the center of
mass would be outside the surface of the Sun. I had always assumed that the
Sun was so massive that the center of mass was almost fixed and certainly
always within its surface. I guess I hadn't factored in the fact that the
planets are so far away and the Sun so small that their masses could have such
large measurable outcomes on the center of mass of the system.

~~~
LifeLiverTransp
I wonder wether this center of mass moving around in the sun, is producing -
time delayed of course- the "sun-weather" aka, the dark spots and
solarflares..

Something else:

Technical the orbit, is a spiral of the whole system around the center of
mass, slightly deteriorated by remainders of old interactions.

Which got me thinking, if you run a astronomical simulation backwards- can you
computate old interactions and "gone" missing bodys from that? Use celestial
motion as a sort of archive-trail to go back along?

~~~
ses1984
I think our ability to look backwards is limited by the precision of our
instruments and the precision of floating point computation.

Any error in our measurements would be magnified as we look backwards to the
point that the simulation would at some point significantly diverge from
reality, I wonder how many years back you can look before that happens?

Also floating point error would compound on top of that.

~~~
username444
Is there a reason you couldn't use whole numbers in place of decimals?

~~~
creatornator
The problem still exists. It's less so specific to floating point numbers and
more so a product of the limited number of states _any_ data type can hold.
Integers and floating point numbers are both still 64 bit on most modern
computers, unless you are using big-int structures which are much more
computationally intensive.

~~~
username444
Granted, I'm no mathematician, but...

If we have a limit of 2^64 for $x, can't we set $y = representive of multiples
of $x's upper limit?

$y = 5 = 5x2^64

What's the reason for not being able to sub-divide numbers and operations into
smaller, more manageable forms?

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fenomas
In college I took a great "Philosophy of Physics" class, where on the first
day the professor asked everyone to write one page on whether the Earth
revolves around the sun, and the final project was another paper on the same
topic, so we could see how nuanced a question it was and how our views had
evolved over the course.

With that said, I don't think there's any valid answer besides "you can define
your terms and points of reference such that it does or doesn't; there's no
particular physical reality to the matter".

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tempguy9999
Claiming that "there's no particular physical reality to the matter" I don't
understand at all. I can make sense of that to a limited degree as a variation
on the saying "all models are wrong..", but you seem to have extended that to
say that there is no 'right' possible (ie the 2nd part of that saying, that
"...but some models are useful").

A naive reading of what you said suggests eg. no spaceflight is possible
because no astronomical (or any...) calculation is reliable. Can you clarify?

~~~
fenomas
What I mean is, if two people disagree about whether the Earth orbits the sun,
there aren't any experiments where we'd expect them to predict different
results. They're both describing the same physical reality, viewed from
different frames of reference.

It's analogous to talking about someone throwing a ball on a train. A person
on the train can say the ball moves relative to the train, and someone outside
can say the ball and train are both moving relative to the Earth, and a guy in
a spaceship can say that all three are actually moving relative to the sun.
But choosing one of those claims over the others is purely a matter of
convenience or perspective, not of physical reality.

~~~
mattigames
But that is not what "there is no physical reality to the matter" means, if
one chooses not to use any relative point there is still an objective distance
between each object with respect to every other object and accelerations for
each of those distances.

~~~
fenomas
"The matter" in that quote is the matter of whether the Earth orbits the sun
or vice versa. Naturally there's observable physical reality to distances and
whatnot, but if you use the laws of physics to make predictions about those
observables, the math works regardless of which body you assume the others
revolve around.

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geoalchimista
Only someone without basic physics literacy would jump from the fact that
"planets revolve around the center of mass of the solar system" to the
clickbait conclusion "earth does not orbit around the sun". They missed that
approximation is a prerequisite for a scientifically meaningful representation
of the real world.

~~~
methodover
I liked it. The clickbaity title leads the reader to learn what a barycenter
means.

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Causality1
"Everything in the solar system orbits around that point."

Depends on exactly how pedantic you want to be. Technically everything in the
solar system orbits the barycenter of the galaxy and intra-system movement is
the same type of low-distance high-frequency wobbling the author chose to
dismiss with regards to the motion of moons around planets.

~~~
tomglynch
Does this mean even the sun orbits around that point?

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saagarjha
Yes.

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nothrabannosir
Fascinating. Basic physics question: the linked Wikipedia page on barycentre
suggests that the further away a planet (in a 1 sun 1 planet system), the
further the barycentre is from the centre of the sun. How does that work in
the context of the declining force of gravity as a function of distance? I
would have thought that the further a planet , the less pull it can exert on
the sun. Does gravity not work that way?

[https://en.m.wikipedia.org/wiki/Barycenter](https://en.m.wikipedia.org/wiki/Barycenter)

~~~
afiori
The barycenter determines the point around which the system rotates, the
intensity determines the speed.

~~~
irchans
I believe this is false in the sense that Mercury's orbit is more centered
about the center of the Sun rather than the barycenter of the solar system
(center of mass).

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afiori
That might be, but the real statement would be more like that the solar system
rotates around the barycenter of the solar system, for sure the moon revolves
"more" around the barycenter of the earth-moon pair than that of the whole
solar system.

