Talk:Orbit

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Contents 1 Miscellaneous 2 open orbits 3 Kepler 4 Article Name 5 Example calculations 6 Table of orbital data 7 Stability of planetary orbits 8 Old Requested move 8.1 Survey 8.2 Discussion 9 Understanding Orbits Sections 10 Tilted Cannon? 11 Requested move 12 Section on Understanding orbits 13 Inequalities 14 Yet more on understanding orbits 15 Tidal effects near GSO 16 Historical maldefinition of 'orbit' 17 Newton's Cannonball question

[edit] Miscellaneous

where can i find information about earth's orbit around the sun? no link seems to lead to this information. -tom Are all planets orbiting the sun anti clock wise? -peg

Yes, all planets (and major moons) orbit in the same direction, and nearly in the same plane. This is now generally accepted to be due to the primordial rotation of the disk of gas and dust, out of which the Solar System formed. Wwheaton (talk) 22:37, 15 February 2008 (UTC)

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Most recent update was mine. My main intention was to fix the incorrect assertion in the previous version that inner planets has more circular orbits. This was in the introduction, so I placed the correction there. As a result the intro now looks a bit bloated with material duplicated below. -- Alan Peakall 17:03, 5 Sep 2003 (UTC)

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This page was moved to orbit (physics) and turned into a disambig page for a bit; this does not really make sense, since almost all inbound links were in the gravitational orbit sense. I moved the disambig page to orbit (disambiguation), and moved orbit (physics) back here. -- The Anome 00:10, 30 Aug 2004 (UTC)

You could also ask User:BenjBot to solve such a problem. I would not say that orbit in physics is the first thing for orbit. Tosha 04:00, 30 Aug 2004 (UTC)

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It's also a brand of gum... Someone should add that.

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Earth's orbit is neither clock wise nor counter-clock wise, and it is both. One cannot truely answer the question because it is relative to your perspective. The orbit is either clockwise or counter-clockwise depending on whether you look at it from the "top" (if it is truely the top), or if you look at it from the "bottom" (if it truely is the bottom).

Yes; in fact the standard convention is that "north" is the direction of the thumb when the fingers of the right hand point in the direction of the motion. According to this, all bodies revolve (and rotate) counterclockwise by definition, except when referenced to some larger external system, as eg, a contrary moon or asteroid w/r the Solar System. Wwheaton (talk) 22:37, 15 February 2008 (UTC)

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The equation of the orbit described by the particle is thus:

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Should the second : be a : ? Otherwise it doesn't make any sense, what would l be? DavidMcKenzie 16:00 21 July 2005

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Remembered what the l was: it's the semi-latus_rectum. Added a link to that and cleared up the ambiguity between the 1 and the L. DavidMcKenzie 16:51 21 July 2005

[edit] open orbits

Is it common for astronomers to call hyperbolic and parabolic motion orbits? To the layman, this is confusing. Orbits in common language implies periodic motion. If this is a common way for astronomers to speak, there should be an introductory sentence that explains this. Like this: "Astronomers commonly refer to any motion of one body relative to another as an orbit, even if the motion is not in a circular or eliptical path." It seems this article could use some translation into common English! -- Samuel Wantman 06:01, 30 July 2005 (UTC)

For astronautical engineers, spacecraft engineers and astronomers, I think it is common. Looking at the definition of "orbit" states that it is a "path" and I cant really see an implication of periodicity. The Greeks are probably the ones who coined the term "orbita" (path) for the wanderers (planets) as they probably did not observe open orbits (how could they have?). Open orbits were probably a mathematical result first before they were observed, hence the misnomer. Wicak 09:19, 3 August 2005 (UTC)

[edit] Kepler

I wonder if the statement "Kepler analyzed mathematically" is correct? As I recall reading, he made many many measurements over years, before arriving at a mathematical result. Empirical deductions would be a more accurate description. Wicak 09:12, 3 August 2005 (UTC)

[edit] Article Name

Shouldn't this article (Planetary orbit) be renamed Orbit (Astronomy) or Orbit (Celestial mechanics)? Planets are not the only thing that orbit. The star also orbits around the planet and two stars may orbit around eachother. Zhatt 16:40, 27 September 2005 (UTC)

Technically, that's not quite correct. The star does not orbit around the planet, nor does the planet orbit around the star. In reality, both the planet and the star orbit around the center of mass of the planetary system. In practice, however, the mass of the star is almost always many many orders of magnitude larger than the mass of the planet, so that the center of mass of the planetary system very nearly coincides with the center of mass of the star. So as a practical matter, the planet revolves around the star, not vice versa.

