Talk:Astronomical unit

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"ua" vs. "au"[edit]

Since the NIST Guide in the links uses "ua" as the symbol for this unit, and the IAU Style Manual in the links uses "au" as the symbol for this unit, why does this article use a different symbol, "AU"?

Why doesn't it at least mention those other symbols? Gene Nygaard 03:35, 17 Dec 2004 (UTC)

In practice, "AU" or "au" is universal. Using "ua" would cause no end of confusion. The IAU is the authority for astronomy.
As for whether to use AU or au, it seems that Astronomy magazine uses "AU" and Sky and Telescope uses "a.u." (!) So as they say, "go figure". -- Curps 21:41, 26 Dec 2004 (UTC)
One reason why it may be suitable for ua (and why the above guide uses ua) is the international country code for Australia is au, causing confusion. But, as said previously, AU is most used. So that's how it stands.

Taylor 08:27, 3 August 2005 (UTC)

"Ua" is the international code for Ukraine. So using "ua" does not solve the problem, which luckily never existed in the first place. -- Heptor talk 18:54, 29 April 2013 (UTC)

removed Francophobic comments re: the Bureau des Poids et Mesures, and replaced it with the more neutral explanation. — Preceding unsigned comment added by [[User:{{{1}}}|{{{1}}}]] ([[User talk:{{{1}}}|talk]] • [[Special:Contributions/{{{1}}}|contribs]])

The IAU style manual of 1989 pdf file given presently as ref 3 says in Section 5.15, pS24, says "The symbol for the astronomical unit is au". So i'm correcting the lead, which is wrong. La version francophone n'est pas apparement acceptee par l'UAI. Boud (talk) 22:14, 20 February 2011 (UTC)
It looks like the French translation has crept back in. I removed it again. Heptor talk 21:44, 29 April 2013 (UTC)

I added a 'citation needed' for the comment that units are usually capitalized only for those that are named after a specific person. This is most assuredly not a universal practice, almost all electrical units for example are not capitalized despite being named after someone, like coulomb and ampere. I would say most acronyms are capitalized, and AU is an acronym. Nerfer (talk) 15:54, 18 April 2011 (UTC)

The 2012 IAU RESOLUTION B2 on the re-definition of the astronomical unit of length, recommends "that the unique symbol “au” be used for the astronomical unit." It looks like we've got a difference between the IAU (2012) recommendation and the BIPM (2006) report. I suspect the BIPM and the IAU will sort this out, but for the moment we probably should let both stand with the more recent IAU recommendation getting some priority. --SteveMcCluskey (talk) 17:43, 11 May 2013 (UTC)
A recent edit removed the former international standard abbreviation of the astronomical unit as "ua" and the related citation to the 2006 report of the BIPM. Until the BIPM changes its recommendation, the alternate abbreviation should remain (after it does, it should still remain in the article, although perhaps not in the lead, for historical interest). --SteveMcCluskey (talk) 17:00, 16 May 2013 (UTC)
To clarify my position here, I personally consider "au" to be preferable; I had never heard of the use of "ua" in English until I read the BIPM recommendation (in English) cited in this article. That being said, our job as editors is not to report our preferred usage, but to report the recommendations of reliable sources. In this case, we presently have two authoritative international bodies making contradictory recommendations; as an encyclopedia Wikipedia should report both. --SteveMcCluskey (talk) 17:37, 16 May 2013 (UTC)
Well, not always. In general, when there are several competing points of views, WP has a goal to represent all of them. However, when some of the views are more widespread, WP gives more attention to those views. You should check WP:undue if you haven't already done so. Otherwise fringe theories would have to be given the same amount of space as the widely accepted versions. Heptor talk 10:42, 17 May 2013 (UTC)
I'm familiar with the notion of not giving undue weight to fringe theories, but there's no way I'd consider a formal publication of an international body that has regulated standards on weights and measures since it was first established in the 19th century by an international treaty to be a fringe theory. Its publications represent a prime example of a Reliable Source. The way I'd look at the differences between the IAU and the BIPM on the abbreviation (and length) of the astronomical unit has to do with the slow movement of international bureaucracies in responding to changing definitions. This is an issue of the slowness of assimilating scientific changes, not of a fringe theory.
Remember, the new IAU abbreviation and length was only established in 2012; the BIPM only issues new versions of its The International System of Units (SI) rarely. The current version is the eighth and draft material for the ninth edition is on it's web site. These changes will concern a fundamental reformulation of the international definitions of the kilogram, ampere, kelvin, and mole in terms of fixed numerical values of the Planck constant h, elementary charge e, Boltzmann constant k, and Avogadro constant NA, respectively. These changes are big and fundamental to the concerns of the BIPM; I suspect the BIPM will not issue a new brochure addressing these changes (and the new IAU definition of the astronomical unit) until they hash out their internal concerns. --SteveMcCluskey (talk) 13:12, 17 May 2013 (UTC)

"ua" length[edit]

BIPM defines an astronomical unit differently than the International Astronomical Union. Does this mean that we are talking about a completely different unit of length here? It looks like the BIPM definition is largely ignored in the astronomical community, both in terms of the definition of length and the recommended abbreviation. I added a section "other views", where I simply stated that BIPM has another set of definitions. Heptor talk 10:35, 17 May 2013 (UTC)

OK, I see that your concern is also with the BIPM's use of an old approximate measure of the length of the AU instead of the new defined length, as well as over the abbreviation. I'd recommend putting that somewhere in the historical section, perhaps under something like recent changes, along with a discussion of alternate abbreviations. I've gathered a few notes on these historical changes on my sandbox but I'm not ready to attack this yet. I'll see what I can do over the weekend. --SteveMcCluskey (talk) 13:12, 17 May 2013 (UTC)

estimates of AU (old discussion)[edit]

I came across a web-page that show the best AU is different from 149 597 870 691 ± 30 metres.

The address list below. And the contents show between two lines.

Name Astronomical Unit
Symbol AU
Value 1.495 978 706 60 ± 0.000 000 000 20 × 1011 m
Category Astrophysical
Comments The astronomical unit is defined as the mean distance between the Earth and the Sun.

The mean distance between the centre of mass of the Sun and the centre of mass of the Earth-Moon system is 1.000 0023 AU.

The best measurement of the astronomical unit comes from phase-modulated continuous-wave radio signals reflected off other planets. It was continuous-wave signals returned from the Viking probes on Mars that allowed the determination of the astronomical unit to 20m.


Do you know how to read scientific notation? The values are the same, except the one at your link is 30 m less, and it’s ±20 instead of 30. It’s an imperceptible difference, really (unless you do it for a living). —Frungi 04:57, 25 February 2006 (UTC)

How did they do it in ancient times[edit]

How did people measure the distance of the sun from the earth accurately in ancient times? Does anyone know? The preceding unsigned comment was added by (talk • contribs) 13:51, 19 January 2006 (UTC).

