Talk:Parallax

The current definition of parallax is mistaken and misleading in respect of the italicised terms below:

"Parallax, or more accurately motion parallax, is the change of angular position of two stationary points relative to each other as seen by an observer, caused by the motion of an observer. Simply put, it is the shift of an object against a background caused by a change in observer position. If there is no parallax between two objects then they occupy the same position. The term derives from the Greek παραλλαγή (parallagé), meaning "alteration"."

There can also be parallax when the observer does not move but rather the viewed objects move. The classic case in astronomy is the observed daily parallax of the Moon, Sun and planets against the stars that arises when observed from the surface of the Earth. These would be the same when observed from the same observer location on the Earth's surface whether or not it is they and the fixed stars or the Earth that rotates daily. --Logicus (talk) 19:51, 24 December 2007 (UTC)

--Logicus (talk) 14:23, 10 May 2008 (UTC)

This diagram illustrates the historically crucial fact that, contrary to the mistaken standard definition of ‘parallax’ as an apparent displacement of an object due to a locomotion or change of position of the observer, in fact a fixed observer can also perceive parallax purely due to the motion of the objects themselves or even just to the observer’s oblique or eccentric viewpoint without any motion of the observer nor of the objects observed. Indeed even in secondary school physics pupils are taught to avoid scientific instrument needle reading errors due to parallax caused by an oblique viewpoint of the dial and needle. The diagram shows for the case of geocentric-geostatic cosmology how the fact that a fixed observer on the Earth’s surface at the equator is displaced from the Earth’s centre by an Earth radius entails that at moonrise in the East the Moon would appear below a given fixed star, whereas at moonset in the West it would appear to have moved ahead of it, and when overhead it would appear in line with it. But for a hypothetical observer at the centre of the Earth they would always appear in alignment. The parallax of the Moon, Sun or of any planet or comet is the angle subtended at that body by the Earth’s equatorial radius. (The parallax is exaggerated here by the lunar and stellar distances being drawn radically under scale for expository purpose.) It seems a daily Lunar parallax apparent to a fixed observer on the surface of a supposedly central and non-rotating Earth was first recorded in the 2nd century BC by Hipparchus in his book On Sizes and Distances.

Because greater distance implies less parallax, calculations of planetary and cometary daily parallax and thus of their relative distances played crucial roles in ‘the astronomical revolution’. For the conclusion that Martian parallax at opposition is greater than Solar parallax, and thus that Mars is sometimes nearer the Earth than the Sun is, refuted the Ptolemaic geocentric solid orbs model because the Martian orb would have had to interpenetrate the Solar orb. In the Ptolemaic model depicted in Copernicus’s ‘ De Revolutionibus’, Mars is never nearer the Earth than the Sun is, but in the semi-heliocentric Tychonic model it is nearer when at opposition. Tycho mistakenly thought his observations showed that Mars had greater parallax at opposition than the Sun’s, which he took on faith to be 3’ without ever measuring it himself. And the calculation that comet parallax is less than Lunar parallax, whereby comets are superlunary and so must pass through various planets’ celestial orbs, refuted solid orbs more generally. Modern values for the maximum daily parallaxes of the Moon, Mars and Sun are respectively almost 1 degree (i.e. approximately two Moon diameters), some 23 arcseconds and some 9 arcseconds. Thus the Moon, for example, is apparently displaced by almost 2 degrees from moonrise to moonset.