As an example, in our own solar system, the Sun makes up 99.85 percent of the total mass of the solar system, and Jupiter accounts for another 0.10 percent. The remaining eight planets account for only 0.04 percent combined, and comets, asteroids, and dust account for the balance. Source: Abell, Morrison, and Wolff, Exploration of the Universe, fifth edition (Saunders College Publishing, 1987), p. 234.

In a binary system, the two stars may have masses of similar order of magnitude, so that it is correct to say that each star orbits around the other, or more accurately, each star orbits around their common center of mass.

-- Metacomet 05:49, 30 December 2005 (UTC)

I agree that the article name is wrong though. Much of the focus is on satellite orbits; the common theme is gravitational orbits. Orbit (gravitational) is my suggestion. Joffan 23:25, 17 July 2006 (UTC)

I suggest either Orbit (astronomy) or Orbit (physics). The Land 00:10, 18 July 2006 (UTC)

you guys never sorted this out, but a name change is needed here.. what are we supposed to do with articles like Satellite orbit? it apparently doesn't fall under this one.. Mlm42 14:30, 4 October 2006 (UTC)

[edit] Example calculations

This section has been moved temporarily to another location while it is under development. -- Metacomet 05:51, 31 December 2005 (UTC)

[edit] Table of orbital data

This section has been moved temporarily to another location while it is under development. -- Metacomet 05:51, 31 December 2005 (UTC)

[edit] Stability of planetary orbits

My understanding is that the stability of planetary orbits, being an n-body problem, is a open question. Wasn't there a prize offered for solving this that was never claimed? --Michael C. Price ^talk 19:50, 22 September 2006 (UTC)

[edit] Understanding Orbits Sections

In the Understanding Orbits section I've described a "range of" parabolic and hyperbolic orbits.

Is that accurate?

Or -- from a given firing height, with a given mass -- is there:

• only one possible parabolic orbit and a range of possible hyperbolic orbits, or,
• a range of possible parabolic orbits and only one possible hyperbolic orbit, or,
• only one possible parabolic orbit and one possible hyperbolic orbit?

Note both a parallel firing direction, and the "tilted cannon" discussed in the next Talk subject.

[edit] Tilted Cannon?

I'm wondering what happens when the cannon is tilted up or down (is not fired parallel to a tangent touching the surface of the Earth).

Is it never possible to launch a circular orbit at an angle like this -- or will gravity correct the path to a circle, due to the speed and mass of the object?

Is is possible to launch any circumnavigating (elliptical) orbit at an angle, or will the curve always hit the Earth?

Does such a tilt influence what the escape velocity is for the object, and the parabolic vs. hyperbolic infinite orbit shape?

If you tilt the cannon one of two things can happen. 1) you tip the cannon up so much that you don't gain enough horizontal velocity to "miss" the Earth and you end up impacting the surface somewhere along your trajectory. 2) if you only tip a small amount and therefore manage to achieve a stable orbit your orbit will be elliptical, furthermore the same elliptical orbit could be achieved by simply placing a different cannon at the periapsis of your orbit and firing horizontal with faster than the circular orbital velocity for that altitude. It might be worth working these concepts into the main article if they haven't been already. As for the escape velocity I don't think it matters but I'm not 100% sure on this. The escape velocity is computed by setting the kinetic energy of the object as it leaves the planets surface equal to the change in potential needed to make it fly off to infinity. Since KE does not depend on direction it should not matter but I'm not certain of this. --AndrewBuck (talk) 06:57, 20 November 2007 (UTC)

[edit] Requested move

This article has been renamed from Orbit (celestial mechanics) to orbit as the result of a move request. This is clearly the primary meaning of "orbit". --Stemonitis 09:13, 9 May 2007 (UTC)

[edit] Section on Understanding orbits

This section, with the cannon ball diagram, is really great to have in the article. But it appears to have several technical misunderstandings. First: assuming the cannon ball is small compared to the Earth (and what cannon ball isn't?) the mass of the cannon ball does not change the velocity required to acheive orbit. (Does make a difference even if the cannon ball were the mass of the Moon?) Also, for any given firing point, there is only one velocity which will result in a parabolic orbit. Would fixing both of these be non-controversial? (Sdsds - Talk) 23:41, 8 May 2007 (UTC)