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praptak
The Moon also revolves around the center of mass of the solar system. In a
sense more than it does around the earth. In particular Moon's orbit around
the barycenter is convex - the popular picture of Moon's orbit having loops is
wrong in that sense.

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rocqua
> the popular picture of Moon's orbit having loops is wrong in that sense.

Do you mean the moon never moves 'backwards'?

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praptak
It never goes backwards and it never "bends in" towards Sun.

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arnarbi
I think it's better to say it never "bends out" away from the sun, which is a
different way to statue that it's orbit around the sun is convex.

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fourier_mode
Isn't the title incorrect? As in: it does orbit around the sun, but the center
does not coincide exactly with the center of the sun. Or maybe there are other
used definitions of the term "orbit around"?

~~~
acjohnson55
I suppose there's no hard line between a single object being the actual center
of orbit and a many-body problem. I think the interesting thing to me is that
the barycenter is often far enough from the center of the sun to not be within
the sun at all. But that's about it.

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AlEinstein
If one is going to play the “well technically“ game then we need to describe
the motion with relativistic physics rather than Newtonian physics.

~~~
taneq
Also you can't just treat the solar system as a point mass if you're inside
it. Earth does orbit the sun but it is also perturbed to some degree by every
other mass in the universe.

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Merrill
The sun moves at about 828,000 km/hour around the glactic center. Thus, it
moves about 7250 million km each year.

The distance from the earth to the sun is about 150 million km.

Therefore, the earth's "orbit" is actually a very elongated helix.

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EForEndeavour
With respect to the galaxy's frame, sure. With respect to the earth or sun,
not so much.

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acomjean
One of my simulation software projects for a class last century was to try and
recreate the slight “wobble” stars have from suspected orbiting planet pulling
on them.

The simulation proved they could but it took a fairly large planet in a
closish orbit.

Planet finding techniques have improved considerably.

One problem I remember having was Setting the initial state so the planets
didn’t just slide off my coordinate system with a constant velocity.

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lisper
All scientific theories are approximations, but some are better approximations
than others.

For example: one of the reasons that the flat-earthers get as much traction as
they do is that "the earth is flat" is actually a reasonable approximation in
some circumstances. "Flat" is actually just a special case of "round" where
the radius is infinite. If the distance scale that you are concerned with is
small (e.g. the size of your own body) and you don't care about small errors,
then infinity is a not-entirely-unreasonable approximation of the radius of
the earth.

Of course, this all falls apart as soon as you start to care about anything
that is more than a few thousand body-lengths away from you. But it's a
mistake to say that the flat-earthers are _wrong_ in any absolute sense. They
aren't. They're just using a bad approximation for the realities of modern
life.

Unless you're sending a spacecraft to another planet, "the earth orbits around
the sun" is a perfectly fine approximation.

~~~
zuminator
> But it's a mistake to say that the flat-earthers are wrong in any absolute
> sense. They aren't.

I'd say they're absolutely wrong in the sense that they make the flawed claim
that people who assert that the Earth is an oblate spheroid are absolutely
wrong, or worse, are engaged in some kind of conspiracy to hide the truth.

~~~
lisper
Fair enough. My point is just that the claim that the earth is flat is not
entirely indefensible under the right circumstances, and so any proclamation
that a particular position is wrong (e.g. "The earth does not orbit the sun")
calls for a certain amount of humility and awareness of context.

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kbutler
Well, actually, it isn't orbiting just around the center of mass of the solar
system (solar system barycenter), but that is just one of the two foci of the
earth's elliptical orbit. The error is of a similar magnitude as assuming the
center of the sun vs barycenter of the solar system.

If you're going to be pedantic, get it right.

This article is a much better technical explanation of the orbit, including
both the moving center of mass and the elliptical orbit:
[https://socratic.org/questions/is-the-sun-directly-in-the-
ce...](https://socratic.org/questions/is-the-sun-directly-in-the-center-of-
earth-s-orbit)

Oh, and the earth's orbit is also distorted by proximity to other
gravitational masses, most notably the moon, but we're not going to be /that/
pedantic.

Side note: the phrase "Well, actually," was banned at my house. Not sure
why...

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vortico
I don't know what definition of "orbit" the author is using, but what he
explains is how scientists define it.

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irchans
The article quotes Cathy Jordan,"Technically, what is going on is that the
Earth, Sun and all the planets are orbiting around the center of mass of the
solar system".

I believe that the statement is a good approximation for the orbits of trans-
Neptunian objects or any other orbiting object that is more than 40 AU from
the Sun, but I think it is a bad approximation for Mercury, and not a good
approximation for the Earth. When I say it's a bad approximation, I mean that
it would be more accurate to say "the Earth and Mercury orbit the center of
the Sun" than it is to say "the Earth and Mercury orbit the barycenter of the
Solar System (center of mass)".

Any comments by someone more qualified that me would be appreciated (I took
about 6 courses in physics and 2 courses in astronomy in college and I
graduated over 20 years ago).

