Talk:Parallax

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This is a good description of parallax. But it's a difficult thing to explain and I think a picture would help.

The article should be expanded in relatioship with cameras.

I have a question at talk:Single lens reflex camera. --KQ

Mention added! - MV

Contents 1 secular parallax 2 parallax 3 Example Diagram 4 References? 5 Stellar parallax 6 I'm old! 7 Confusing wording 8 confusing symbol choice 9 Stellar parallax 10 Linking 11 Oscilloscopes 12 Definition of parallax mistaken 13 The Scale of the Universe -- Why? 14 Formula

[edit] secular parallax

Is this another type of parallax of just another name for a certain type already listed in the article?Tmchk | Talk 01:05, 20 May 2007 (UTC)

[edit] parallax

you described it rather akwardly I think. The simple definition I have learned describes it I feel more simply. Parallax is the apparent movement of one object relative to another due to the movement of the observer.(o.k don't have permisson to copywrite this but hey its a leaving cert definition and is used by thousands of students every year) Show diagrams of the simple pen experiment where you hold two pens out in front of you, one above the other. The bottom one slightly further away from your face than the top. move your head and you will see parallax. But you know this already right?

OK, I added your simple definition (reworded). A drawing would be good, but I am no good at drawing - MV

What about diurnal parallax of solar system bodies?

[edit] Example Diagram

I agree that the page needed a clear, concise diagram with similar explaination - I threw something together in (don't laugh) Microsoft Word... Should be good enough until a real artist decides to make something reasonable.

The previous "informal introduction" was far too wordy and very unclear, even to someone who knows precisely what parallax is. Hopefully this should rectify the situation.

Someone improve my formatting (and image) if you like :)

[edit] References?

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added reference for Zizek Drabauer 04:39, 12 July 2007 (UTC)drabauer

[edit] Stellar parallax

The problem with the argument against heliocentrism was not that a huge size of the universe didn't occur to anyone; it's that the size was incredible. Aristarchus himself explained that the size of the sphere of fixed stars was effectively infinite compared even to the Earth's orbit. Archimedes rejected that, mainly on a quibble with the language Aristarchus used. Tycho understood the argument but thought he had good reasons to believe that the universe couldn't be as big as the lack of parallax implied. --Dandrake 00:25, Jun 11, 2005 (UTC)

[edit] I'm old!

1 quibble: recent research puts age of the universe @13.8GY, doesn't it? Trekphiler 07:23, 29 December 2005 (UTC)

[edit] Confusing wording

What does "The parallax in arc seconds" mean? I think the equals sign is substituting for the word "equals"? This wording needs to be clarified. --AySz88^-^ 21:55, 9 February 2006 (UTC)

[edit] confusing symbol choice

In an article so heavily dependent on trig and geometery, is it really very clever to denote the parallax angle by π (as opposed to its more usual use as the ratio of circumference to diameter)? This forces the reader to decide whether the error in distance is proportional to the error in angle divided by the angle squared, or by a constant approximately equal to 9. And yes, of course one can see the derivation quickly... but nothing is gained by this momentary confusion, and someone not clear on the concept (i.e., the sort of person likely to be looking it up) is less likely to be confident in the derivation.

Unfortunately astronomers have been using π to denote parallax for well over a century. While it would possibly reduce some amount of momentary confusion, I think consistancy with other sources trumps in this case. --anon 7sept2006

[edit] Stellar parallax

To what range can parallax methods reliably determine the distance to a star? Is it any more than 6000 light years? If it is, than this is an excellent method for proving the universe is older than what biblethumpers declare. Wouter Lievens 12:09, 14 June 2006 (UTC)

It's about 1000 parsecs, and that comes to 3300 light years.Magmir 02:40, 23 February 2007 (UTC)

It wouldn't help anyways. I asked one about this, and he claimed the light from the stars was already set in motion. In other words, the light from a 10,000 lya (light years away) star would have been created at 6,000 lya to appear to be 4000 ly further away. -- trlkly 09:33, 28 November 2007 (UTC)

[edit] Linking

Science Buddies at www.sciencebuddies.org has many resources and ideas for science fair projects that involves ideas on this website including parallax.

[edit] Oscilloscopes

This is where I first met the word parallax: In an old CRT oscilloscop, the electron beam (cathode ray) draws a pattern on the phosphor inside the glass, which the operator reads along a grid pattern drawn on the outside of the glass. If the operator moves his head, a parallax reading error might occur. The same kind of reading error can occur in moving needle instruments (analog voltmeters, ampèremeters), which is why precision instruments had a mirror in the scale. When the needle covers its own mirror image, the operator's head is straight above the scale, without any parallax. --LA2 23:33, 26 January 2007 (UTC)

[edit] Definition of parallax mistaken

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)

[edit] The Scale of the Universe -- Why?

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)

[edit] Formula

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)