Talk:Proper motion

Untitled
Perhaps the information in http://csep10.phys.utk.edu/astr162/lect/motion/proper.html can be blended in? Tenbaset 11:17 Apr 4, 2003 (UTC)

Bigger picture
It would be interesting to note if there are any patterns in the proper motions of nearby stars and galaxies (I see to remember there are, that it is not just a random bouncing around). -- Beland 04:34, 29 October 2006 (UTC)


 * Yes, why is there proper motion? Galactic rotation? Big Bang? Random bouncing? Kortoso (talk) 22:02, 23 October 2014 (UTC)

Proper Motion of exterior galaxies unobservably small
Assumptions gof observable proper motions of galaxies were based on old star catalogs which assumed that faint stars with extremely small or unobservable proper motions were the reference for zero proper motion. Newer observations assume exterior galaxies do not show any proper motion. They are simply too distant. Their lateral velocities cannot be more than a few hundred kilometers each second, which is only 10^5 * 31,557,600 kilometers each year. In a million years that is only around one light-year and they are all millions of light-years distant or more. With sin(y) = y approaching zero rapidly, the angle they might move is so small as to be unobservable with any current techniques. This is a relatively new assumption, which NASA uses.

From Lick Northern Proper Motion Program: NPM1 Reference Galaxies by Klemola A.R., Hanson R.B., Jones B.F. 

"The NPM1 proper motions (of stars) were measured with respect to an absolute reference frame defined by some 50,000 faint galaxies (mostly 16 < B < 18 mag). "  (http://adc.gsfc.nasa.gov/adc-cgi/cat.pl?/catalogs/1/1200/) SyntheticET (talk) 21:03, 7 January 2009 (UTC)

Coordinates of proper motion
It seems to me that the α-component of the proper motion given in the Wiki tables of all celestial objects ("Proper motion (μ): RA=xxx mas/yr") is $$\cos(\delta)\frac{d\alpha}{dt}$$ and not $$\frac{d\alpha}{dt}$$. This gives rise to some confusion: if this value is interpreted as $$\mu_\alpha$$, the valid formula for $$\|\mu\|$$ does not involve the cos(δ) term (namely, it is simply $$\|\mu\|^2=\|\mu_\delta\|^2+\|\mu_\alpha\|^2$$ instead of $$\|\mu\|^2=\|\mu_\delta\|^2+\cos(\delta)^2\|\mu_\alpha\|^2$$ as asserted in this article). The references given for this formula are certainly correct but based the other definition of $$\mu_\alpha$$. Some computations like footnote 58 in Vega's page seem to be based on this mistaken interpretation of the proper motion tables and are therefore unvalid. Accordingly, some clarification seems to be needed. Alpha carinae (talk) 18:19, 24 January 2010 (UTC)

Improper motion
From the article:
 * The term "proper motion" derives from the historical use of "proper" to mean "belonging to" (cf, propre in French), so there is no such thing as "improper motion" in astronomy.

But surely proper motion was meant to be distinguished specifically from the obvious motion shared by all the stars (caused by the movement of the celestial sphere or the rotating of the earth), which is indeed improper in the sense of not belonging (property rights are all about exclusiveness) to any star. Cesiumfrog (talk) 05:24, 30 January 2011 (UTC)

Macrobius
Macrobius is said to have spoken in 400 A.D. The article on him says he was born in 395 A.D. It seems unlikely that he was interested in astronomy at five years of age. — Preceding unsigned comment added by 86.179.62.137 (talk) 10:11, 16 September 2011 (UTC)
 * Where do you see his birth at 395? Kortoso (talk) 22:29, 26 October 2016 (UTC)
 * I have seen 370 A.D. and El Kef, in Tunisia. — Preceding unsigned comment added by 86.178.127.25 (talk) 12:30, 4 June 2018 (UTC)

Gamma Caeli
The article says that Gamma Caeli will be the next star to move to another constellation. But Gamma Caeli isn't a star; the article says it's the designation for two stellar systems several hundred light years apart, Gamma¹ Caeli and Gamma² Caeli. Which one is it?--Prosfilaes (talk) 06:50, 18 April 2012 (UTC)