I think so. If the cannon ball were the mass of the moon, there would be a sizable reaction force on Earth. The way, the truth, and the light 00:32, 9 May 2007 (UTC)

Somebody fixed it [1].--Patrick (talk) 13:10, 20 November 2007 (UTC)

[Gibberish section deleted here] Wwheaton (talk) 22:16, 15 February 2008 (UTC)

[edit] Inequalities

In trying to edit the Pierre-Simon Laplace and George Biddell Airy articles I keep coming across references to "inqualities" in planetary motions. What is the exact definition of an inequality in this context?Cutler 19:53, 12 September 2007 (UTC)

[edit] Yet more on understanding orbits

I have just reverted one error in the last previous edit, because a closer reading of the examples cited shows that for that case (slightly slower than C, but faster than B), the periapsis (perigee for Earth) is actually opposite the firing point, per the original.

I also added a note that, (a) on the one hand, all Newtonian orbits are closed ellipses that repeat exactly, but (b) any non-Newtonian effects produce orbits that are generally not closed, though bound. I see now that this fussy point should probably be moved to the following section, which I will do shortly.

On an unrelated issue, just to explain, I have capitalized "Earth" within the section, according to the usage of astronomers (my personal background), being a proper name (of a planet), as decreed by the IAU. But I believe this may be one of those Wiki-specific conventions (as between English and American usage), suggesting consistency within an article as originally begun. I would advocate such capitalization in astronomical or related technical contexts, but am not rigidly committed to either; nor even to consistency, if it would be disruptive.

Also, does anyone have opinion or advice on the usage of orbit for the open-trajectory cases? I would tend to call those "trajectories", and reserve "orbit" for bound trajectories (which in practice are never precisely closed). As the section stands, "orbit" is used for all. Wwheaton (talk) 22:11, 15 February 2008 (UTC)

[edit] Tidal effects near GSO

I'm watching "If there were no Moon" (produced by Discovery Channel and narrated by Patrick Stewart), which talks about tidal forces pushing the moon away gradually. I've found discussion on orbital decay due to tidal forces when an object is below synchronous orbit. Though it may be just the opposite, I'm not quite getting how it works when an object is in or slightly above SO. Could someone please add a detailed technical description to the main article?

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—Preceding unsigned comment added by Greenwikiengineeer (talk • contribs) 17:22, 30 December 2007 (UTC)

I've moved this from the head to the end of the article, where later discussion should normally go.

These tidal effects work by exerting a gravitational torque (roughly, a kind of twisting force), so there are two logical steps to necessary to make them act. They would not have any effect at all between bodies that were perfect spheres, or spherically symmetric, say made of concentric spherical layers. So the first thing is that one or both bodies have to be non-spherical (or at least, not axisymmetric around the axis perpendicular to the plane of the orbit). Typically this happens because the tidal forces themselves pull stronger on the nearer sides and weaker on the further sides, which tends to draw the bodies out into a slightly teardrop shape, with the points of the tears towards each other. For the Moon's effect on the Earth, this effect is roughly the size of the oceanic tides (plus a smaller bit due to tides actually raised in the rock, with amplitudes of the order of centimeters I think). While the teardrop shape is typical due to the tidal forces, any irregularity will do: any bump say, like a mountain, or whatever.

Then, as the other part, to get the twisting effect, the axis of the typically tear-shaped distortion has to be out of alignment with the line connecting the centers of the two bodies. Then one body's gravitational field can try to force the other into alignment, pulling harder on the bump pointed in its direction. Such misalignment occurs either because the orbit is not perfectly circular (so that the angular speed around the orbit is not constant), or because one or both bodies is rotating at a rate different than the orbital angular velocity.

The final element is inevitable if the first two conditions (asymmetry and misalignment) are fulfilled. Because Newton's Third Law requires the gravitational force on the twisted body to have an accompanying reaction back on the other, there will be a force on the other body (even if it is symmetrical itself), trying to move it to align with the axis of the teardrop. This will cause the orbital velocity of that other body to change, either increasing or decreasing, as it tries to move into alignment.