The diagram provides a corrective pedagogical alternative to the somewhat misleading and confusing diagram of daily parallax in Kuhn’s 1957 ‘The Copernican Revolution’ [See Fig 39, p207 1959 Vintage Books edition] that misrepresents this parallax as due to the transportation or to different locations of the observer rather than to their displacement from the centre of the Earth, and also mistakenly claims “The large size of the moon and its rapid orbital motion disguise the parallactic effect.” It was crucially Kuhn’s inattention to observations and his neglect of analysing the logical role of crucial observations and observational conclusions in the heliocentric revolution, such as daily parallax, the phases of Venus and stellar aberration, combined with the traditional failure of ‘ignoring the elephant in the room’ initiated by Galileo in his 1632 ’Dialogue on the two chief world systems’, which utterly failed to compare or even mention the most scientifically and culturally relevant geocentric model of the day, namely the Tychonic geo-heliocentric third world system favoured by the Church, with the heliocentric model, and further combined with the traditional failure of historians to reliably date and quantify the majority conversion in the scientific community from geocentrism in its post-Ptolemaic geo-heliocentric models that became dominant in the 17th century after the 1610 telescopic confirmation of the phases of Venus, to pure heliocentrism in the 18th century after the discovery of stellar aberration, that led him to invalidly conclude this ‘scientific revolution’ was empirically irrational. This in turn induced his subsequent Wittgensteinian-Koyrean irrationalist general theory of scientific revolutions as incommensurable ‘paradigm‘ changes rather than empirically rational progress, published in his 1962 ‘The Structure of Scientific Revolutions’, with its non-logical sociopsycho-babble approach to the history of scientific discovery.

In conclusion, the standard definition of parallax typically to be found in dictionaries, encyclopedias and in science and history of science textbooks, which (mal)defines it as a function or consequence of motion, and in particular only of the motion of the observer, is thus at least twofold mistakenly restrictive, for there can also be (i) a parallax of moving objects for a static observer, such as assumed in Ptolemy’s lunar parallax observations, and also (ii) static parallax when there is no motion at all, neither of the observer nor of the observed, such as when it is caused purely by an eccentric or oblique viewpoint in a mistaken scientific instrument dial reading, as taught in secondary school elementary experimental physics, or even in primary school numeracy teaching of accurate clock reading. And of course most people are also basically intuitively familiar with static parallax from sport, where the very purpose of a linesman is to eliminate static parallax due to an oblique or eccentric viewpoint of other spectators in estimating the instantaneous location of a ball or person, for example, in relation to some line, such as the line in a goalmouth, for example, in the most notorious of all soccer parallax disputes, namely “Was it in ref. ?”. Thus we must distinguish between motion parallax caused by a motion either of the observer or of the observed, and static parallax not due to any motion at all, neither of the observer nor of the observed, but rather just to the observer’s oblique or eccentric but static viewpoint.

However, defining the general notion of parallax involved in both motion and static parallax seems difficult, but at least to essentially involve the notion of obliquity or eccentricity of viewpoint, or at least of the deviation of one line of sight from another, rather than that of motion.

I invite Wikipedians to attempt an adequate general definition of parallax here with this analysis in mind, and whilst I try to come up with one.

--Logicus (talk) 14:39, 11 May 2008 (UTC)

Edited and expanded 12 May --Logicus (talk) 18:12, 12 May 2008 (UTC)

Further edited and expanded 14 May--Logicus (talk) 18:15, 14 May 2008 (UTC)

Be bold and change the page rather than talking about it. As long as you cite reliable sources, the change will stick. However, this article is primarily about the modern scientific concept of parallax and thus the modern definition seems to be the relevant one, although a history section might be an interesting addition to the article. ASHill (talk | contribs) 15:15, 11 May 2008 (UTC)

Thanks ASHill, but still developing. You are surely miss the main point in claiming the current definition is ‘the modern scientific one’. For it is simply wrong, whether modern, ancient or antedeluvian, as my text explains, even for schoolgirl physics. —Preceding unsigned comment added by Logicus (talk • contribs) 18:16, 12 May 2008 (UTC)

Then you missed my point. If there's a change to be made, make it! I've gathered that you think a change should me made to the definition, and I'm encouraging you to change it with the warning that it should be a modern, scientific definition and it must be properly sourced. If you want to put your actual proposed text here on the talk page before implementing it, that would be fine. Frankly, I'm not going to carefully read a verbose explanation of a suggested revision (without the actual proposed new text) on the talk page, so I haven't read your comments well enough to know if your proposed definition is the modern scientific one. ASHill (talk | contribs) 18:30, 12 May 2008 (UTC)

Contrary to what ASHill claims, there is no requirement that the definition be "the modern scientific one", whatever that means, but merely to be correct. Logicus —Preceding unsigned comment added by 80.6.94.131 (talk) 18:16, 15 May 2008 (UTC)

The SOED Definition best ?:The principal definition of 'parallax' of the 1968 Shorter Oxford English Dictionary, namely

“Mutual inclination of two lines meeting in an angle.”

seems a plausible candidate for a general definition if adapted as follows:

‘The inclination of two lines of sight meeting in an angle.’