 * Since we're talking a move to another constellation (not another galaxy) I'd be inclined to expect it means the one that is substantially closer to Earth. Apparently this is Gamma-one, and is also substantially brighter (more visible) than gamma-two, and hence, more likely to be the one meant when just giving the simpler name. It also turns out that gamma-2 is supposed to be more commonly referred to under a different designation, X Caeli, since it was catalogued as variable. The sourcing for the date of constellation-shift might be weak but notwithstanding that, it seems it is indeed already at the edge of that next constellation, and I'd concede benefit of doubt over what was being referred to. (I guess you've noticed gamma-one isn't just a star either - apparently a giant with a companion star, that's even less-bright than gamma-2, orbiting it - not to mention any further planetary objects, etc.) Cesiumfrog (talk) 10:28, 10 July 2012 (UTC)

Confusing definitions and two pictures
Does anyone else beside me have problem understanding the explanations of two pictures in the article: and ) in relation to each other?

When the term is defined it clearly says that it is 'rate of angular change in position over time and that it's measured by arcsecond per year (i.e. it's the unit for proper motion), but then later on it says this:

"The proper motion is a two-dimensional vector (because it excludes the component in the direction of the line of sight) and is thus defined by two quantities: its position angle and its magnitude."

I mean, how could a change of arcsec./year be (also) represented as a vector? This is not possible, the proper motion should be defined (as it says properly) by two other components: an angle (which in turn is "measured" and drawn by vectors) and magnitude, but not a vector! So to me the second picture in particular is confusing since in it there is nowhere to be found where is that change (of arcsec./year); instead though the proper motion is now shown as a vector.

--Wayfarer (talk) 01:12, 28 July 2014 (UTC)

Further improvements
To start, I think the introduction section should be reworked to start from the simple concept this really is. Formulas aren't really necessary for this, though they could stay as an added mathematical formulation, just as long as it is centered around explaining the concept, not the math. --JorisvS (talk) 08:52, 8 April 2015 (UTC)

Causes of proper motion?
Nowhere do I see the cause of proper motion here. Surely somebody has a theory at least. Kortoso (talk) 22:30, 26 October 2016 (UTC)

Macrobius (2)
Macrobius's remarks about possible proper motion of the stars seem to have been in the "Commentary". A quotation might be a good idea. — Preceding unsigned comment added by 2A02:C7D:B300:C700:E0CF:70EA:52F3:1DB1 (talk) 13:45, 16 April 2017 (UTC)

Why the star trails picture?
I'm pretty sure the picture of star trails is not an example of proper motion. This picture is due to the spinning of the earth, not the relative position of stars from the Solar Systems centre of mass.
 * You are correct. I had noted the same thing and came here to change it. The photo can be used as a good example of what proper motion is not. Also, the sentence, "Each of these trails is formed by the motion of an individual star, captured over a long exposure time" was taken directly from the article cited, which is itself in error (it should have said "apparent motion"). I replaced the sentence, and kept the reference to a different fact from the same article.Al Leluia81 (talk) 18:05, 17 August 2017 (UTC)

Recent article Introduction modifications
Thanks for these recent useful article edits that have simplified the text. However, statement : "No one measures proper motion in RA in terms of seconds of time (though RA itself is expressed in hours, minutes, and seconds). "Right ascension" and "declination" are not capitalized in Wikipedia's articles."

No true. RA is measured in time in transit telescope in seconds then converted to arcsec in calculations. Seconds are also convenient determining time by transit, where proper motions of the transit star are used as a correction in seconds RA. Many of the older star catalogues give this in seconds not arcsec. I have temporarily replaced the word 'measure' with 'given'.

"Right ascension" and "Declination" are usually capitalised, and I can find no evidence / reason that they are "...are not capitalized in Wikipedia's articles."

The 2nd paragraph should lead with "Two components for proper motion in the..." not "Components for proper motion in the..." This is more specific and avoids confusion with the traverse motion and true motion. e.g. Proper motion is a single vector made by the magnitude of two components. True motion is the single vector made by the magnitude of three components.