So to answer your question: the tidal distortion of the Earth due to the Moon rotates with the orbital period of the Moon, once per month, and so it acts to speed up satellites that are slower (beyond the Moon), but to slow down those that are faster, inside the Moon's orbit. Because the lunar distortion is so small, for artificial satellites the effect alternates back and forth (as they pass the bump), and so almost cancels, and the result is negligible on human time scales. (There is also an effect due to the Sun's tidal field that seriously affects lunar satellites, often crashing them in a year or two.) But if the Moon were near GSO, the tides would be huge (GSO is ~10 times closer, so about 1000 times what they are today, since, if r is the distance, the tidal distorting force is roughly proportional to 1 / r³). This would cause the Moon to spiral inward if it were inside GSO (because the axis of alignment of the teardrop on the Earth would lag behind the Moon), and the Earth's rotation to speed up, until they came into mutual rotational lock. This situation is seen in many binary stars in close orbits, that are slightly (or sometimes extremely) tear-drop shaped, and rotate in synch with their points aligned (sometimes almost touching). But if the Moon were a little outside GSO. the Earth would rotate faster than the Moon's orbital revolution, so the bump would always be ahead of the Moon, spiraling it outwards, and slowing the Earth's rotation, which is what seems to have happened in historical fact.

This explanation is longer than I meant! But I hope it helps. Wwheaton (talk) 16:26, 20 February 2008 (UTC)

[edit] Historical maldefinition of 'orbit'

The definition of an orbit in this article as follows

“In physics, an orbit is the path that an object makes around another object or a barycenter while under the influence of a central force, such as gravity.”

is historically untenable for the very simple reason that it seems it was not until the 17th century well after Kepler that planetary orbits were conceived as in any part caused by a centripetal force such as gravity, possibly first by Roberval.(???), except perhaps for the Moon’s orbit around the Earth.

Certainly the article’s claim that

“The basis for the modern understanding of orbits was first formulated by Johannes Kepler whose results are summarized in his three laws of planetary motion.”

is historically false on this definition, since Kepler did not believe planetary orbits were caused by gravity nor any other centripetal force nor central force such as gravity, but by a combination of the Sun’s rotating sunspecks and a magnetic force that perturbed circular orbits into ellipses.

A somewhat historically better definition which does not depend upon central forces and therefore includes the notion of a planetary orbit not caused by any central force(s), but by such as rotating celestial spheres, and which thus does not exclude the notions of planetary orbits for millennia before the 17th century, is to be found in the Shorter Oxford English Dictionary. But an adequate and accurate definition of 'orbit' is certainly extremely difficult.--Logicus (talk) 18:01, 20 April 2008 (UTC)

The definition in the lead should be the modern one. You might want to be editing the section on the historical developement of the concept. The way, the truth, and the light (talk) 18:19, 24 April 2008 (UTC)

Logicus: No the main definition should just be correct, whether modern or not. Defining 'orbit' such as meaning 'the path of a planet' in such a way that it excludes planetary orbits as conceived before the 17th century and even by Kepler is clearly unacceptable. The criterion of 'modernity' for a definition objected by TWTTTL is a red herring: the Shorter Oxford English Dictionary definition that TWTTTL deleted is perfectly modern in its definition, which is as follows:

“ In astronomy an orbit is the path of a heavenly body, the curved path described by a planet or comet round the sun and by a satellite about its primary. “

Moreover, contra TWTTTL and Wikipedia, the current modern Wiktionary definition of orbit is :

"A circular or elliptical path of one object around another object."

which also omits the current Wikipedia definition’s untenable requirement of orbits being caused by a central force.

However this Wiktionary definition is also untenable but for another reason, namely that orbits need not be circular nor elliptical, if indeed any orbits are, but may also be parabolic or hyperbolic as some comet orbits may be, or a rosetta like Mercury's orbit, or any other figure going around something. Thus the Wiktionary definition must be reduced to

'The path of one object around another object.'

However, a valid objection to this definition is that in Aristotelian celestial physics planetary orbits were conceived as paths centred around a point, namely the centre of the universe, rather than around a body, and it seems this also applies to the alleged ‘modern’ conception as defined around a barycentre rather than around a body

Thus I propose the following minimal but historically inclusive adequate definition:

‘The path of one object around a point or another object.’ —Preceding unsigned comment added by Logicus (talk • contribs) 18:14, 30 April 2008 (UTC)

I agree that the limitation to central forces is too restrictive. The gravitational field of Saturn is clearly non-central, for example. Any potential force between two bodies will give bounded trajectories, which may or may not be periodic. And even the restriction to potential forces is a bit tight. For example, we might reasonably say the NEAR spacecraft orbited Eros, but it could gain or lose energy as it did so, due to the objects extreme asymmetry and rotation, giving a time-dependent potential. I personally think the term should be limited to trajectories that are at least spatially bounded, at least for a moderate time, but they clearly do not have to be periodic. I think the definition ought not exclude the bound trajectories of stars in disk or elliptical galaxies and star clusters, even though those may not remain bound forever.