In ancient times, people didn’t believe in the solar system. Except for Galileo, which was why they ganged up on him. —Frungi 04:14, 25 February 2006 (UTC)

Well, not really. Ancient Greeks did know how to calculate the distance of the Sun from the Earth (they already knew that the Earth revolves around the Sun). They also knew that the Earth was round and they knew how to calculate its diameter. Aristarchus of Samos, who lived circa 310 BC - 230 BC used a method for determining the Earth-Sun distance, which had to do with measuring the angle in the sky between the Sun and the Moon when it is precisely in its first quarter phase. This was a particularly difficult thing to measure accuratly at that time. You would also have to know the Earth-Moon distance (which he did) to perform the calculation. Even though his measures weren't extremely accurate, the model was correct. And it's quite impressive that a man, some 2200 years ago, without telescopes, instruments or even a calculator, actually managed to find out how far away the Sun is. Leschatz 15:17, 5 March 2006 (UTC)

Message regarding "acceptance" of the value I edited here:

The value I wrote in for

"AU = 149 597 870 696.0 +/- 0.146"

is from Russia, figured from their ephemeris called "EPM2004" (of 2004). The newest NASA value (figured using their ephemeris called "DE410" of 2003) is

"AU = 149 597 870 697.4 +/- 0.3"

which is do to work headed by Myles Standish of the JPL. Both values were presented at the IAU Colloquium 196. In the transcript of the discussions between the Russians and Myles Standish at that meeting, the Russians tell him his value has a uncertainty of "1.4" ( and thus by implication, that their value is better). He said "perhaps the extra digit they have is significant, I am not sure".

 Thus I choose to use the Russian value for the AU.
                       Sincerely, EGB 

The Americans have more spacecrafts in deep space and better radio ranging technology. The Russians have first class mathematicians though, nobody would argue with that. In any case, deciding which is the better value is something even the IAU didn't want to do. DonPMitchell (talk) 04:06, 28 May 2010 (UTC)

astronomical units[edit]

how long does it take to travel one astronomical unit in measures of time (minutes, hours, days, months, etc.) ??????????????? —The preceding unsigned comment was added by (talk) 00:19, 6 March 2007 (UTC).

>> Depends how fast you're moving. (Darktachyon 15:24, 5 May 2007 (UTC))


The conversion of AU to angstroms is entirely unnecessary. (Darktachyon 15:24, 5 May 2007 (UTC))

Proposed WikiProject[edit]

Right now the content related to the various articles relating to measurement seems to be rather indifferently handled. This is not good, because at least 45 or so are of a great deal of importance to Wikipedia, and are even regarded as Vital articles. On that basis, I am proposing a new project at Wikipedia:WikiProject Council/Proposals#Measurement to work with these articles, and the others that relate to the concepts of measurement. Any and all input in the proposed project, including indications of willingness to contribute to its work, would be greatly appreciated. Thank you for your attention. John Carter 20:58, 2 May 2007 (UTC)

Definition of AU[edit]

The JPL appears to define the AU as the distance travelled by light in 499.004 783 806(10) s... I don't know the subject well enough to wish to amend the article, but could someone more knowledgeable check this out? Thanks! Physchim62 (talk) 09:50, 29 May 2007 (UTC)

That's true, because what is actually measured is the light travel time. In fact, this comes down to the same thing as defining it as a number of metres, because the metre is defined as the distance travelled by light in 299792458 seconds 1/299792458 of a second. I don't think it's an important enough distinction to be worth mentioning here. Cosmo0 16:32, 19 September 2007 (UTC)
Yeah, um, no: the meter isn't defined as light's traverse of nearly 300 million seconds, turning it into molasses; it's its traverse 'cross one-nearly-three-hundred-millionth of one second. Wink. —Preceding unsigned comment added by (talk) 03:58, 28 May 2008 (UTC)
Oops. Cosmo0 (talk) 09:49, 28 May 2008 (UTC)
The above JPL time constant in the link source is now displayed as 499.004 783 836 s, derived by dividing the AU distance by the speed of light. The site now states: "Last Updated: 2012-Oct-24." A 3 m margin of error in the AU would correspond to a 0.010 μs (3/299 792 458) margin of error for time. — Glenn L (talk) 11:44, 8 November 2012 (UTC)


I changed the intro to make the first sentence a little clearer to the lay reader and moved the bit about an AU being slightly less than the semi-major axis of the earth's orbit to below the full definition. I think it would confuse some readers if the first sentence of the article defined an AU as being “almost equal to” something else. Cosmo0 10:18, 22 September 2007 (UTC)

million vs billion[edit]

I noticed that the distance is written as almost 150 billion kilometers, but it says just after that it is 150 million. I am not an expert, and so will not make an edit one way or the other, but would someone check the value?Llama (talk) 01:53, 14 April 2008 (UTC)

It is written as 150 million km or 150 billion meters - but be careful - in the U.S. a "billion" is 10^9 but in England it is 10^12 - so best not to use the term. Use "mega", "giga" and so on. Anyway it is OK and consistent as it is.Carrionluggage (talk) 03:15, 14 April 2008 (UTC)
Is there any way we can make this less confusing somehow? It does seem to catch a lot of people out, judging by the number of times it gets 'corrected'. Do we even need to give the value in both metres and kilometres, given that converting from one to the other is not exactly difficult? Cosmo0 (talk) 20:48, 15 April 2008 (UTC)
I agree, there is no need to have both. In my eyes, meters should be prefered over kilometers as they are the SI unit for distance.--Fogeltje (talk) 06:38, 16 April 2008 (UTC)
Meters might be the SI unit, but to me it sounds a bit ridiculous because it gives the impression that the distance is known (and constant) to an accuracy that warrants the use of "meters". Even for distances on earth, in the order of kilometers, one would never use "meters"... JH-man (talk) 07:34, 16 April 2008 (UTC)

Gotta agree with JH; meter may be a human-scale distance unit, but we ought to go with larger scales, kilometers at least. This after all being astronomy, and not dustin' crops, boy. —Preceding unsigned comment added by (talk) 04:05, 28 May 2008 (UTC)

Looks like there's an error in the Examples section[edit]

Kind of a somewhat minor nitpick, I suppose, but consider the following two lines:

  1. 50,000 AU: possible closest estimate of the "Outer Oort Cloud" limits (1.0 ly)
  2. 100,000 AU: possible farthest estimate of the "Outer Oort Cloud" limits (1.6 ly).

