Talk:Right ascension

message: I did an overhaul of the second paragraph, to make it flow better and to make it more accurate.  RA is not a "time" as the previous version had all-but-implied; it's an angle that is customarily (but not always!) measured in units of time for the sake of convenience.
Also, I'd never heard the term Sidereal Hour Angle, which is apparently legitimate terminology in celestial navigation.  If one were to confuse this with the definition of Hour Angle used in ordinary astronomy, serious confusion would result.  Therefore, I added a sentence to clarify this. Jthorstensen 22:53, 29 April 2007 (UTC)
thread_title:  Rewrites in the second para . 

message: I'd like to see some reference points to help understand how this really applies.  What RA would galactic center have?
: See Galactic Center.—RJH (talk) 20:39, 10 November 2009 (UTC)
thread_title:  Give the concept meaning 

message: It seems so stone age and so difficult to measure. Do we use Stone Henge to calibrate this? The first point of Aries must point fairly precisely toward a particular star, or this system would be even more moronic than it sounds. What star is it?  —Preceding unsigned comment added by 67.161.72.57 (talk) 09:22, 23 March 2009 (UTC)
replies: text: It isn't based on a particular star because the coordinates will change over time due to precession of the Earth. Thus the RA coordinates are given for a particular epoch. Cf. Galactic coordinate system.—RJH (talk) 20:42, 10 November 2009 (UTC)
replies: text:  RJHall is correct, and here is (I hope) another helpful note: the vernal point is where the center of the Sun appears to cross the celestial equator at the exact moment of the March equinox.  This point changes slightly every year, for exactly the reasons RJHall mentioned: precession and nutation.  So you could say that on March 21 of any year, the Sun has a right ascension of approximately 0 hours. CosineKitty (talk) 22:27, 3 February 2010 (UTC)

text: Shouldn't the article say that the equator crosses the ecliptic in Pisces rather than Aries?  (thumbnails added to right.)  JDoolin (talk) 12:39, 7 January 2013 (UTC)
thread_title:  Is really how you measure the zero point of Right Ascension? 

message: What a wonderful clear article this is.  Thank you for eschewing the mumbly dense language that so often is used to write these things.  Much appreciated.  NaySay (talk) 12:49, 8 May 2010 (UTC)
replies: text: Absolutely, no other language uses such precise jargon (rarely translated as deemed to be "dumbing down", or worse "an approximation") as English astronomy.- Adam37 Talk 21:16, 23 June 2018 (UTC)
thread_title:  Great article 

message: In an old astronomy book, right ascension is listed using a typographic ligature of A and R. I wonder how common that practice was?—RJH (talk) 19:11, 19 May 2010 (UTC)
thread_title:  Early notation 

message: I am wondering whether the example in the current article is correct. There one reads:
"For example, if a star with RA = 01:30:00 is at a location's meridian, then a star with RA = 20:00:00 will be in the meridian 18.5 sidereal hours later."
I see it in the following way:
After 24h the second star should return to 20:00.
1:30h is 18,5h earlier than 20:00.
It should therefore be reached after 24h -18,5h =5,5h.
Am I missing an important thing?
Theoritician (talk) 16:44, 13 September 2010 (UTC)
thread_title:  Error in example? 

message: This is not the correct term, as can be verified from the references to to nautical almanacs. Star hour angle (SHA) is the correct term, and is the angle measured to the West from the first point af Aries to the star. RA is not expressed in sidereal time, but in UTC and can be converted to SHA by SHA.= 360 -RA when RA has been converted to an angle at 15 degrees per hour. Reference should also be made to the Nautical Almanacs giving the position of the first point of Aries for every second of each year with an accuracy of one tenthe of a minute of arc. Ref. Nautical Almanacs as listed.  Dg
101.171.213.76 (talk) 05:04, 24 January 2013 (UTC)
thread_title:  Sidereal hour angle 

message: I notice my question wasn't addressed, and that's probably because I put my question out of order, (sorry about that) but above I posted a question about Aries vs. Pisces.  Why is the zero-point of right ascension called the first point of Aries, when the zero Right Ascension point on the star maps appears in Pisces? JDoolin (talk) 02:20, 1 February 2013 (UTC)
replies: text: is this to do with it being an anachronism? It was called the first point of Aries back in the era of Astrology, some 2000+ years ago, due to precession and nutation the point of zero Right Ascension has apparently moved however anachronistically its still called 'first point of Aries'.shaidar cuebiyar (talk) 00:00, 25 February 2013 (UTC)
thread_title: Aries vs Pisces?