I am tentatively reverting to User:Logicus's minimal form, pending a little more discussion, hopefully with input from others. Cheers, Wwheaton (talk) 21:52, 30 April 2008 (UTC)

After nosing around a bit, it seems to me that the Wiki Orbit (dynamics) article is a superset of our meaning here, in the sense that anything we consider an orbit here would be there too. I suspect we ought to check out that article (more carefully than I have done, yet) to see where we want to draw the line. It seems to me that stable bounded solutions of the gravitational many-body problem should be included here (so we get the Earth/Moon system), certainly, maybe even if only stable for a long time, not quite forever. But I think we should also include non-gravitational forces in the lead-sentence definition, even if we want to narrow the article's scope down to the gravitational case shortly after. Here I am thinking about orbits in general relativity (which even applies to Mercury's orbit as a practical case), orbits close to black holes, the orbits of charged particles in a magnetic field such as orbits in a particle accelerator, the Van Allen Belts, the orbit of a particle in a harmonic potential, etc, etc. My guess is that we should have links out to the more specialized cases, and limit ourselves to celestial mechanics here, but try to include the realistic cases that occur in the astronomical context, at least by linkage. Cheers, Wwheaton (talk) 22:56, 30 April 2008 (UTC)

This article used to be named Orbit (celestial mechanics) and Planetary orbit; see the RMs above, and it was always recognized that this should be about the astrodynamical concept, while the other uses should be indexed by orbit (disambiguation).

Also, it seems that User:Logicus is here to promote Aristotelian physics and has a fundamentally wrong argument: namely, that our definition of orbit must include all historical concepts that have now been superseded. I'm not responding to him any more until he gets a clue. The way, the truth, and the light (talk) 23:29, 30 April 2008 (UTC)

I agree that historical usage is of limited use here, as there are many more meanings than I would have imagined.

Fundamentally I think we should scope and title articles so that a naive but reasonably intelligent reader/user can find his way to what he wants as quickly, easily, and surely as possible. And also be comprehensive enough that someone on a more esoteric quest has a reasonable hope that finding his goal is possible following our pointers. The single sentence introductory paragraph does seem a bit sparse to me, but do you (TWTTTL) agree that restriction to central forces is too strict? I would like to see all cases that actually come up often in an astronomical context included, even though some (eg, n-body problem) are too specialized to deal with in detail here, and I think we could require that the trajectories in question be bounded (at least for a reasonably short time interval). Orbits of stars in galactic potentials, and also of charged particles in magnetic fields, I would propose to admit via the first sentence, but reroute later in the lead paragraph. I also think the disambiguation page needs some additions, as does our "See also" list. Being the main entry for "Orbit" seems to me to carry some responsibility to be comprehensive in helping folks to find the right track. I do think Logicus's version is sufficiently inclusive, but some additional words to map out the territory in more detail might be good. An alternative would be to leave all this to the disambiguation page, but a little more info than we give now could save the reader some false steps. Wwheaton (talk) 03:08, 1 May 2008 (UTC)

[edit] Newton's Cannonball question

In Newton's Cannonball experiment, am I correct in thinking that if the cannonball manages to pass the 'half-way mark' it will go into an uninterrupted orbit, and won't hit the Earth say 3/4 round? Marky1981 (talk) 10:06, 24 April 2008 (UTC)

Yes, with a couple of minor caveats. First of all, the mountain must be high enough that it is completely above the atmosphere, so there is no drag loss. Second, the approximation of exact Newtonian physics, for an exactly spherical planet, must apply. Then (assuming the barrel is precisely horizontal), the firing point will either be the lowest (perigee) or the highest (apogee) point of the orbit, and the orbit will be a closed ellipse, precisely repeating itself ad infinitum. The critical issue is that the perigee must be high enough to clear the top of the atmosphere. This "top of the atmosphere" is a bit fuzzy, but in practice 100 km is too low to remain in orbit for more than a short time, and 1000 km is high enough that such orbits last many years.

As a practical application, note that if two satellites collide and fragment into pieces, all the pieces will be left in orbits passing through the collision point, and will therefore have perigees no higher than that. Then the height of that point essentially determines the maximum possible lifetime of the fragments against atmospheric drag. (But since the pieces will generally all have different periods, they will typically not collide again.)

Thanks for the detailed explanation. Marky1981 (talk) 09:51, 25 April 2008 (UTC)