It doesn't appear to me that it can be mathematically true that 50,000 AU equals 1.0 ly (to one significant digit) if 100,000 AU = 1.6 ly (to one significant digit). Basic multiplication would suggest that either 50,000 AU is approx 0.8 ly (maybe .7 or .9 due to rounding), or 100,000 AU is about 2.0 ly (I could see it being 1.9 ly due to rounding). Anyhow, later in the article, we see: 1 light-year ≈ 63,241 AU so that gives us 50,000 ≈ .8 ly. —Preceding unsigned comment added by (talk) 18:11, 23 June 2008 (UTC)

Fixed. Cosmo0 (talk) 20:00, 23 June 2008 (UTC)


There is an inconsistency in the timeline of the history section. It says that the richer and cassini performed the first estimate of the AU, yet below it says Horrocks did it 10 years earlier... This also desperately needs citations!! —Preceding unsigned comment added by Lzkelley (talkcontribs) 21:39, 25 January 2009 (UTC)

Thanks for pointing that out. The confusion seems to arise because Horrocks attempted to estimate the AU, but his method was flawed and he got the wrong answer. I guess whether you still consider that an estimate or not is a moot point, but I've revised the text to say that "The value of the AU was first estimated with reasonable accuracy by Jean Richer and Giovanni Domenico Cassini in 1672" (no bolding in main article). I didn't have time to look for citations, but I agree it needs some. Cosmo0 (talk) 12:38, 27 January 2009 (UTC)

What's that (6) in the figure?[edit]

I've checked the reference, and it says the value has an uncertainty of 6 metres, so plus/minus 3 metres? But I had no clue what the (6) meant until I downloaded the reference pdf, then searched it, then found the table on page 4!

I've no idea how to use this:

  • {{val|1.49597870691|(6)|e=11|ul=m}}

Could someone educate me? Is it being used correctly here?

I see that the rounded figure is given in kilometres, then the accurate figure in metres. Definite room for confusion there - I made the mistake of copy & pasting the accurate figure and missed the metres unit - oops! HarryAlffa (talk) 21:03, 3 May 2009 (UTC)

OK! First time use of these template thingys! Above gives a significance of the figure, not an uncertainty. I took away the parenthesis around the (6), which makes it +-6, should it be +-3? HarryAlffa (talk) 21:22, 3 May 2009 (UTC)
I think ±6 is correct, at least according to the source cited. NIST, however, give the value as 1.49597870691(6)×1011 m, while claiming to get it from the same source. So either they're using a bizarre notation or one of them is wrong. Apparently this means the same thing in the notation they use, although it's ambiguous IMO.
Be careful with the template, though: how you wrote it implied an error of 6×1011 m! I've reformatted it as 149597870691±m, which seems to be the most sensible way to fit in the small error.
Yes! The very same occured to me before bedtime. I thought that maybe a word in the template authors ear to put the +-val after the exponent! HarryAlffa (talk) 13:33, 5 May 2009 (UTC)
Or simply use the parentheses convention rather than the misleading +/-… Physchim62 (talk) 06:01, 6 May 2009 (UTC)
And yes, the use of both metres and kilometres in the introduction has been the source of much confusion in the past!
Cosmo0 (talk) 13:07, 5 May 2009 (UTC)


On the sidebar of the page, it shows this:

SI units
149.60×10^6 km 149.60×10^9 m
Astronomical units
4.8481 × 10−6 pc 15.813×10^−6 ly
US customary / Imperial units
92.956×10^6 mi 490.81×10^9 ft

I don't know how to edit it, but this should be:

SI units
1.4960×10^8 km 1.4960×10^11 m
Astronomical units
4.8481 × 10^−6 pc 15.813×10^−6 ly
US customary / Imperial units
9.2956×10^7 mi 4.9081×10^11 ft

This can be found on any site explaining significant figures, including wikipedia: —Preceding unsigned comment added by (talk) 22:46, 7 March 2010 (UTC)

But see Engineering notation.
I have no idea why this convention is used, but the box is generated by a site-wide template so it can't be changed here. Cosmo0 (talk) 21:39, 8 March 2010 (UTC)

Jabir ibn Aflah and Mu’ayyad al-Din al-’Urdi[edit]

The article claimed that these two astronomers realized that Ptolemy grossly underestimated the distance from the Earth to the Sun. I took this out because it is a misreading of the source, which is p. 237 of A History of Arabic Astronomy: Planetary Theories During the Golden Age of Islam, George Saliba, New York University Press, 1994, and states:

We have also seen above that Ptolemy had assumed the apparent size of the solar disk to be invariable at all geometric distances of the Sun, namely 0;31,20°. In addition to this approximation, there were also the faulty numbers associated with Ptolemy's grossly underestimated real geocentric distance of the Sun. As a result of that, at least two medieval astronomers, namely Jābir Ibn Aflaḥ (fl. 1150) and ʿUrḍī (d. 1266), argued against Ptolemy's order of the planets and, by using Ptolemy's own figures, defended an order in which Venus was to be placed above the Sun.

The source says that Ptolemy's figure for the earth-sun distance caused problems with his cosmological model, which Jābir Ibn Aflaḥ and ʿUrḍī reacted to. But, it does not say that they were aware that this figure was a gross underestimate. To further explain, let's look at what ʿUrḍī did. According to van Helden (p. 33, Measuring the Universe, University of Chicago Press, 1985, ISBN 0-226-84881-7):

Al-ʿUrḍī recalculated all the distances from parameters which he redetermined, making sure to add the diameter of each heavenly body to the calculated thickness of its sphere. He found that there was not enough room between Mercury and the Sun for the sphere of Venus. Venus therefore had to be above the Sun, so the order of the planets became Moon-Mercury-Sun-Venus-Mars-Jupiter-Saturn...Since his greatest solar distance was 1,266 e.r., about the same as Ptolemy's, Venus's greatest distance became 8,486 e.r., which was Mars's least distance.

So, al-ʿUrḍī rearranged Ptolemy's spheres, but did not significantly change the Earth-Sun distance.

As for Jābir Ibn Aflaḥ, Richard Lorch explains in his paper "The astronomy of Jābir Ibn Aflah" (doi:10.1111/j.1600-0498.1975.tb00315.x) that, according to Jābir, Ptolemy had determined a maximum solar parallax of 2′51″, and that if Venus and Mercury were inside the sphere of the Sun, they would have still larger parallaxes. In his treatise, Jābir therefore places Mercury and Venus above the Sun (see also [1]), but there is no sign that he greatly enlarged the Earth-Sun distance (if he had done this, there would be no need to move Mercury and Venus.)

Spacepotato (talk) 02:43, 29 May 2010 (UTC)

AU and m[edit]

NOTE: this discussion is now of historical interest only since the 2012 IAU definition fixes the AU at 1495977870700 meters exactly.2600:1000:B00C:57A5:7D9F:81E9:F7A3:51F0 (talk) 03:51, 18 October 2012 (UTC)

The article subsection Conversion factors states:

1AU = 149,597,870.700 ± 0.003 km

The IAU website states:

One AU is exactly 149,597,870.691 kilometres

The JPL website says:

AU = c * tauA = 1.49597870691 x 1011 (± 3) m

The 2002 IERS documentation says (Table 1.1):

c�τA = 149597870691m ±3m [3] Astronomical unit in meters

The multi-volume Landolt-Bornstien compendium (p.4) says:

AU = 1.49597871464 × 1011 m (no error bars)

Shirley & Fairbridge say (Table A8, p.49):

AU =1.4959787066 x 1011 (no error bars)