Of course it needs expanding in its application to the various different kinds of parallax, but this seems to be the general notion common them all.--Logicus (talk) 16:05, 16 May 2008 (UTC)

Maybe this has been discussed, or maybe someone has a clear idea, but: why is this section here? As written it describes three methods of distance estimation, none of which is based on parallax. It also discusses the end goal, estimating the size of the universe, which again has nothing to do with the topic in question.

I don't believe this section belongs. If no one can give a good justification, I am going to delete it. atakdoug (talk) 19:59, 7 January 2008 (UTC)

I agree, and the material is all covered in cosmic distance ladder anyway. I added a bit to the intro mentioning that parallax is the basis of the distance ladder. Ashill (talk) 09:24, 15 January 2008 (UTC)

And I deleted the section, which I forgot to mention. Ashill (talk) 09:26, 15 January 2008 (UTC)

The current equation seemed very lacking. Surely a formula to work out the error given the perpendicular distance and parallel distance, or the angle from perpendicular, combined with the distance from the observed object and frame of reference, would be very desirably and helpful? It would certainly have helped me. Larklight (talk) 10:13, 30 January 2008 (UTC)

"In astronomy, parallax is the only direct method by which distances to objects (typically stars) beyond the Solar System can be measured" - what about SN1987a? 98.221.27.220 (talk) 01:14, 14 August 2008 (UTC)JohnS

There's no direct measurement. Any "direct" measurement of the distance to SN 1987A requires an estimate of the intrinsic luminosity of the object. With parallax, once you know the distance from the Earth to the Sun, you can measure distance purely geometrically, without assumptions about something about the nature of the object. —Alex (ASHill | talk | contribs) 01:30, 14 August 2008 (UTC)

I was wondering whether this article could discuss statistical parallax, or if it should be on a separate page? (See part D of this, for example.) Thank you.—RJH (talk) 23:23, 18 October 2008 (UTC)

When planets move around the sun, the sun itself also moves (but of course slower and in a smaller ellipse), and Jupiter causes the Sun's position to change by about 1.5 million km within 6 years. Earth orbits Sun in a close orbit compared to Jupiter, so it can be expected to follow this movement of the Sun (in addition to orbiting it). This should produce an additional parallax of about 7 milliarcseconds at α Centauri - this should be measurable e. g. with Hipparcos. Does this additional parallax have a special name? Maybe the article should mention it. Icek (talk) 22:32, 20 November 2008 (UTC)

This section only mentioned naval artillery. However, the same holds for field artillery. Here, too, fire control must correct for the exact gun positions. I changed the section accordingly. (I am not an military engineer myself, but I presume that, given the exact coordinates of both the target and each individual gun, it must be quite simple for the fire control software to calculate the aiming parameters for each separate gun, thus automatically correcting for parallax.) HHahn (Talk) 16:34, 10 February 2010 (UTC)

In the meantime, several people have modified this paragraph, mainly by adding statements on rangefinders, shooting sights, etc. Someone then split this up into different paragraphs, which is fine, as the sighting problem for one personal firearm is quite different from (and smaller than!) the multi-gun artillery problem described in this paragraph.

However, unfortunately my previous addtion of field artillery has been dropped somehow. In actual fact, the parallax problem may be even bigger in field artillery than in naval artillery, as field guns may be positioned wider apart that those on a warship, where the distance is usually limited by the ship's size.

