Talk:Astronomical unit

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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)

removed Francophobic comments re: the Bureau des Poids et Mesures, and replaced it with the more neutral explanation.

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. http://www.scenta.co.uk/tcaep/nonxml/science/constant/details/Astronomical%20Unit.htm

Science
Constants
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.

YuWong yuwong36@yahoo.com

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)

Contents

[edit] How did they do it in ancient times

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 81.179.39.220 (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

[edit] astronomical units

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 71.168.192.207 (talk) 00:19, 6 March 2007 (UTC).

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

[edit] conversion

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

[edit] Proposed WikiProject

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)

[edit] Definition of AU

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 69.148.169.120 (talk) 03:58, 28 May 2008 (UTC)
Oops. Cosmo0 (talk) 09:49, 28 May 2008 (UTC)

[edit] Definition

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)

[edit] million vs billion

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 69.148.169.120 (talk) 04:05, 28 May 2008 (UTC)