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stabbles
In a 2-body system on the complex plane, can't you basically take any point as
the origin and describe the motion of te bodies in terms of a complex Fourier
series, with the interpretation that the bodies orbit a circle on a circle ...
on a circle ... around your chosen point?

Edit: Basically Ptolomy did that the 2nd century AD.

~~~
layoutIfNeeded
The word you’re looking for is: epicycles

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edoo
Since gravitational waves travel at the speed of light there should be a delay
as well where the gravity of one body is affecting another from where it was
not where it is. For earth that is where the sun was 8 minutes ago.
Technically the galaxy might be orbiting something too.

~~~
gbear605
In fact, this is not how it works. My understanding is that whatever the
gravitational force is carried by (ie. gravitational waves) is also affected
by the velocity and rotational velocity of the gravitating object (in this
case, the Sun), so the Earth really is attracted to the Sun’s current
position, not where it was 8 minutes ago. Of course, this only works if no net
forces are applied to the sun in those eight minutes, but that’s almost
definitely the case. I believe, although it’s been a while since I did the
calculations, that if this were the case then the galaxy would gradually
become funnel shaped before eventually dissipating after the center gets too
far away from the rest. It would have similar effects on the solar system as
well.

~~~
Simon_says
> current position

Careful there, buddy.

~~~
gbear605
Okay, yes, current position is not quite correct, but you can replace each
instance of it with “the position of the Sun after the same amount of time as
it takes for light to get to the Earth from the Sun, according to an observer
that is stationary and on neither,” but that’s a mite clumsy (and still not
totally correct, but good enough)

~~~
Simon_says
What does “stationary” mean?

~~~
gbear605
Poor wording there as well :)

Perhaps “not experiencing net force“

~~~
Simon_says
You have reinvented inertial reference frame, but that doesn’t give you what
you want either.

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nocturnial
Fine, it orbits sagittarius A'*' with some very small wobbles caused by sol.

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dwighttk
>Every single object in the solar system, from the gargantuan sun to the
tiniest speck, exerts a gravitational pull on everything else.

Actually: in the universe, not just solar system.

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option
every person who finished some school should understand that speaking about
motion without reference frame is meaningless.

Is such frame is proper to Earth (yeah, go google what proper frame is) then
the Sun does revolve around Earth. If the frame is not proper to Earth or Sun
then they both rotate around common center of mass which is (relatively) near
Sun’s center but does not coincide with it.

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tomglynch
> "Everything in the solar system orbits around that point."

Does this mean even the sun orbits around that point?

~~~
ah27182
Yep. Thats the cool thing about the center of mass: its always the center of
rotation for a set of moving bodies.

~~~
pbhjpbhj
Of course the point isn't a point, the solar system is moving relative to the
galactic centre, which is moving relative to other galaxies. And space itself
is changing.

So as, pointed out elsewhere, we choose our points of reference and they're
pretty arbitrary when you consider the whole Universe.

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nurettin
How much, in terms of percent variation, could this be? 1%? 2%? When is it
going to become significant?

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RickJWagner
Wow, is that catnip-clickbait for the Flat Earth Society...

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TallGuyShort
Then technically, nothing orbits any other single thing.

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TeMPOraL
As the saying goes: all models are wrong; some are useful.

~~~
toomuchequate
By the way, be careful who you tell this to.

The anti-intellectuals will spin this to prove their own points without any
understanding of math or physics.

I've seen this happen, and they are very confident, their 'friends' don't know
either. When it comes time, will anti-intellectuals trust scientists or their
neighbor?

~~~
__MatrixMan__
I disagree with the need for a warning. I think it would be better if this
were _more_ commonly used.

So often debates arrive at a stasis like:

> "You're wrong"

> "No YOU'RE wrong"

And there they sit, each side certain that the other is an idiot.

The alternative is to admit that both parties are right according to their
model, and that both models are wrong (because being right is not what models
are for). I think this is better because the "which model is more useful"
question sets up a lot more potentially fruitful interaction between opposite
sides.

The danger you're referring to only occurs in a setting where science is
implicitly authoritative in the first place. If we drop that assumption,
science still produces the most useful models, but finding the most useful one
for your project becomes less adversarial.

~~~
seandougall
I want to believe that, but how do you evaluate models’ usefulness if you
refuse to acknowledge facts and data, and just yell “fake news” when they
point to a conclusion you don’t like?

~~~
rexpop
What do you hope to gain from combative dialogue with such people? Are you
trying to change how they vote, or shop?

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projectramo
This article will probably single handedly produce several hundred “well,
actually” responses on Twitter.

~~~
TeMPOraL
There's nothing wrong with "well, actually", despite what the silly meme would
have us think.

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LifeLiverTransp
Imagine if you had a really, really fine tuned sensor for gravity, you could
move a mass on one side of the system, and detect that move on the other side,
communicating with gravity waves.

~~~
xgulfie
You could, but it wouldn't move faster than light anyway.

~~~
LifeLiverTransp
Yes, but it would behave like a wave. You could make a gravity sonar for
planets with it.

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kingkawn
Not technically, only actually