On the other hand, I presume that in modern systems the fire control computer knows the exact coordinates of each gun, as well as those of the target. So it can calculate the targeting parameters for each single gun based on these coordinates (together with the wind parameters etc.). This way, the actual parallax problem is complete solved within the fire control computer.

I therefore changed the paragraph title to "Artillery" and added field artillery in the text.

HHahn (Talk) 11:32, 22 August 2011 (UTC)

Surely the issue of parallax only arises if you actually want your artillery fire to converge? Artillery is commonly used as an area weapon, particularly so in the case of field artillery, where convergence is the last thing you want - the spread of fire over an area rather than on a point being of the essence. Regrettably I am unable to cite any particular sources. — Preceding unsigned comment added by 90.243.8.250 (talk) 11:27, 7 February 2019 (UTC)

This section is entitled Parallax compensation for firearm and archery sights, but doesn't mention archery. —Preceding unsigned comment added by 91.84.176.32 (talk) 21:10, 28 September 2010 (UTC)

Is this correct? ${\displaystyle \sin p={\frac {1AU}{d}}}$

Shouldn't it be: ${\displaystyle \tan p={\frac {1AU}{d}}}$

This is probably a matter of defiition. And don't forget that when the angles involved are very small, both sin (φ) and tan (φ) converge to the same value.

HHahn (Talk) 10:09, 17 July 2010 (UTC) (corrected HHahn (Talk) 10:13, 17 July 2010 (UTC))

This statement is false: "Annual parallax is defined as the difference in position of a star as seen from the Earth and Sun". When a star near the ecliptic is occulted by the Sun (a syzygy), there is no difference in position between geocentric and heliocentric observations. A slightly less incorrect statement would be "the difference in position of a star as seen from the Earth and the solar system barycenter during astronomical quadrature". Unlike observations made from the sun, Earth based observations are strongly effected by stellar aberration. Only the "absolute parallax" is significantly effected by aberration. Absolute parallax is the change in angle measured relative to a non-rotating frame of reference. "Relative parallax" is the change in angle relative to adjacent points of reference.

Also, annual parallax is not "the angle subtended at a star by the mean radius of the Earth's orbit around the Sun". That is actually the "unit of parallax angle". Quoted parallax angles are not the actual measured parallax, but what the parallax would have been if it had been measured with a baseline of 1 au. The Sun is offset an average of about 456 kilometers from the Sun/earth/moon barycenter. This means the Earth's heliocentric orbit is not 2 au across, it is 2*(au-456 km) across. As mentioned by Icek above, the Sun/Earth/Moon barycenter is orbiting the solar system barycenter with a period of about 12 years, this adds an additional parallax which must be accounted for when calculating the star's distance.

The following is the first sentence of the As a metaphor section. I think it should be revised, does anybody agree?

"In a philosophic/geometric sense: An apparent change in the direction of an object, caused by a change in observational position that provides a new line of sight." — Preceding unsigned comment added by 207.177.213.116 (talk) 14:26, 5 August 2011 (UTC)

The section "Parallax compensation for firearm and archery sights" starts out just plain wrong. "the distance between the sighting mechanism and the weapon's bore" is "Sight Height". I am not seeing any reference that it is referred to as "parallax". "Parallax is the apparent movement of the target relative to the reticle when moving your head up and down," (or left and right for that matter) per this document (page 16). "parallax compensation" does not correct for Sight Height, it corrects for miss matches in planes of focus at different distances, i.e. it is a correction in the sight to keep it aligned in relationship to itself (its optical axis). Fountains of Bryn Mawr (talk) 13:31, 23 August 2011 (UTC)

Proper motion is not measured in arc seconds -a unit of length-. The correct units are arc seconds per year. JuancitoxTw (talk) 16:53, 29 February 2012 (UTC)

Is there any scope for the use of parallax in animation? For example, the 1988 film Akira has an opening scene in which parallax is exploited to produce the effect of panning by moving different layers of background across each other at different speeds. — Sasuke Sarutobi (talk) 13:36, 7 January 2014 (UTC)