Why the variety of values? Some of these numbers are close in value, but the IAU says their number is exact. Some sources quote no error bar. Is this an experimental number or a theoretical number based upon using Newton's laws with a particular gravitational constant?? Brews ohare (talk) 05:52, 23 July 2010 (UTC)

The IAU website is misleading: the current IAU best estimate (2009) is 149,597,870.700(3) km, as quoted in the article. The value of 149,597,870.691(3) km comes from the widely used DE-405 ephemeris (1998). Shirley & Fairbridge gives some discussion on other values.
The answer to your second question is a bit more subtle. For any given (modern) ephemeris, the astronomical unit is a fixed number (equivalent to a fixed value for the heliocentric gravitational constant): the ephemeris is a model of the solar system, and so internally exact. So the IAU writer wasn't actually wrong when s/he said that the AU is exactly 149,597,870.691 km: that's true for anyone using the DE-405 ephemeris (which is just about everyone). However, the people who compile ephemerides (the Jet Propulsion Laboratory in the case of DE-405) usually make an estimate of the uncertainty based on the residuals between the model and the available observations, and the ±3 m is JPL's estimate of the uncertainty. I say "estimate" because you cannot use a simple Gaussian treatment of measurement uncertainties – too many of the input data are correlated with one another – and there is some debate on the best way to make the estimate. The Russian team have produced an ephemeris with a claimed uncertainty in the AU of ±0.3 m, but the current consensus appears to be that they were too optimistic in their treatment of uncertainties, hence the return to ±3 m for the IAU best estimate. Physchim62 (talk) 11:47, 23 July 2010 (UTC)
It would appear that the article is rather lacking in its discussion of these points. Certainly something about the exact internally consistent model and the error in applicability of this model should be there. BTW, it looks a bit tough to determine the "experimental" error in applying a model that assumes an infinitesimal mass particle in an exactly circular orbit. Where in nature does one find this item? Brews ohare (talk) 14:05, 23 July 2010 (UTC)
Planetary masses are small compared to the Solar mass and, to calculate the perturbation due to finite planetary mass, you only need to know the mass ratios. Physchim62 (talk) 15:59, 23 July 2010 (UTC)

In short, if one selects a specific value for k, it would seem that there isn't any choice in the matter: the AU is determined by Newton's laws independent of any experimental input. On the other hand, if one takes k to be an experimental number, then the definition of the AU is not as stated, i.e. is not for a specific k-value. What is the case? Brews ohare (talk) 14:24, 23 July 2010 (UTC)

But of course! If you have a fixed value for k you have defined A, that's the whole point of a definition! But how do you compare that value with Earth-based units? You can't, unless you have a measurement both in astronomical units and in Earth-based units, so that you can calculate the conversion factor. Physchim62 (talk) 15:59, 23 July 2010 (UTC)

Rewrite needed reflecting 2012 redefinition[edit]

Just did a reread of this article, and there are many sections that need revisions. Some discuss the proposals for change that were implemented in 2012; others relate the au to other physical constants in ways that are no longer appropriate to the new definition. As Captaine et al. said supporting the proposal for redefinition of the au:

  • k will not have a role any more; it should be deleted from the IAU System of astronomical constants,
  • the experimental determination of the ua in SI unit, will be abandoned,
  • GMSun will be determined experimentally

This is a good starting point for future deletions from this article.

They further pointed out the following consequences of the new definition, which might be incorporated into future revisions:

  • Eliminates deviation from SI
  • Eliminates dependence of the unit on theories of motion
  • Provides a self-consistent set of units in the relativistic framework
  • Avoids time-dependent units if time variation of solar mass is considered
  • Permits direct determination of time variation in solar mass parameter GMSun in SI units

In sum, a major rewrite is called for. --SteveMcCluskey (talk) 14:48, 17 May 2013 (UTC)

Why are there two tables in the 'Equivalencies' section?[edit]

I wonder why there are two tables in the 'Equivalencies' section. I'll prefer the smaller {{unit of length.. rather than the big cumbersome one. It's ok to have more decimals once, but if the conversions have been done by a Wikipedian it's OR - and should be excluded. --Regards, Necessary Evil (talk) 09:36, 8 January 2014 (UTC)

See Talk:Light-year#Why are there two tables in the 'Equivalencies' section?. Jimp 11:43, 8 January 2014 (UTC)

AU vs au vs ua[edit]

Discussion moved from Talk:Voyager 1[edit]

I just converted the symbol AU to au because au seems to be preferred by articles like astronomical unit and parsec (the conversion was incomplete because some instances had to be AU because of some indiosyncrasy of the convert function, but this is by the way). I am now having second thoughts because I think the correct symbol should be neither of these, but ua, as this symbol is the one preferred by international standards bodies (e.g., BIPM and IEEE). Thoughts anyone? Dondervogel 2 (talk) 19:33, 31 December 2014 (UTC)

If a change like that is going to touch this article, it should probably be made to others as well. I suggest the right venue for discussion is the project talk page. Evensteven (talk) 20:54, 31 December 2014 (UTC)
You mean the astronomy project? Dondervogel 2 (talk) 21:10, 31 December 2014 (UTC)
I agree this is not the best place for this discussion, best to take it to somewhere more people involved in the subject will see it like WikiProject Astronomy and indeed the astronomical unit talk page. In fact this has already been mentioned there, with ua seemingly not favoured at the time. ChiZeroOne (talk) 21:32, 31 December 2014 (UTC)
Good suggestion. I will raise it at the Astronomical unit. Dondervogel 2 (talk) 21:37, 31 December 2014 (UTC)

New discussion at Astronomical Unit[edit]

The above discussion was started at the Voyager 1 talk page and an editor pointed out that the matter had previously been discussed here. I'd like to raise the issue again. It seems to me that the international standard symbol is ua so that is what should be used in articles related to astronomy. What do others think? Dondervogel 2 (talk) 21:43, 31 December 2014 (UTC)