Do we require an WP:RfC here? Arianewiki1 (talk) 21:59, 5 February 2018 (UTC)
 * I'm not sure if you're arguing for or against the capitalization of right ascension and declination, but it is grammatically wrong to capitalize them. Those are common terms, not proper nouns, and should follow the capitalization rules of common terms. Headbomb {t · c · p · b} 01:42, 6 February 2018 (UTC)

, you originally added the bit about "seconds of time" on April 7, 2015, as a historical remark. It may be true that proper motions in RA were once given in seconds of the kind used routinely for RA (that is, 1/3600 of an hour), but I don't think anybody nowadays bothers with that when measuring or giving μα. Do you think Gaia at some point finds the value in 3600ths of an hour and then converts it? I don't. Anyway, as I say, you put it in as a historical remark, and someone along the line changed it into a normal statement as though it's the case now. That wasn't your intention or the intention of Birney whom you cite. I think it's just confusing, unless one talks about transit telescopes and all that.

As for the capitalization, I simply checked by typing Right ascension and looking at the article that I got.

On your next point, you had (simplified by taking out parentheses): Two components for proper motion in the equatorial coordinate are measured for Right Ascension in seconds of time and in Declination seconds of arc or ″.

Now we have: Components for proper motion in the equatorial coordinate system are given for right ascension and declination in seconds of arc.

I say it doesn't make sense to say "Two components are measured for RA in seconds of time and in Declination seconds of arc". The sentence as it stands now is better English. You're welcome to make a "Request for comment" if you want.

, I put in that bit about the Starboxes. Because after reading the beginning of the article I was wondering whether I had misunderstood the Starbox numbers. Turns out I had not misunderstood. But I thought it would be good to make it clear. I had to go to Template:Starbox astrometry in order to find the answer.

Eric Kvaalen (talk) 20:39, 7 February 2018 (UTC)


 * Thanks for the reply. This all hinges on the wording. Here saying 'measuring' proper motion is in seconds in RA. e.g. when using a transit telescope, whose use when determining time is corrected in seconds RA. However, the 'expression' maybe in mas/arcsec or arcsec/100years. The article said 'measured' not 'expressed.'
 * Secondly the Introduction summaries the rest of the bulk of the article which mentions the conversion of cos δ. I'd assume it is important to be implicit, so the corrected and uncorrected values are not confused. "Two components are measured for RA in seconds of time and in Declination seconds of arc." and "Two components are expressed in RA and Declination in seconds of arc" are both technically correct. (That is why I removed 'measured'.)
 * Note: Gaia gives position in RA and Dec in h m s, ° ′ ″. Also correcting RA for different epochs for precession, aberration, etc. all needs to be expressed in seconds not mas. Formal surveying calculations still adopt the practice.
 * Definition : Right Ascension "the distance of a point east of the First Point of Aries, measured along the celestial equator and expressed in hours, minutes, and seconds." (RA is not an angle!!!) Declination: "the angular distance of a point north or south of the celestial equator." Arianewiki1 (talk) 23:03, 7 February 2018 (UTC)
 * RA is an angle, not a distance. Headbomb {t · c · p · b} 00:16, 8 February 2018 (UTC)
 * How can RA be an angle? It is written in hours minutes seconds and represents in time. Sure it can be converted to degrees (angular distance, but that isn't RA. This issue applies to celestial longitude and hour angle. Many sources say this Oxford English dictionary.  Is longitude also an angle? Furthermore, we need two points to make an angle, but RA is only a point.  (Thanks for pointing this out, because Right ascension article is actually wrong.) Arianewiki1 (talk) 06:27, 8 February 2018 (UTC)
 * In this context, hours, minutes, and seconds are angles. See Right ascension. 1 hour = 1/24th of a circle, 1 minute = 1/3600th of a circle, 1s = 1/86400th of a circle. And yes, longitudes and latitudes are angles. Headbomb {t · c · p · b} 10:59, 8 February 2018 (UTC)

, I do not understand your English. Do you have anybody who could check what you write? Eric Kvaalen (talk) 07:57, 8 February 2018 (UTC)
 * , I do not understand your French. Just saying.
 * An object, say at 12h 22m 20.5s, cannot be an angle, but can be converted to an angle (only for calculations) as 180.372361° eastward from the First Point of Aries. As already said in Equatorial coordinate system, "Analogous to terrestrial longitude, right ascension is usually measured in sidereal hours, minutes and seconds instead of degrees, a result of the method of measuring right ascensions by timing the passage of objects across the meridian as the Earth rotates." So reading your response above - is the Oxford English dictionary actually wrong, and an unreliable source?
 * Note 1: I've looked at a dozen references like Meeus' 'Astronomical Algorithms' pg.87, 'Penguin Dictionary of Astronomy', pg. 325-26, 'Explanation of Astronomical Ephemeris/Almanac' (2009) pg.418 etc. ) I own all of these. All do say right ascension is a "unit of time". None say it is an angle. Do we really have to test this?
 * Note 2: The capitalisation issue is that I cannot find where Wikipedia says it is lower case. Arianewiki1 (talk) 13:11, 8 February 2018 (UTC)