Sounds like a good idea for an addition to the article. Its actually not Parallax, its a simulation of it called "multiplane", made using a Multiplane camera. In modern computer graphics and phone interfaces, etc these simulations are called Parallax scrolling. I removed an overlong (redundant) section on it a while ago since it was out of place but I can see a section on all aspects of "simulation" it in a section title like "Simulated parallax", maybe right above the "As a metaphor" section. Fountains of Bryn Mawr (talk) 15:28, 7 January 2014 (UTC)

A couple of recent edits have centered on removal and then restoration of this topic in the second lead paragraph. Now, I'm not greatly expert in the subject, and currently have no reliable source to offer, but neither has user David J Wilson supplied one, and the article has none either. Perhaps it's time there was one (or more), for the article's sake. At present, I recognize the diurnal parallax measures as having been in use historically, but I am under the impression that they are largely superseded these days by more direct techniques, often taken by spacecraft, and involving laser or radar reflection to produce very precise measures. I know for certain that the Apollo 11 mission left a reflector at the Sea of Tranquility, and that earth-based radar signals used it to measure distances to the moon beginning immediately (1969). Has something of that kind not also been done on any of the multiple Mars landers? And we've had flybys of all the planets. Is earth-based parallax measure really sufficient to provide useful information that is accurate enough to improve on what we know from those sources? I ask, because perhaps someone here already knows. But if answers are not readily available that way, I may go in search of something concrete myself, because (inexpert as I am), this just seems somewhat dubious (or perhaps, merely dated). Evensteven (talk) 20:05, 30 March 2014 (UTC)

Let me illustrate the point about accuracy. It is my understanding that the best parallax resolutions currently available resolve to measures on the order of milliarcseconds, and that those are not available from the surface of the earth (due to atmospheric distortions), but from spacecraft only. Yet planetary disks are themselves only on the order of tens of arcminutes wide at most. How much larger could a useful diurnal parallax be? Observation times within the solar system must be highly critical, since the apparent movement of planets and other bodies occurs at a much quicker pace. So, a measure of diurnal parallax on the order of 10 arcminutes, to within one mas, yields (generously) 7 significant digits of accuracy, 19 or 20 significant bits if one wants to go binary. Yet radar measures of lunar distance give accuracies to within hundredths of cm across distances of hundreds of thousands of km. That's pretty easily 13 significant digits. There is no possible parallax measure under 360 degrees that can be made at that level of accuracy. How then is diurnal parallax useful to present-day solar system measures? Evensteven (talk) 20:54, 30 March 2014 (UTC)

Mea culpa. You are quite right. I have now restored your edit and added a footnote about the historical use of diurnal parallax to measure distances to local celestial objects.

David Wilson (talk · cont) 12:38, 31 March 2014 (UTC)

Say, thanks for verifying this item. As you can see, I didn't really know directly, but just had snippets of different things that seemed to imply it couldn't be otherwise. I also have a nagging memory that I heard this once, but haven't been able to pin that down - not at all good enough for this purpose either. Evensteven (talk) 15:29, 31 March 2014 (UTC)

From the article: "One way to determine the lunar parallax from one location is by using a lunar eclipse. A full shadow of the Earth on the Moon has an apparent radius of curvature equal to the difference between the apparent radii of the Earth and the Sun as seen from the Moon. This radius can be seen to be equal to 0.75 degree, from which (with the solar apparent radius 0.25 degree) we get an Earth apparent radius of 1 degree. This yields for the Earth–Moon distance 60.27 Earth radii or 384,399 kilometres (238,854 mi) This procedure was first used by Aristarchus of Samos[17] and Hipparchus, and later found its way into the work of Ptolemy.[18] "

While this seems to me like a correct exposition of Aristarchus' (and Ptolemy's) method, but I don't understand how the determination of lunar distance necessarily has anything to do with determination of parallax. The paragraph points to a Gutzwiller article I have no access to. However, here is a quote from O. Neugebauer from A History of Mathematical Astronomy, which can be found here ^[1] (pg. 635):

"He [Aristarchus] asserts that the moon is illuminated by the sun, that the (circular) lunar orbit has a smaller radius than the then solar orbit, and that parallax may by ignored, allowing us to identify our eye with the center E of the earth which is also the center of lunar and solar orbit."

