Talk:Proper motion

message: 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)
replies: text: Yes, why is there proper motion? Galactic rotation? Big Bang? Random bouncing? Kortoso (talk) 22:02, 23 October 2014 (UTC)
thread_title:  Bigger picture 

message: 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. <Astron. J. 94, 501 (1987)>
"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)
thread_title:  Proper Motion of exterior galaxies unobservably small 

message: 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)
thread_title:  Coordinates of proper motion 

message: 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)
thread_title:  Improper motion 

message: 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)
replies: text: Where do you see his birth at 395? Kortoso  (talk) 22:29, 26 October 2016 (UTC)
replies: text: 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)
thread_title:  Macrobius 

message: 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)
replies: text: 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)
thread_title:  [[Gamma Caeli]] 

message: Does anyone else beside me have problem understanding the explanations of two pictures in the article: Relation between proper motion and velocity components and Components of proper motion in celestial sphere) 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)
thread_title:  Confusing definitions and two pictures 

message: 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)
thread_title:  Further improvements 

message: 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)
thread_title:  Causes of proper motion? 

message: 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)
thread_title:  Macrobius (2) 

message: 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)
thread_title:  Why the star trails picture? 

message: 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)
replies: text: Eh? This statement makes no sense. Arianewiki1 (talk) 01:04, 5 June 2018 (UTC)
replies: text: 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)
thread_title:  Earlier remarks 

message: 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)
thread_title:  HIP  67593 

message: 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)
replies: text: Fixed. Headbomb {t · c · p · b} 19:27, 5 November 2018 (UTC)
thread_title:  Meaning of Δt in Intro 

message: 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)
thread_title:  Relative motion and intrinsic motion 

message: 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)
thread_title:  Did Halley really prove proper motion?