 * Now noticed something odd. In Right ascension the first line [1] gives links to U.S. Naval Observatory Nautical Almanac Office (1992). Seidelmann, P. Kenneth, ed. Explanatory Supplement to the Astronomical Almanac. University Science Books, Mill Valley, CA. p. 735. ISBN 0-935702-68-7. Page 725 should be pg. 735. It says in the Glossary "right ascension angular distance on the celestial sphere measured eastward along the celestial equator from the equinox to the hour circle passing through the celestial object. Right ascension is usually given in combination with declination." yet on pg.418 it says twice: "right ascension is given in units of time." Yet pg. 11 says: "Right ascension, like hour angle, is usually expressed in time measure from 0h to 24h, and both right ascension and longitude are measured in the positive (or right-handed) sense. The complement of right ascension with respect to 24h is known as sidereal hour angle (SHA); in navigational publications it is usually expressed in degrees." Page 735 is contradicted by many other sources I've read. Here the reference differs even within this same source!! Arianewiki1 (talk) 14:14, 8 February 2018 (UTC)
 * When you say "Look at 6 O'Clock" that means "Look behind you", not "Wait until 6:00 am/pm to look at this". It's exactly the same thing here except with a 24 hour "clock", rather than a 12 hour "clock". Right ascension is an angle, not a time. There is a correspondence with the two because earth rotates approximately at a rate of 24 hours per rotation, or 86400 s/rev (the exact rate is 86164.098903691 s/rev, a ~0.27% difference). Headbomb {t · c · p · b} 14:43, 8 February 2018 (UTC)
 * Your are seemingly failing to understand what right ascension represents (I.e. It relates only to the equatorial grid/coordinates equivalent to the Sidereal time on the meridian.) But is also now not a matter of opinion but citations. Most sources don't say this. e.g. Right ascension, like hour angle, is usually expressed in time measure... is NOT "Right ascension is an angle, not a time. Again. So reading your response above - is the Oxford English dictionary actually wrong, and an unreliable source? Worst Right ascension and Equatorial coordinate system are now contradictory. Is a RfC the only option here against such overwhelming evidence? Arianewiki1 (talk) 23:24, 8 February 2018 (UTC)
 * Both articles are in full agreement. From "Right ascension" (abbreviated RA; symbol α) is the angular distance measured eastward along the celestial equator from the vernal equinox to the hour circle of the point in question.", emphasis mine. See also File:Components of proper motion.svg, where RA / &alpha; is again clearly an angle, used in both articles. h/min/s corresponds to positions on the hour circle, aka angles. Headbomb {t · c · p · b} 23:55, 8 February 2018 (UTC)
 * "Both articles are in full agreement." Nope. Again. Equatorial coordinate system, says: "Analogous to terrestrial longitude, right ascension is usually measured in sidereal hours, minutes and seconds instead of degrees, a result of the method of measuring right ascensions by timing the passage of objects across the meridian as the Earth rotates." B/W " ....measured in sidereal hours, minutes and seconds instead of degrees, " We are talking about RA not Proper motion, which is converted. Cited references agree. You don't have consensus. RfC to follow. (Good luck.) Arianewiki1 (talk) 07:17, 9 February 2018 (UTC)
 * What wording, specifically, do you disagree with in the article here? Headbomb {t · c · p · b} 03:14, 10 February 2018 (UTC)
 * Deliberately asking 'load questions' or 'unanswerable questions' can be deemed bad faith, and can inhibit getting article consensus. Issue is "Right ascension is time NOT "an angle"'. This affects some of this article statements and structure, in this case, also across many other articles. Considering now Admin assistance to correct this impasse. Thanks. Arianewiki1 (talk) 00:45, 13 February 2018 (UTC)
 * If you cannot answer a question about what specific wording you disagree with, or consider asking that question to be "loaded" then I'll point you to WP:NOTFORUM. Headbomb {t · c · p · b} 01:14, 13 February 2018 (UTC)
 * Seriously. You make these non-consensus changes. AFTER the starting this discussion. Now your saying : "If you cannot answer a question about what specific wording you disagree with,..." What version then?
 * The edit version (minus references for brevity) should be as follows: "Two components for proper motion in the equatorial coordinate (of a given epoch, often as J2000.0), are measured for Right Ascension (RA or α) in seconds of time and in Declination (dec. or δ) seconds of arc or ″ (μδ). Right Ascension is then converted into seconds of arc (μα), whose combined value is computed as the total proper motion (μ), > which is usually expressed in arc seconds per year (arcsec/yr) or per century, where 3600 arc seconds equal one degree."
 * All edits made after this do not actual have consensus. Arianewiki1 (talk) 04:22, 13 February 2018 (UTC)