Furthermore, I could find no quotes reinforcing the Gutzwillerian view. I'm pretty new here and don't know if I'm interpreting Neugebaeur correctly (I am in no way an astronomer by training), but I feel like mentioning Aristarchus in an article on parallax is highly misleading and should be removed. Ptolemy made the same initial point as Aristarchus, but did attempt to calculate parallax by other mechanisms. I personally think the entire paragraph and illustration should be removed.

Even the wikipedia article on Aristarchus' On The Sizes and Distances correctly shows a right triangle^[2]. While the measurement of distance is relevant to parallax, they are not equivalent. If someone does not explain to me why I am mistaken within the week I will delete the paragraph. At least, I think an additional comment should be added signalling Aristarchus did not measure parallax.

LilChaki (talk) 20:56, 4 May 2016 (UTC) J.S. Chaki Fauber

There are certainly several problems with the text of the paragraph. I'm not sure I completely understand the nature of your objections, but I agree that the characterisation of the relation between the procedure described in the paragraph and that followed by Aristarchus is misleading. While I don't have any objection to the current paragraph's being removed, I also don't think it would take a huge amount of work to rectify the problems with it.

David Wilson (talk · cont) 01:13, 10 May 2016 (UTC)

The term motion parallax should be worth a separate article in the English Wikipedia according to its historical and cultural importance and not only incompletely mentioned in the article parallax. The historical and cultural significance of motion parallax arose from cultural practitioners in the field of photography and cinematography in views from moving carriages, trains, cars, chain carousels, and planes taking off. Children and adults often ask why the near objects on the passing car move faster than those on the horizon. I therefore propose to adopt parts of the German article as I have done below with an addition of my own.

Motion parallax

In perceptual psychology, motion parallax is the effect that occurs optically when various objects are distributed at different distances from one another in a landscape and the observer moves sideways parallel to these objects while looking in the direction of the horizon.

Principle

The light pattern of near objects moves faster across the retina than that of distant objects. This effect occurs, for example, when looking out of the side window of a moving car or train. The result is that those objects that are close to the viewer move faster than those far away from the viewer. The movement of the objects approximates to zero the closer they are to the horizon, which moves only minimally (depending on the overall speed). Selective motion blur only in the near range of the camera is a specific effect of motion parallax. Seen from the observer, when looking out of a moving suburban train, the church is on a circular centre point, with increasing observation distance (radius) from this, the circumferential speed increases while the angular speed remains the same. Objects such as trees and bushes on the track bed, which are closer to the observer on the train, have a higher circumferential or tangential speed to his reference point, the church, and appear blurred by the speed of travel (motion blur). Light spots in the trees and bushes along the track bed provide a view of the church and the neighbouring buildings. Some leaves and branches only briefly obstruct the view due to the change in perspective caused by the speed. In some places on the white outer walls of the houses and the church in the picture, they cause blurred shadow traces as a result of the partial shortening of the exposure time, as in photographic dodging. https://upload.wikimedia.org/wikipedia/commons/2/29/Bewegungsunschärfe_durch_Bewegungsparallaxe_beim_Blick_aus_der_S-Bahn_nach_Osterburken.jpg File:Bewegungsparallaxe_Motion_parallax_from_Heidelberg.webm Georg Buzin (talk) 14:04, 16 June 2023 (UTC)

Images from the post above, in viewable form:

--Srleffler (talk) 06:36, 17 June 2023 (UTC)

I’m assuming this undetectable transitioning image in the background of my phone display that was placed on my devices is a motion parallax or similar of the sort. I would like this removed. This motion parallax or something similar is being placed on devices and compromising them. Rmedia65 (talk) 17:53, 15 August 2023 (UTC)

And Wikipedia is responsible how? Lithopsian (talk) 20:14, 15 August 2023 (UTC)