Did the Earth really revolve around the Sun a billion times?

[Rakehellion]

It seems like on that timescale, the slightest little thing could send it off course. The gravity of Jupiter, collision with an asteroid, any slight disturbance could alter our course, and over a few years, eventually send us crashing into the Sun.

Are we still orbiting in pretty much the same way as we were at the beginning of time?

 

[Revolution 11]

You would think all this at first glance but who said physics had to be intuitive? Ironically, Jupiter's gravity prevents many asteroid collisions with the Earth.

For the stability of our orbit, first assume that we are in a steady-state scenario. If our orbit was unstable, you would think that a billion years later it would have changed in some substantial manner. The fact that it hasn't means we are quite stable.

There were probably millions of rocks and planetoids around our young Sun that did not assume stable orbits and either fell into the Sun, collided with other objects (eventually forming the planets), or were ejected out of the solar system. There are also still massive amounts of objects in possibly unstable orbits around the Sun, two groups which include the Oort Cloud and the Kuiper Belt. These objects may or may not be stable in the long-run, the odds changing with each object collision.

As for random falling out of orbit into the Sun, first consider how mind-bogglingly massive the Earth or any other planet is. Then consider how much energy you need to push the Earth around the Sun in a speed that will finish a complete orbit in 1 Earth year (for Earth in this case).

Objects external to our solar system could disturb a planet's orbit but must require a large planet-level amount of mass. A event like the Andromeda Galaxy colliding with the Milky Way in our distant future could change planetary orbits but more likely not, considering the vast distances involved. Certainly our solar system's position in the galaxy would change.

 

[SecurityTheatre]

It seems like on that timescale, the slightest little thing could send it off course. The gravity of Jupiter, collision with an asteroid, any slight disturbance could alter our course, and over a few years, eventually send us crashing into the Sun.

Are we still orbiting in pretty much the same way as we were at the beginning of time?

Things like this that seem a paradox, really aren't.

You have to think of how planets formed. Imagine the early sun being a tiny ball, surrounded by a massive flat disc of gas, dust and debris.

Now, over aeons, much of this dust, gas and debris ends up in unstable orbits and is fired off into the Universe, or is absorbed into the sun. Small packets of this dust that happens to be in a stable orbit begin to attract and clump together, eventually forming planets.

As a result, the planets are in a remarkably stable orbit, essentially because they were comprised of only that material which was already stable enough to sit still for aeons and form planets in the first place. The effect is much the same (and is misunderstood in much the same way) as the process of evolution. It's preposterous to think of a rat turning into a marmot, but if you collect together the trillions of random variations that happen over millenia and realize that only the few most successful ones will propagate (and the rest will be ejected from the gene pool), it actually makes a lot of sense.

I think the planets slow down gradually due to some small tidal forces, but I think the effect is pretty minimal... As in... tens of billions of years, minimal.

 

[SecurityTheatre]

Are we still orbiting in pretty much the same way as we were at the beginning of time?

Also, to be clear, our solar system is not a "first generation" solar system. It was likely comprised of the guts and debris from a supernova of a previous solar system. Who knows, maybe that one had intelligent life too...

 

[Mark R]

It is also the case that a suitably large body in orbit around a central body, will tend to disturb nearby orbits, destabilising them and often forcing anything in those orbits into a more stable orbit (frequently a multiple of the size of the main orbit), or ejecting it from the system.

This "clearing" effect is part of the definition of what makes a planet a planet, as opposed to an asteroid.

Planets, in general, are big. This means that they won't tend to be disturbed much by anything much smaller than a planet, or even other planets in the solar system; in other words, most orbits are stable. So although there will be a gravitational pull between 2 orbiting planets, because the orbits won't have the same time period, the orbits will go out of sync, and the gravitational effects will cancel out over the long term. The exception is if the orbits are extremely close, and have such similar orbital periods, that you get a prolonged gravitational attraction at one point, which is sufficient to destabilise one or other orbit.

 

[SecurityTheatre]

So although there will be a gravitational pull between 2 orbiting planets, because the orbits won't have the same time period, the orbits will go out of sync, and the gravitational effects will cancel out over the long term. The exception is if the orbits are extremely close, and have such similar orbital periods, that you get a prolonged gravitational attraction at one point, which is sufficient to destabilise one or other orbit.