As I understand it au is proper, but I have been using AU for years as it stands out better. -- Kheider (talk) 21:55, 31 December 2014 (UTC)
In what sense is au "proper" if ua is the international standard? Dondervogel 2 (talk) 22:21, 31 December 2014 (UTC)
I believe that both au and AU are considered acceptable alternate usage by the international standards organizations. They are at least widely recognized. Evensteven (talk) 22:52, 31 December 2014 (UTC)
This is the English speaking Wikipedia. -- Kheider (talk) 22:56, 31 December 2014 (UTC)
Indeed. I've been around astronomy most of my life and have never seen nor heard of "ua" being used. It may be the preference of the international standards organisations, but it is not in use in practice, at least in my experience. We're here to be useful to the readers as a whole, not to ourselves or the elite experts. "AU" works best for us. Huntster (t @ c) 23:37, 31 December 2014 (UTC)
Just to be honest, I always prefer AU because that is an abbreviation. I've never considered light year to be ly. However, "ua"? I know my astronomy coverage is not the Solar System where this unit is used, however throughout my 9 years experience in astronomy I never saw "ua" was used. I think it was a French term, but even before Bessel measured the distance to 61 Cygni they used AU. So if ua is the standard, I've never seen it, even from works 400 or so years ago. SkyFlubbler (talk) 01:53, 1 January 2015 (UTC)
I'd oppose any change to using "ua" , for one thing, it means Ukraine to me; for another WP:UE/WP:UCN/WP:JARGON, in English, it is usually "AU" or "A.U." or something like that. That "ua" isn't found in English to any great degree and with the English language popular press using A.U. , the use of "ua" falls into JARGON, and isn't the common form of the term in English. -- (talk) 05:33, 1 January 2015 (UTC)
I believe that "ua" is an abbreviation of the French term, unité astronomique. Evensteven (talk) 08:47, 1 January 2015 (UTC)
My question is not about abbreviations, in English, French or any other language. It is about the correct symbol. The international standards bodies IEEE and the BIPM both use ua, which seemed to me a strong argument. However, I have just been informed that the IAU has adopted au as its preferred symbol, and this is a strong counter-argument. Can we agree to use au (and not AU) on WP? Dondervogel 2 (talk) 11:50, 1 January 2015 (UTC)
I would tend to disagree with "au" as well. Common usage suggests remaining with "AU", in addition to it being an acronym which tend to be capitalised. Huntster (t @ c) 13:29, 1 January 2015 (UTC)
I agree with Huntster. "AU" seems to be a wp:Common usage and it stands out better. The "au" standard was only recently adopted. And if anything this should be determined on a per article basis just as British vs American spelling is. I hate it when the wiki-police start enforcing a standard all across Wikipedia based on the consensus (opinion) of a handful of users. -- Kheider (talk) 14:27, 1 January 2015 (UTC)
@Dondervogel, we know IEEE and BIPM are standards, but as far I know IAU is a more reliable standard source primarily because it consists the legion of best astronomers on Earth, so strictly speaking when IAU does not say ua even though the former two says so, certainly it's not ua! And besides, Wikipedia is about what most people thought of, and not all people on Earth can speak French, although most can understand English, so most people will understand it as "astronomical unit" or AU rather than "unité astronomique" which I can't even pronounce. I'm not told that IEEE and BIPM are the official declarers of standard units, and this is even the first time I was told about them. SkyFlubbler (talk) 14:21, 1 January 2015 (UTC)
I'd quite strongly disagree with "au" (and "ua" isn't even English, but French; in other languages other symbols tend to be used: the Italian and Spanish WPs give (also) "UA", the Dutch one gives "AE", and the Greek one "α.μ."). I would opt for consistently using "AU" instead. The capitalized version is what I come across most. For example, JPL's database uses it. And "au" still tends to confuse me. --JorisvS (talk) 17:29, 1 January 2015 (UTC)
My personal preference is also for AU (caps, not small), regardless IEEE etc. The fact is, this is not an international unit (SI), and there is not international consensus on what should be used. I agree that the best international group to take recommendations from is theoretically IAU, but they also do not control common usage in any place or language. I think we must fall back on common English usage as the arbiter. There are no other standards widespread enough to apply to an international encyclopedia. Evensteven (talk) 17:57, 1 January 2015 (UTC) Addendum: And I agree that on the basis of common usage, there is no need to enforce a WP-wide standard either. Evensteven (talk) 18:02, 1 January 2015 (UTC)
My personal preference is also for AU based on common usage, fairly strongly. Usage in (English language) textbooks and the professional literature is overwhelmingly "AU". I don't recall ever seeing "ua", and "au" is uncommon and mostly in older texts. The IAU resolution surprises me, but given how uncommon such usage is anyway, I don't think it's binding on us anyway. I do also agree that there's no need to enforce a Wikipedia-wide standard, but I'd really object to someone going through and replacing all instances of "AU" with "au" or "ua". —Alex (ASHill | talk | contribs) 18:43, 1 January 2015 (UTC)
Yes, I don't think there is open support yet for changing away from AU, and where it's in place it should stay so until there is a call for it to change. Evensteven (talk) 18:54, 1 January 2015 (UTC)
Just a small observation – the convert template already uses "AU" as the unit symbol: {{convert|1|AU|km|abbr=on}} gives output 1 AU (150,000,000 km). I don't think there is a single convention that is broadly followed by everyone, so "AU" is likely as good a choice as any. Archon 2488 (talk) 19:04, 1 January 2015 (UTC)
AU is what I've seen all over the place. Cas Liber (talk · contribs) 21:01, 1 January 2015 (UTC)

Need for harmonisation[edit]

I was assuming when I started that there would be a general preference for harmonisation across WP articles, but some of the comments above lead me to question this assumption, so let’s settle the question of principle first. Regardless of what the adopted symbol, how much support is there for the thesis Wikipedia articles should adopt a single unit symbol for Astronomical unit? Dondervogel 2 (talk) 21:32, 1 January 2015 (UTC)