message: We have figures giving the Right Ascension of all of the planets, but not that of the Earth (even though the Earth's should actually be something like 0° or 90°, right?). However, isn't this measurement confusing, since it only measures the direction that a planet's north pole is facing exclusively in relation to the Earth's orbit? Isn't there an accurate measurement which can show what direction a planet's north pole is facing in relation to its own orbital plane? BigSteve (talk) 20:33, 14 March 2013 (UTC)
replies: text: That is called obliquity.- Adam37 Talk 21:23, 23 June 2018 (UTC)
thread_title:  Comparability 

message: The table giving hours, minutes, and seconds in radians lists them as π/12, π/720, and π/43200.  While years of training have taught us to always reduce fractions, in this case it seems to me that listing them in their unreduced form as 2π/24, 2π/1440, and 2π/86400 might make the entries less mysterious, thereby reducing cognitive load.  Comments? Vaughan Pratt (talk) 18:28, 20 June 2015 (UTC)
replies: text: Maybe. To me 2π/86400 isn't really any more meaningful than π/43200. Radians are somewhat intuitive for distance around the circumference, but not so much for angles. Tfr000 (talk) 20:46, 21 June 2015 (UTC)
replies: text: Yes, I prefer degrees myself.  I also agree that my suggested version would not be meaningful for those who did not know that 2π radians is a full turn of the Earth in one day, or that a day has 24 hours, 1440 minutes, and 86400 seconds.  I had in mind those who did know those things and would therefore immediately see the rationale for those fractions instead of having to do arithmetic to verify them.  Vaughan Pratt (talk) 02:32, 6 July 2015 (UTC)
thread_title:  h:m:s in radians? 

message: The article states that 1m of right ascension is equivalent to 15 minutes of arc. But isn't that only true at the celestial equator? Close to the poles, isn't 1m of right ascension much less than 15 minutes of arc?  In which case, the wording might be "1m of right ascension, or 15 minutes of arc at the celestial equator". Assambrew (talk) 19:50, 12 June 2020 (UTC)
replies: text: Right ascension is by definition an angle measured in the equatorial plane, or as the article puts it, "the angular distance of a particular point measured eastward along the celestial equator" (my emphasis). So this is not a meaningful question - you can't measure something along the celestial equator and close to the poles. HairyDan (talk) 19:58, 18 July 2021 (UTC)
thread_title:  Minutes of RA to minutes of arc 

message: "The Earth's axis rotates around a small circle (relative to its equator)..." - what on earth is "relative to its equator" supposed to mean in this sentence? The earth's axis moves relative to distant stars, or to an inertial frame of reference - not to the equator (which moves with it)
HairyDan (talk) 19:54, 18 July 2021 (UTC)
thread_title:  Effects of precession 

message: From DELTA-DOR QUASAR CATALOG UPDATE PROCEDURE (a publication from the CCSDS, which defines spacecraft communication standards, relating to precision navigation and orientation)
> 5.3.1.1 The Angular positions shall be specified by a pair of angular coordinates: RA and DEC. It should be noted that while right ascension used to be defined as the angular distance along the celestial equator from the intersection of the equator and the ecliptic, but this is no longer true once one becomes concerned with accuracy levels < 100 milliarcseconds (500 nrad).
> 5.3.1.2 Since 1 January 1998, right ascension, and most importantly the origin of RA, have been defined by conventional agreement as to the value of the RA of extragalactic radio sources [continues…]
This suggests to me that the article should have a section describing this more precise definition of the RA origin, based on the source cited in this reference. Does that sound right? --Raisins31415 (talk) 04:53, 29 December 2021 (UTC)
thread_title:  Precise definition of origin 

message: This is somewhat of a followup to Roland’s comment on https://en.wikipedia.org/wiki/Talk%3ARight_ascension#Is_this_way_of_calculating_it_correct?
I’m using JPL Horizons to calculate them, with UTC second precision, and I’m getting good results for the æquinoctēs, namely two seconds each in which the declination is exactly 0 (which follows the definition): 2022-03-20 15:33:23 and :24 (RA 359.99996 and 359.99997 respectively); 2022-09-23 01:04:25 and :26 (RA 180.00052 and 180.00053), but for the solstices, the declination is at its maximum/minimum for long (between 21:05 and 22:40, minute precision, on 2022-12-21). I can figure out the solstice timing from the RA (precisely 90 at 2022-06-21 09:13:52, precisely 270 on 2022-12-21 21:48:14), but I’m puzzled about why the RA is not 0 and 180 on the æquinoctēs… it makes me cringe using two different data to determine the same thing half of the time. mirabilos (talk) 07:14, 30 January 2022 (UTC)
thread_title:  Æquinōx and solstice… right ascension or declination? 

message: I know how to calculate the RA for right now or any given time using available formulas. But how do you determine the RA for right now by observation? What do you observe to determine the RA for right now? For instance, I can predict by calculation that at a future time the RA will be at a certain value, but how can I verify by observation that the prediction was right when the predicted time arrives?  --Roland (talk) 18:35, 22 February 2024 (UTC)
thread_title:  How to determine the right ascension (RA) right now by observation?