This.

And it's possible that such a "co-orbiting" planetoid is what caused the creation of the moon, which was, by most accounts, spawned by a massive planetary impact event.

It's also worth pointing out that slight changes in the orbit of a planet in a near-vacuum does not cause it go hurtling into the sun (or spiral into the sun), but it simply makes the orbit slightly more elliptical. Now, passing closer to the sun during apogee made us slow down substantially (due to solar atmospheric drag, or similar), these orbital perturbations might be significant, but as it is, being part of an elliptical orbit doesn't kill our momentum.

The only way to make us go shooting into space, or spiraling down rapidly into the sun would be to change the planets orbital velocity by many thousands of kilometers per hour. The kinetic energy required to accelerate a planetoid by several dozen km/s is absolutely astounding.

 

[BurnItDwn]

The earth did not exist at the beginning of time.
The earth is only about 4.55 billion years old.
The Sun is only about 4.57 billion years old.
The Milky Way is about 13.2 billion years old.
The Universe is about 13.8 billion years old.

Over 9 billion years of time have existed before the formation of our sun and the earth.

Earths orbit is not constant. There are variations to the orbit which are experienced in cycles due to influence from other celestial bodies.

The earth has probably revolved around the sun for 4-5 billion times. However, I really do not know this number.

 

[aigomorla]

there is a theory a very well hashed theory, a giant asteroid collided with the earth and broke off a portion now known as the moon.
http://en.wikipedia.org/wiki/Giant_impact_hypothesis

ud figure if our orbit went terribly wrong... it would of happened at that event.

 

[sm625]

The earth is falling towards the sun. It never stops falling, it cannot stop falling. It is self stabilizing. So it can and will go on for billions of years. Until the earth accretes and creates enough matter to cuase its orbit to expand until its orbital path collides with something. Or until the sun grows larger and bakes the planet into nothingness.

 

[Agent11]

Although Nibiru is crap, it is theoretically possible for a rogue planetoid to collide with or severely alter the orbit of the Earth.

Such a hypothetical planetoid would have to get past all the gas giants of the outer solar system as well as Jupiter, and we would most likely be able to detect erratic behavior of comets, asteroids, and planets if such an event were to occur.

Being as there is lots of matter on earth besides Hydrogen and Helium, as well lots of other solar bodies made up of such it would be safe to assume that our solar system is the result of matter formed over several supernovae.

If someone wanted to get real fancy they could probably try to calculate how much matter a star of any given volume would produce on its death, and attempt to determine how many of what size would be necessary to account for the estimated matter in the solar system.

 

[Brian Stirling]

The earth is falling towards the sun. It never stops falling, it cannot stop falling.

Are you sure about that? The moon was once much closer to the earth and over time the energy lost (tidal energy has to come from somewhere) has seen the orbit of the moon increase and it continues to increase at the rate of something like 1.5 inches per year.

So, a safe bet is that the earth has revolved around the sun about 5B times +/- a couple billion times.


Brian

 

[DrPizza]

Are you sure about that? The moon was once much closer to the earth and over time the energy lost (tidal energy has to come from somewhere) has seen the orbit of the moon increase and it continues to increase at the rate of something like 1.5 inches per year.

So, a safe bet is that the earth has revolved around the sun about 5B times +/- a couple billion times.


Brian

Yes, he's sure about that. The moon is also falling toward earth. That was Newton's greatest realization.

 

[SecurityTheatre]

Are you sure about that? The moon was once much closer to the earth and over time the energy lost (tidal energy has to come from somewhere) has seen the orbit of the moon increase and it continues to increase at the rate of something like 1.5 inches per year.

So, a safe bet is that the earth has revolved around the sun about 5B times +/- a couple billion times.


Brian

In a strictly Newtonian sense, it most certainly is falling, even if its orbit gets gradually larger, that just means the angle of its fall is slightly different.

Marshall explains it fairly nicely here:
http://what-if.xkcd.com/58/

 

[Biftheunderstudy]

It's also worth it to point out that the many body gravitational problem ( many means >2 ) is a chaotic system. This, I mean in the mathematical/physics sense, and so can never be truly "stable".

