Talk:Right ascension

Is this way of calculating it correct?

If you are on the Tropic of Cancer and looking directly straight up, at the zenith, on the March equinox, then the object is at 0 hours R.A.

If you wait an hour and look up it is at 1 hour R.A.

Is this correct?

Similarly, if you wait a month, 1 year/12 days, and do the same thing then it is at 2 hours.

Looking up on the September equinox will make a star a little past 12 hours. —Preceding unsigned comment added by Slothman32 (talk • contribs) 11:16, 19 September 2010 (UTC)

No, it's not right. At the Vernal Equinox, the right ascension (RA) is 0^o; at the Summer Solstice, the RA is 90^o; at the Autumnal Equinox, the RA is 180^o; at the Winter Solstice, the RA is 270^o; and at yet another Vernal Equinox, the RA is 360^o, or back to 0^o. In other words, it completes a 0^o - 360^o cycle in one year, NOT in one day. That's to say, it takes the RA roughly 3 months to advance 6 hours, or 1 year to advance 24 hours. And keep in mind that the angular speed of RA is not uniform in time.

One way to avoid confusion in visualizing either the Equatorial Coordinate System or the Ecliptic Coordinate System is to assume that the Earth does NOT spin, or rotate, around its own axis but only translates along its orbit around the Sun.

--Roland 02:40, 19 February 2012 (UTC)

To understand the origin of the expression right ascension

The altitude-azimuth (alt-az) coordinate system is appropriate for localization of terrestrial objects but the earth's rotation causes celestial objects to move with time both in altitude and azimuth at a variable rate making it really difficult to "track" objects. Alt-az coordinates of an object vary with time and are different for different places on earth.

It is very convenient to consider a system in which only a single coordinate depends on time. One way is to tilt the alt-az mounting of the observing instrument by an angle equal in order that it is aligned parallel to the earth's axis. This polar aligned axis is called the declination or dec axis which now points toward the celestial pole instead of the zenith. Now once a sky object is located it can be tracked with a single motion (the old azimuth motion).

Declination runs from d=+90° at the north celestial pole through zero at the celestial equator to -90° at the south celestial pole. A given declination is represented by a circle of declination on the celestial sphere except for zero declination, the greatest circle called the celestial equator. At latitude f declination d=f is always on the zenith.

The meridian circles of earth localization system are replaced by great circles of hour angles (HA) measured east (or west) of the local meridian. Each hour corresponds to 15° of arc along the celestial equator. The projection of the local meridian on the celestial sphere is zero hours. Rising objects are east and setting objects west. Due east on the eastern horizon (which contains the celestial equator) is six hours east, the western horizon six hours west. A celestial object with HA 2 hours east (HA=2h00mE) will cross the meridian in two hours. The ancient astronomers measured this quantity referred to the rising point on the horizon and called it ascension.

In the RA-DEC system, now used, the great circles of HA are fixed on the celestial sphere with the zero point defined as one of the intersections of the celestial equator and the ecliptic, the vernal equinox. This coordinate is called right ascension and increases to the east from zero hours at the vernal equinox around the celestial sphere through 24 hours (360°). RA and HA are both measured in hours (h) minutes (m) and seconds (s) of time. The sky is divided into 24 hours so each hour corresponds exactly to 15° of arc (angle), each minute 15' of arc and each second 15" of arc. One degree of arc corresponds to 4 miutes of time. — Preceding unsigned comment added by 165.158.9.93 (talk) 15:20, December 16, 2004

What A Terrible Diagram

The illustrative diagram used for both RA and Dec is horrible! It falls into the category of "crummy diagram for somebody who already knows the subject, and for someone who does not: Just about useless". I mean this in the nicest possible way: It would be great if it could be replaced by one or better yet about three diagrams that serve to illustrate the point better.

First of all the declination angle looks like a vector sticking up from the equator. Second, the ecliptic plane as a heavy yellow band is visually emphasized over the equatorial plane which is the entire point of the coordinate system. — Preceding unsigned comment added by 131.107.0.73 (talk) 11:49, January 17, 2007

Rewrites in the second para .

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)

We're all new at this Prof. Thorstensen :) "Since an arc has 360 degrees, an hour of right ascension is equal to 15 degrees of arc, a single minute of right ascension equal to 15 minutes of arc, and a second of right ascension equal to 15 seconds of arc." Uh, I'm about to flunk (anything less than an 'A' is flunking) my undergraduate for nonmajors astronomy course since I'd rather learn than follow dogma. Anyway, 'Sine there are 360 degrees in the arc of a full circle, an hour of right ascension is equal to 15 degrees of arc, an arcminute is equal to 15/60 degrees of an arc... 60 seconds in an minute and 60 minutes in a day and 24 hours in 360 degrees up, over, and around the sky? arcminute qv http://en.wikipedia.org/wiki/Arcminute 128.195.84.188 05:14, 10 June 2007 (UTC)David [email protected] (soon to be no more...)

Give the concept meaning

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)

Is really how you measure the zero point of Right Ascension?

Pisces

Aries?

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)

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)

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)

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)

Great article

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)

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)

Early notation

In an old astronomy book,[1] 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)

Error in example?

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)

Sidereal hour angle

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)

Aries vs Pisces?

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)

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)

Hey people, all you have to do is click the Wikilink first point of Aries and it explains it right there. Is this what is supposed to be confusing? We can't explain things like that in every single occurance in every single article all over Wikipedia, it would get too cumbersome. That's the point of Wikilinks. Tfr000 (talk) 01:35, 29 April 2013 (UTC)

I removed the tag, for the above reasons. Tfr000 (talk) 17:58, 16 May 2013 (UTC)

Comparability

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)

That is called obliquity.- Adam37 Talk 21:23, 23 June 2018 (UTC) Your earlier point confuses RA, RA is just where in the sky the object is east-to-west, as published at Greenwich on the true Vernal Equinox, examining the entrails of a dead goat, basically. A slightly different figure, tweaked from the east, is given for Rome, near where such matters originated.- Adam37 Talk 21:23, 23 June 2018 (UTC)

h:m:s in radians?

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)

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)

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)

Minutes of RA to minutes of arc

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)

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)

Effects of precession

"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)

Precise definition of origin

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)

This is described in an authoritative source describing how the origin of RA is now redefined relative to the Quasars: https://www.iers.org/IERS/EN/Science/ICRS/ICRS.html Is someone interested in taking a shot at adding a section about this new definition? Custard31415 (talk) 18:49, 20 January 2022 (UTC)

Æquinōx and solstice… right ascension or declination?

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)

How to determine the right ascension (RA) right now by observation?

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)