That you disagree with them for vague unknown reasons doesn't mean they don't have consensus. The current version is The components for proper motion in the equatorial coordinate system (of a given epoch, often J2000.0) are given in the direction of right ascension (μα) and of declination (μδ). Their combined value is computed as the total proper motion (μ). It has dimensions of angle per time, typically arcseconds per year or milliarcseconds per year. Knowledge of the proper motion, distance, and radial velocity allows calculations of true stellar motion or velocity in space in respect to the Sun, and by coordinate transformation, the motion in respect to the Milky Way. There is nothing wrong with this passage, even if you somehow insist on calling RA a time, rather than an angle. Headbomb {t · c · p · b} 11:28, 13 February 2018 (UTC)

I cannot comment on the technical aspects of this debate. However, what I can say is that 'right ascension' and 'declination' should definitely not be capitalised. The article Right ascension does this correctly, only capitalising at the start of a sentence. The reason is that 'right ascension' and 'declination' are not 'proper nouns'.

Manual of Style/Capital letters describes when words should be capitalised. The first paragraph explains that words should not be capitalised unless they fall into one of the exceptions listed there. 'right ascension' and 'declination' should be capitalised in this article if and only if someone can explain why they fall into one of these exceptions. I hope that clears things up a little. —♫CheChe♫ talk 11:21, 13 February 2018 (UTC)

Earlier remarks
The article Chinese star maps seems to say that proper motion was mentioned in 683-727 A. D. This is before Halley's time. — Preceding unsigned comment added by 86.178.127.25 (talk) 12:25, 4 June 2018 (UTC)
 * Eh? This statement makes no sense. Arianewiki1 (talk) 01:04, 5 June 2018 (UTC)
 * Perhaps Arianewiki1 would like to speak more clearly. — Preceding unsigned comment added by 2A00:23C0:FCF6:4801:C43C:4F7C:53E0:BB3 (talk) 09:56, 5 June 2018 (UTC)

HIP 67593
According to SIMBAD the proper motion of HIP 67593 is only 4.636 and -14.598 mas/yr in RA and Dec (from GAIA DR2). So this star should be removed from the table? Hobbema (talk) 01:21, 25 August 2018 (UTC)

Meaning of Δt in Intro
I'm too new at this to make a direct edit, so I'll ask: isn't there a problem with the sentence in the Introduction section- "Suppose in a year an object moves from coordinates (α1, δ1) to coordinates (α2, δ2) in a time Δt. " Does this mean that Δt is always 1 year? — Preceding unsigned comment added by 2600:1700:9580:72F0:8045:9F66:D67:2245 (talk) 19:25, 5 November 2018 (UTC)


 * Fixed. Headbomb {t · c · p · b} 19:27, 5 November 2018 (UTC)

Relative motion and intrinsic motion
Currently the lede says: "Proper motion is not entirely intrinsic to the celestial body or star, because it includes a component due to the motion of the Solar System itself." I'm not sure of the value of this statement, given that velocities are always relative. Perhaps revise?Ordinary Person (talk) 01:04, 26 August 2020 (UTC)

First sentence
The first sentence mentions the "abstract background" of the more distant stars. The word "abstract" has no meaning here.

Did Halley really prove proper motion?
There’s a paper asserting the contrary, and crediting Cassini. https://journals.sagepub.com/doi/pdf/10.1177/0021828619877967 Lostlakehiker (talk) 21:16, 1 July 2023 (UTC)