In fact, with a bit of math you can expand what the contributions from the other planets do to the potential and solve (ish). Then you see that the orbits in many body system are not closed in that they are not perfect ellipses.

When teams working on exoplanets see a system with multiple planets, they run LOTS of simulations to get a handle on the statistical probability that a particular set up is stable over long times. This also includes things like orbital resonances which can stabilize an orbit.

 

[DrPizza]

Chaos doesn't necessarily imply that there are no bounds to a particular behavior. Though, I suppose you're still technically correct mathematically, due to the precise nature of "stable." I suppose it would be like saying "walk in circles around your house." You might walk around your house 100 times before you step in the exact same spot again, but not being truly "stable" doesn't mean you would ever accidentally get lost and wind up on the neighbor's porch.

 

[Brian Stirling]

In a strictly Newtonian sense, it most certainly is falling, even if its orbit gets gradually larger, that just means the angle of its fall is slightly different.

Marshall explains it fairly nicely here:
http://what-if.xkcd.com/58/

OK, we're confusing concepts here. I understood the question was weather the moon is getting closer or farther away from the earth over time and not the general concept of orbits in the sense that all things in orbit about the earth are falling towards the earth.

Yes, all things in orbit about the earth are falling towards the earth and it needs enough speed to keep in orbit.

But, since the question was about the number of times the earth has revolved about the sun it IS more a question about orbital radius and how that changes over time. It seems to me that the earth is on average farther away from the sun now then it was a billion years ago and the length of time required to complete an orbit is based on the orbital radius, all other things being equal.

That said, since the earth/sun system has been around for nearly 5B years it seems certain to me that the total number of orbits is well over 5B.


Brian

 

[DrPizza]

OK, we're confusing concepts here. I understood the question was weather the moon is getting closer or farther away from the earth over time and not the general concept of orbits in the sense that all things in orbit about the earth are falling towards the earth.

Yes, all things in orbit about the earth are falling towards the earth and it needs enough speed to keep in orbit.

But, since the question was about the number of times the earth has revolved about the sun it IS more a question about orbital radius and how that changes over time. It seems to me that the earth is on average farther away from the sun now then it was a billion years ago and the length of time required to complete an orbit is based on the orbital radius, all other things being equal.

That said, since the earth/sun system has been around for nearly 5B years it seems certain to me that the total number of orbits is well over 5B.


Brian

If we define a year as the amount of time for the Earth to make a full orbit around the sun, then it's trivial to claim that the number of years and number of orbits is the same. Though, the length of a year in this circumstance isn't a constant. So, which definition of "year" do we use?

 

[SecurityTheatre]

If we define a year as the amount of time for the Earth to make a full orbit around the sun, then it's trivial to claim that the number of years and number of orbits is the same. Though, the length of a year in this circumstance isn't a constant. So, which definition of "year" do we use?

Generally, a "year" is a complete revolution around the sun (although in practical use we sometimes define it by the number of days/seconds).

A "day" is a revolution of the earth on its axis (although in practical use, we often define it by the number of seconds).

However, a second is traditionally an SI unit defined by some physical constant (like the oscillation rate of an atom).

As a result, the number of seconds in a "true" day, or a "true" year, is constantly changing, and this is why we have "leap year" and "leap second".

Realistically, the number of seconds that elapse in a current year is probably more than elapsed in a year in the distant past. It's likely that tidal effects have slowed the rotation of the earth somewhat during the previous billions of years, and therefore our orbit has changed somewhat....

 

[Brian Stirling]

OK, if a year is defined by the length of time it takes for the earth to revolve around the sun and if we say that definition is not fixed at any point in time like now then when the solar system was 50% as old as it is now the duration of a year would be something less than it is now. However, in most cases when we talk about the duration of a year it IS based on the current, average, time it takes for the earth to revolve around the sun and is therefore fixed at the current average period.

Either way, the earth has revolved around the sun about 5B times.

There are a couple wrinkles here though:

1. The sun loses mass as it burns energy

2. The sun gains mass as it eats things that hit it

3. The earth's orbit must have been f'd up when the mars like planet hit it resulting in the formation of the moon

I don't know if 1 or 2 dominates so the net change in mass could be +/-.