  • I support the thesis. Dondervogel 2 (talk) 21:33, 1 January 2015 (UTC)
  • OPPOSE/DISAGREE. If anything this should be determined on a per article basis just as British vs American spelling is. -- Kheider (talk) 22:39, 1 January 2015 (UTC)
  • oppose (was support, now oppose until I see what will be harmonized on) I don't think there is much question about harmonization within the wiki; the debate in recent years on different symbols/abbreviations for the astronomical unit has centered on whether we should even mention alternate symbols which are used in the literature. Particular venom seems to be reserved for the BIPM "ua" (latin-based ordering), which one does see in the literature occasionally (most often in diagrams from articles translated from another language). I don't think there is any question we should not be using "ua" in the english language Wikipedia, but we do need to mention it in the introduction since it will appear in diagrams from other languages. The other debate seems to center on "au" vs "AU" vs "㍳". I believe the Unicode abbreviation (the last) is awkward, and while you often see AU capitalized to differentiate it from surrounding text, the proper form we should be using in the english language Wikipedia is "au". Tarl.Neustaedter (talk) 22:45, 1 January 2015 (UTC)
  • Standardize at A.U. (abbreviation) or AU (symbol) or written full out astronomical unit in articles which use this unit; remove all uses that use "au" or "ua". "㍳" should not be used, it is a special unicode symbol thus support may be spotty, and may show up as questionmarks, diamonds or blank squares. The authors of articles can choose between AU/A.U./astronomical unit as they please. I will note that the neither BIPM nor IEEE have any oversight over astronomy, while the IAU does. They are not astronomy bodies, so not competent authorities in the field under discussion. (They for instance, do not define planet, assign names to astronomical bodies, etc) This article, "astronomical unit" should mention all forms, and indicate the common form in English being AU/A.U./astroonmical unit; and indicate the auxiliary forms ㍳/ua as being special unicode and not used in English unit symbol. -- (talk) 23:48, 1 January 2015 (UTC)
  • Oppose/Disagree. This seems like a solution in search of a problem. What problem does this proposal address? I agree that the British vs American spelling is a useful analogy. As long as each article is internally consistent, I see no need to enforce a project-wide standard. —Alex (ASHill | talk | contribs) 00:39, 2 January 2015 (UTC)
  • Oppose. Not needed.
Additional Comment. Some of the replies above are going beyond the thesis asked for here: "should WP adopt a single unit symbol". They are replying yes, and it should be this one or that one. Presumably, the reply might not be yes if another unit symbol were chosen over their preferred one, so where does that leave this survey? Evensteven (talk) 08:17, 2 January 2015 (UTC)
Good point. I presumed the intent was to go with the IAU-specified abbreviation, (au), but since that isn't obvious, I'm changing my support until that becomes obvious.Tarl.Neustaedter (talk) 08:25, 2 January 2015 (UTC)
  • Oppose. As others have said, treat this as an ENGVAR-type situation and standardise within individual articles only. Don't try to force the issue all at once across the entire site. Huntster (t @ c) 15:08, 2 January 2015 (UTC)
  • Comment: This discussion seems to be moving beyond the scope of this talk page, and it sounds to me like a discussion that would need to take place at WT:MOSNUM. Even if a consensus emerged here on what symbol to use (which now seems unlikely), it would have no bearing on any page beyond the attached article; we could not make a WP policy directly from it. With that in mind, I'd suggest that further discussion in this thread will not serve any purpose, so it should perhaps be moved to the MOS.
On the other hand, as ASHill observes above, this proposal does seem a bit like a solution in search of a problem, a point which would likely be raised in a discussion at the MOS level. If you want to persuade the wider community to approve something as policy, you usually need to show that there has been some disagreement at the article level which the policy would help to solve. For example, I recently started a discussion at MOSNUM about the correct unit symbol for standard gravity (when used as a unit of acceleration), which was ultimately inconclusive since it was considered premature to standardise at the MOS level without adequate discussion in article-space. So if this thread is to be continued, it would make most sense to switch from discussing standardisation across WP (which we cannot do here in any case) to discussing what convention should be followed in this article. Further standardisation would have to follow after that. Archon 2488 (talk) 23:50, 2 January 2015 (UTC)
Though it's certainly true that any Wikipedia-wide decision would have to be made at the MOS level, this is as good as any a place for an initial discussion, I think. It has been pointed to (and drawn my attention) from wider pages including WT:ASTRONOMY, so I think a broad initial discussion is as good here as elsewhere. Developing some consensus amongst astronomy editors before going to the MOS level is probably appropriate. —Alex (ASHill | talk | contribs) 19:23, 3 January 2015 (UTC)

Arguments for and against harmonisation[edit]

It seems I am the only editor arguing in favour of harmonisation. At the moment there is an apparently random mixture of au and AU across articles (there might be other symbols as well but these are the ones I have encountered). Because I do not comprehend the benefits of perpetuating this mixture, I propose we construct a list of arguments for and against harmonisation. The argument in favour of harmonisation is to reduce unnecessary confusion by use of multiple symbols. This the reason Wikipedia has chosen to adopt a single unit symbol for the metre (m, not mtr), a single unit symbol for the foot (ft, not ft.), a single unit symbol for the second (s, not sec), a single unit symbol for the bit (bit, not b) and so on. There is no parallel with engvar because there is no equivalent bipolar education system that teaches half of us to use AU and the other half au. Dondervogel 2 (talk)

All of those symbols have nearly universally-used standard abbreviations outside of Wikipedia. AU appears not to. ("AU" is by far the most common form, but apparently isn't recommended by any standards body, including the IAU.) —Alex (ASHill | talk | contribs) 16:22, 2 January 2015 (UTC)
I would say that of the examples I gave only one (m, for metre) is widely adopted outside WP. The rest are all over the place (s or sec for second, ft or ft. for foot, b or bit for bit). And there are plenty of other examples I could have chosen (e.g., nmi or NM, not nm for nautical mile; kn, not kt for knot, bit/s, not bps for bit per second) for which there is little or no standardization in the world at large, but for which WP has adopted a consistent symbol (or symbols). Dondervogel 2 (talk) 00:15, 3 January 2015 (UTC)
Indeed, WP has largely standardised on a set of unit symbols – one for each unit, even when real-life usage is inconsistent. It makes sense for the project to be as internally consistent as possible; obviously ENGVAR is an exception, but in the case of unit symbols, there is usually not much of a question of regional variation. But as I said above, this isn't really the sort of thing that can be discussed meaningfully on an article talk page, since it would need broader community consensus before it could become MOS policy. For now, we should focus on what symbols to use in the article on the astronomical unit – the question of what symbol articles on planets should use can be discussed after that. Archon 2488 (talk) 02:05, 3 January 2015 (UTC)
I agree also, but prefer not to expend more effort than necessary on a matter such as this. If discussion really has to be done at all, and unit symbol adoption is to become a mandate, I would agree that MOS policy is the only place where it belongs, as that presents the easiest means of informing and enforcement afterwards, as well as the proper breadth for consideration. Evensteven (talk) 06:59, 3 January 2015 (UTC)
Just a point of note...the MOS is not policy, just a guideline. Huntster (t @ c) 07:33, 3 January 2015 (UTC)
I'm actually quite glad you point that out, since I frequently run into people who insist on citing the MOS as if it were holy scripture that only an infidel could disagree with (yeah, that is pretty much their tone). Archon 2488 (talk) 15:54, 3 January 2015 (UTC)


  1. The choice of a unique unit symbol makes Wikipedia easier to read.


  1. The choice of a unique unit symbol requires Wikipedia to make a judgment that the world has not made. WP is supposed to be reflective, not assertive. Evensteven (talk) 09:46, 2 January 2015 (UTC)
  2. The choice of a unique unit symbol requires Wikipedia to maintain that symbol alone in its articles, requiring some editor to produce an explanation to every new or visiting editor who comes along with something different, like, say, some alternative they encountered in the real world. Evensteven (talk) 09:46, 2 January 2015 (UTC)
  3. Both the Minor Planet Center database and JPL Small-Body Database use AU and it is always good for the articles to match the source. -- Kheider (talk) 15:25, 2 January 2015 (UTC)

I find myself unable to follow the logic of the first two. The third one I understand. Specifically

  1. This is tantamount to saying "I cannot decide because that would require me to decide". WP reports what is written elsewhere, and in that sense it is reflective. But there is no requirement to repeat spelling mistakes (for example) except in a verbatim quote.
  2. On the contrary, it is the absence of harmonization that leads to a requirement to explain each new symbol on each new article.
  3. This is the one I understand. There is a clear conflict between following the unit symbols in sources and adopting one of them.