Brian

 

[SecurityTheatre]

in most cases when we talk about the duration of a year it IS based on the current, average, time it takes for the earth to revolve around the sun and is therefore fixed at the current average period.

I think this is accepted as the margin for error on timescales where this matters is probably greater than the variance in the length of the year...

There are a couple wrinkles here though:

1. The sun loses mass as it burns energy

Not really that much mass, though. I saw a calculation that over the sun's entire 10 billion year life, only about 0.7% of its mass will be converted to energy, resulting in a TINY fraction of change over the ~4 billion years we're talking about.

2. The sun gains mass as it eats things that hit it

I think, compared to the mass of the sun itself, this is pretty trivial. Probably also less than 1%?

3. The earth's orbit must have been f'd up when the mars like planet hit it resulting in the formation of the moon

Presumably, both objects were in some sort of elliptical orbit. If this is the case, their collision simply results in an orbit that is the mean of the two previous orbits (influenced by relative mass).

 

[Biftheunderstudy]

The 0.7% comes from just the difference in mass of 4 hydrogens and 1 Helium and E=mc^2 -- i.e fusion. This is somewhat naive, but it's a good first guess.

The Sun also loses an actually quite significant amount though the stellar wind, wiki puts it at somewhere around 0.01%. All told, not super important to orbital mechanics, but certainly in the realm of being able to model it.

I don't think the sun gains that much mass, the solar wind and radiation are pretty effective at keeping the ISM out of the solar system.

My point about chaos was really that we can't really know anything about the history of our orbits because small changes in the initial conditions lead to drastically different outcomes. We can only get a handle on the statistical distribution of possible orbital configurations.

Hell, we don't even *really* understand how the gas giants formed and how they got where they are.

 

[glugglug]

Conservation of Energy means for the earth's distance from the sun to change, either

a) it would speed up as gravitational potential energy was lost (getting closer to the sun), but this higher velocity would exceed our acceleration towards the sun making us get farther from it, so this balances out (our distance does oscillate a bit over the year, with our speed doing the opposite change).

or

b) it would slow down as it got further from the sun, eventually not moving fast enough to maintain orbit and "falling" back towards the sun (again, just end up with oscillation as in a)

or

c) energy transferred to/some some other object (asteroids, debris etc), or converted to heat. On average, these collisions should be slowing us down over time, bringing our orbit closer to the sun. But unless the collision is with something super massive, the amount of kinetic energy loss we are talking about is tiny (at least relative to the scale of the Earth/Sun relationship). So we don't really notice. But Earth probably is slightly closer to the sun then it was 4 billion years ago.

 

[SecurityTheatre]

Conservation of Energy means for the earth's distance from the sun to change, either

a) it would speed up as gravitational potential energy was lost (getting closer to the sun), but this higher velocity would exceed our acceleration towards the sun making us get farther from it, so this balances out (our distance does oscillate a bit over the year, with our speed doing the opposite change).

or

b) it would slow down as it got further from the sun, eventually not moving fast enough to maintain orbit and "falling" back towards the sun (again, just end up with oscillation as in a)

or

c) energy transferred to/some some other object (asteroids, debris etc), or converted to heat. On average, these collisions should be slowing us down over time, bringing our orbit closer to the sun. But unless the collision is with something super massive, the amount of kinetic energy loss we are talking about is tiny (at least relative to the scale of the Earth/Sun relationship). So we don't really notice. But Earth probably is slightly closer to the sun then it was 4 billion years ago.

or

d) loss of mass/energy from nuclear fusion that is radiated into deep space.

 

[WHAMPOM]

It seems like on that timescale, the slightest little thing could send it off course. The gravity of Jupiter, collision with an asteroid, any slight disturbance could alter our course, and over a few years, eventually send us crashing into the Sun.

Are we still orbiting in pretty much the same way as we were at the beginning of time?

Four billions revolutions around the Sun, much closer and faster at the start just like the moon as mentioned in certain posts. Jupiter- due to the inverse square law of gravity- has little effect on Earth though it may have pulled apart a fifth rocky planet or kept one from forming. The Earth wobbles all the time right now due to the Earth/Moon common center of gravity, something else for you to to look up.

 

[Ken g6]

Been reading about the Nice model? It suggests stuff has changed, but not in the past 3-4 billion years.