Dondervogel 2 (talk) 09:41, 3 January 2015 (UTC)

I think the first point is simply saying that it is not WP's place to standardise in the absence of real-world standardisation (perhaps this is a NPOV-related argument). But as you point out, we already do this for most other units.
I tend to agree more with you on point 2, that harmonisation makes the encyclopedia easier to read, which I think is the real goal. However, it's not inconceivable that if we picked one symbol, a new editor might be confused if our choice was not of the symbol they are more familiar with (I think this was the original point).
There's another problem in your analysis of point 3; WP explicitly doesn't follow the stylistic conventions of the specific sources which happen to be cited in a given article, because it has its own MOS. Normal WP practice is to pick a symbol for each unit and stick with it, even if particular sources use diverse practices. Archon 2488 (talk) 15:25, 3 January 2015 (UTC)
A further point is that AU is an uncommon-enough unit that it should be defined at first use in every article (ie "Jupiter is 5 astronomical units (AU) from the Sun"), so whatever definition is used in the article should be consistent. (I don't know More common units like m and kg don't need to be defined. —Alex (ASHill | talk | contribs) 19:18, 3 January 2015 (UTC)
The MOS standard practice is "In prose, unit names should be given in full if used only a few times, but symbols may be used when a unit (especially one with a long name) is used repeatedly, after spelling out the first use (e.g. Up to 15 kilograms of filler is used for a batch of 250 kg)." So this is what I'd recommend for the astronomical unit. For more unusual units, there is also the option to gloss the unit at first mention (so at first mention of the astronomical unit, you could link to this article). The convert template supports this: {{convert|12|AU|km|lk=in}} gives output "12 astronomical units (1.8×109 km)". Archon 2488 (talk) 21:23, 3 January 2015 (UTC)


The gist of the above comments is that editors at Astronomical Unit a) prefer not to harmonise and b) prefer not to discuss harmonisation, making it pointless to continue the discussion. Goodbye and farewell. Dondervogel 2 (talk) 09:49, 3 January 2015 (UTC)

User:Dondervogel 2 has resumed this discussion at WT:MOSNUM#What is the correct symbol for astronomical unit?. —Alex (ASHill | talk | contribs) 16:19, 4 January 2015 (UTC)
It looks like user: Evensteven not Dondervogel2 that resumed the discussion there. DV2 just left a note about the discussion occurring here. -- (talk) 04:00, 5 January 2015 (UTC)
No. At the time, I continued there because I had thought the discussion had moved. Let it be as you and Dondervogel2 wish, and continue here. My only issue with venue was to ensure wide participation by interested parties, and apparently everyone has settled on where to have it. Evensteven (talk) 06:16, 5 January 2015 (UTC)

March 2015[edit]

This has come up again with new edits to this article. I'm pretty sure that "AU" is the most common usage (despite the IAU standard), so saying "sometimes AU" in the lede of the article isn't right. Maybe the article should say explicitly, either in the main text or in a footnote, that the IAU specifies "au" but "AU" is in widespread usage (assuming we can find a source that says that, which I assume we can)? Note that this is a different issue from which symbol we use to choose for usage throughout Wikipedia. In the encyclopedia article on the astronomical unit, we should describe the situation in the real world even if we do follow the IAU recommendation as a matter of style when simply using AU as a unit. —Alex (Ashill | talk | contribs) 14:24, 20 March 2015 (UTC)

In fact, I see that we already do say this in the body, though I don't think the text in the body really reflects common usage. I'll see if I can find a better source. —Alex (Ashill | talk | contribs) 14:28, 20 March 2015 (UTC)
I disagree with putting AU before au in the lede. The international standard symbols are ua and au. I know AU is common in Wikipedia but I blame that on the Convert template, which does not support use of the international standard. But it is not a good reason to have AU predominate here. If there is a source saying that AU is more common than the international standards in scientific literature, then that source needs to be cited. Dondervogel 2 (talk) 16:32, 20 March 2015 (UTC)
AU is common on Wikipedia simply because it's common everywhere. But sources that discuss this are hard to find (and I see no sources that say that "au" or "ua" are commonly used either; we simply have sources that say they're recommended — a very different thing). Instead, most sources simply use one or the other without discussion (presumably since the distinction is not that interesting; both are clear enough). Even the sources that specify rarely say why or make any comment about common usage; at least, I haven't been able to find any that do. —Alex (Ashill | talk | contribs) 16:53, 20 March 2015 (UTC)

June 2015[edit]

This has been brought up again, this time at WT:MOSNUM. Editors may wish to discuss there. —Alex (Ashill | talk | contribs) 12:37, 14 June 2015 (UTC)

Centimetres in the infobox[edit]

Centimetres were added to the infobox. I really fail to see the point in mentioning how many centimetres there are in an astronomical unit. Jimp 15:05, 8 February 2015 (UTC)

AU vs au[edit]

I know there has been a lot of discussion about "AU vs au", but there is currently a discrepancy in the Manual of Style and this article. See:

which states "AU (not A.U., au, ua)" and links to astronomical unit, where "au" is preferred over "AU". This article or the entry in Manual of Style has to change. Last time I edited the article, AU was changed into au, so I won't do that myself anymore, but I will continue to bring it up until this cognitive dissonance is recognized. -- Rfassbind – talk 22:27, 25 August 2015 (UTC)

The MOSNUM rule you refer to was added quite recently (a few weeks ago). Before the rule was added I would have argued for au here (and probably did), on the grounds that au is preferred by both BIPM and IAU. Now there is consensus at MOSNUM for AU, I see no reason not to bring this article into line. Dondervogel 2 (talk) 23:13, 25 August 2015 (UTC)
Yup, the determination of the consensus out of that discussion was that there is a consensus to use AU. I agree that following that consensus in this article is appropriate, and I'll do it. I think keeping the discussion of the various symbols is helpful; I'm not sure if the discussion needs to be changed. The problem is that for a style consensus, Wikipedia editors conducting original research to determine that AU is most commonly used is fine, but we obviously can't use that original research in the article itself. We failed to find any sources that say whether AU or au (or some other symbol) is most commonly used, although I think the finding of our original research was pretty clear that AU is most commonly used. —Alex (Ashill | talk | contribs) 23:25, 25 August 2015 (UTC)
Do note, Manual of Style says how wikipedia should format its usage. That's an internal standard. What we cannot do is remove content which disagrees with the manual of style. We can change internal use within the article to use AU in cases where the usage is not specific to a standards body - but anything referring to a standard must continue to use the abbreviation that standard requires. Tarl.Neustaedter (talk) 01:39, 26 August 2015 (UTC)
Sure, the issue (with the article) is about "preference", not "removal". Rfassbind – talk 15:07, 26 August 2015 (UTC)

New image[edit]

I am moving the following discussion from my talk page. I oppose the addition of the image because it is confusing. Jc3s5h (talk) 22:03, 28 September 2015 (UTC)

What's confusing about the image? It is clearly a line connecting the Sun and Earth. Pretty simple. The image, of course, shouldn't be the explanation itself. It is just an illustration. Huritisho (talk) 16:15, 28 September 2015 (UTC)

Perspective images are not usually used for astronomical drawings. If the reader interprets it like most drawings that appear in serious articles about astronomy, the reader will think the distance from the Earth to the Sun is about three times greater than the distance from the Earth to the Moon. Jc3s5h (talk) 16:40, 28 September 2015 (UTC)
Hm. Is that really a big deal? Regardless of the position of the Moon, the Sun-Earth distance will still remain the same. The article isn't about the position of the Moon afterall. The line is there indicating the Sun-Earth distance and I think it is a perfect image to use in that article. Sorry, but I do intend to add it again. If you really want to remove it, start a discussion in the talk page. Cheers! Huritisho (talk) 18:37, 28 September 2015 (UTC)
The point is that the image doesn't provide any meaningful information. If it provided scale, that might be relevant, but simply having three white dots with a line between them marked a (non-AU) distance doesn't add anything. Tarl.Neustaedter (talk) 22:47, 28 September 2015 (UTC)
I do not see how the image helps the article. To the casual observer the moon can look like it is 50 million km from Earth. -- Kheider (talk) 23:26, 28 September 2015 (UTC)
It is just an illustration to make the article more friendly. The real information should obviously be contained in the article. What if I edit the image and remove the Moon and it's orbit. Would that be ok? Huritisho (talk) 01:00, 29 September 2015 (UTC)
I agree that the image is not helpful. Even without the moon there are a few stars in the background which clearly don't fit in the scale of the drawing. If you wanted an image to show what an astronomical unit is and how it fits in the scheme of things, I'd suggest a scale drawing of the Sun and the orbits of the four inner planets (Mercury through Mars) with the planets enlarged for visibility. The AU could then be illustrated by a line from the Sun to the orbit of the Earth. --SteveMcCluskey (talk) 16:29, 29 September 2015 (UTC)

──────────────────────────────────────────────────────────────────────────────────────────────────── @SteveMcCluskey: Gosh, the stars are clearly just an illustration. They have nothing to do with the Sun-Earth distance and nobody is even going to look at them. You know what? Remove the image. You guys are too full of perfectionisms. I give up. Huritisho (talk) 16:45, 29 September 2015 (UTC)

New new image[edit]

As suggested by -SteveMcCluskey, I added that image. The text could be a little bigger, I think... Huritisho 17:25, 5 October 2015 (UTC)


The word "kilometre" first appeared in the article on 6 June 2003 it was spelt in the Commonwealth way (as opposed to the US way "kilometer"). For eight years, the article continued to use "metre", "kilometre", "centre", etc. until this piece of vandalism by Crashkidd34 on 17 June 2011. The vandalism introduced an inconsistency in spelling which persisted until 8 February when the long-established style was restored. Hours later, this was switched to US spelling with the comment "restore American spelling per WP:ENGVAR; it was used primarily before recent series of edits". This was done presumably in good faith, there's nothing wrong with going with the most commonly used style in an article; however, ENGVAR doesn't mention that. What ENGVAR does point out, though, is "When an English variety's consistent usage has been established in an article, maintain it in the absence of consensus to the contrary.". It's irrelevant that the vandal managed to get most of the "metre"s, "kilometre"s, etc., it was still vandalism. I don't blame Ashill for not checking this out but the established style is Commonwealth spelling. Jimp 04:13, 5 October 2015 (UTC)

"On average" or "about"[edit]

In the caption for the new image, it initially said the distance between the Earth and the Sun was "on average" 1AU, then that was changed to say "about" 1AU. That distance, however, changes, and "on average" sent that message across; "about" doesn't, and additionally, it makes one assume it's roughly similar, while in fact it is exactly that distance (albeit on average) as the AU was originally defined that way. So, I'd like to restore "on average" in the absence of objections. LjL (talk) 12:35, 8 October 2015 (UTC)

I don't think the AU has been defined strictly defined as "on average" in recent times. Sure, if you measured the distance at many different instants that in some sense were "fairly distributed" you would get close to 1 AU, but I don't believe that has been the definition in the last several decades. I don't know what the original definition was back when the term was first coined. Jc3s5h (talk) 13:33, 8 October 2015 (UTC)
In the context of that caption, though, it's not about what the definition is, but what the distance actually is. The article itself states: "Originally conceived as the average of Earth's aphelion and perihelion, it is now defined as exactly 149597870700 metres (about 150 million kilometres, or 93 million miles)." So yes, originally it was defined as the very average; now that has been frozen into an exact number. LjL (talk) 13:37, 8 October 2015 (UTC)
Before it was frozen as exactly 149597870700 metres it had other definitions:

the radius of a circular orbit in which a body of negligible mass, and free of perturbations, would revolve around the sun in 2π/k days, k being the Gaussian gravitational constant. ("Glossary" of Explanatory Supplement to the Astronomical Almanac, 2013) astronomical units, the mean distance of the Earth from the Sun, calculated by Kepler's law from the observed mean motion m and adopted mass m is 1.00000003.[Hence the mean distance of the Earth from the Sun is not exactly 1 AU.] (Explanatory Supplement to the Ephemeris, 1961, p. 96)

Jc3s5h (talk) 13:57, 8 October 2015 (UTC)
If "on average" alone is not adequate because it's not exactly 1AU on average, then I propose having both: "on average about 1AU". LjL (talk) 14:00, 8 October 2015 (UTC)
That's OK with me. Jc3s5h (talk) 14:10, 8 October 2015 (UTC)

1976 definition[edit]

In 1976, in order to establish a yet more precise measure for the astronomical unit, the IAU formally adopted a new definition. Although directly based on the then-best available observational measurements, the definition was recast in terms of the then-best mathematical derivations from celestial mechanics and planetary ephemerides. It stated that "the astronomical unit of length is that length (A) for which the Gaussian gravitational constant (k) takes the value 0.01720209895 when the units of measurement are the astronomical units of length, mass and time". Equivalently, by this definition, one AU is the radius of an unperturbed circular Newtonian orbit about the sun of a particle having infinitesimal mass, moving with an angular frequency of 0.01720209895 radians per day; or alternatively that length for which the heliocentric gravitational constant (the product GM☉) is equal to (0.01720209895)2 AU3/d2, when the length is used to describe the positions of objects in the Solar System.

There are several problems with this paragraph.

  • The IAU defined the a.u. like this in 1964. The 1976 system of constants continued it.
  • It wasn't based on best available measurements. It was based on the definition Gauss gave to his gravitational constant, as described above. If it had been based on measurements, it would have been some fixed value like it is now, like 149597870700 m. Defined as above, it is essentially an equation. Its value varies depending on what you plug into it.
  • heliocentric gravitational constant points to standard gravitational parameter, which is indeed equal to GM in the case of the Sun. However, k2 is not equal to GM as stated here, it is equal to G. This error seems to have gotten into many Wikipedia articles.

See Gaussian gravitational constant for references for all of this. Tfr000 (talk) 17:35, 10 January 2016 (UTC)

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External links modified[edit]

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I have just modified 2 external links on Astronomical unit. Please take a moment to review my edit. If you have any questions, or need the bot to ignore the links, or the page altogether, please visit this simple FaQ for additional information. I made the following changes:

When you have finished reviewing my changes, please set the checked parameter below to true or failed to let others know (documentation at {{Sourcecheck}}).

Question? Archived sources still need to be checked

Cheers.—InternetArchiveBot (Report bug) 10:13, 20 October 2016 (UTC)