IAU votes to redefine the astronomical unit – giving it a constant value

Sep 19, 2012 by Bob Yirka report
Credit: NASA

(Phys.org)—Members of the International Astronomical Union have voted to approve a change to the definition of the famous "astronomical unit" aka, AU, from one based on variable data, to a definite number. The change has been a long time coming and will allow those in the field to describe their work more easily and will allow professors to forego the lengthy explanation of the prior definition to new students.

The AU is at its root a measure of the distance between the and the sun; which was first calculated by the famous astronomer who noted the position of Mars while standing in Paris and compared it to its angle in the sky when viewed from a site in South America at the same moment in time. Using parallax he was able to come up with a very close estimate of just how far the Earth was from Mars, and then the Earth from the sun, which he said should be about 140 million kilometers. That eerily accurate figure came to be used throughout astronomy as a standard for describing distances in the visible . To make it more accurate, the calculation was changed in 1976 to include a tie to the sun's mass, which for newcomers to the field only made things more difficult to understand. It also didn't take into account the fact that the sun is gradually shrinking.

Subsequent new developments in science and technology have led to much more precise ways to measure the distance between solar objects, which served to make using the old calculation even more obsolete. The only reason it's lingered as long as it has, astronomers say, is because everyone was used to it and thought changing things might be too disruptive.

That line of thinking apparently gave away to common sense at the latest meeting of the IAU, as members voted to make the AU an exact 149,597,870,700 meters, which is the average mean distance between the Earth and sun when viewed from the Earth. This last point is important because the old calculation violated Einstein's Theory of Relativity in that using it should have given different answers depending on where the measurement was taken, i.e. from different objects in the solar system.

Most in the field seem to be relieved to finally have the AU changed to a constant, believing it will help make their work more accurate and because it will, of course, require a lot less explaining to those new to astronomy.

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User comments : 22

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chardo137
2.7 / 5 (3) Sep 19, 2012
This is a much bigger change than making Pluto a minor planet, but I'll bet that the popular press doesn't even pick up on it.
nkalanaga
3.8 / 5 (6) Sep 19, 2012
No, because to most people it doesn't matter. With Pluto, the IAU was redefining language. In effect, they were telling people that they, not the speakers of English, would define the word "planet". Since "planet" is older than English, much less the IAU, people didn't like that. Also, to say that a "dwarf planet" isn't a planet, while a "giant planet" is, is illogical. Thus, they were also trying to change the way English works.

On the other hand, the exact distance to the Sun doesn't matter to the average person. What the IAU chooses for the AU doesn't change anything in the physical world, or in everyday language, so non-astronomers don't worry about it.

But, you're right, to an astronomer this is a much bigger deal.
Jitterbewegung
2 / 5 (4) Sep 19, 2012
"149,597,870,700 metres"

They should have made it

149,597,375,794.93

then it would factor into a lightyear.
Sanescience
3.7 / 5 (3) Sep 19, 2012
"Most in the field seem to be relieved to finally have the AU changed to a constant, believing it will help make their work more accurate"

I find it difficult to understand why a scientist would use a unit of measurement at the core of his research that would make his work "less accurate".
aidoru_4u
2 / 5 (2) Sep 19, 2012
On the other hand, the exact distance to the Sun doesn't matter to the average person. What the IAU chooses for the AU doesn't change anything in the physical world, or in everyday language, so non-astronomers don't worry about it.


I'm an avreage person, hey should have made it 150,000,000,000 :-)
Deadbolt
1 / 5 (1) Sep 19, 2012
No, because to most people it doesn't matter. With Pluto, the IAU was redefining language. In effect, they were telling people that they, not the speakers of English, would define the word "planet". Since "planet" is older than English, much less the IAU, people didn't like that. Also, to say that a "dwarf planet" isn't a planet, while a "giant planet" is, is illogical. Thus, they were also trying to change the way English works.

On the other hand, the exact distance to the Sun doesn't matter to the average person. What the IAU chooses for the AU doesn't change anything in the physical world, or in everyday language, so non-astronomers don't worry about it.

But, you're right, to an astronomer this is a much bigger deal.


When did they say specifically that Pluto being a dwarf planet makes it not a planet? I thought the public were complaining because of a misconception.
fmfbrestel
5 / 5 (3) Sep 19, 2012
With Pluto, the IAU was redefining language.

No, they were not. They changed a technical definition so they wouldn't be compelled to officially label Eris, Haumea, Sedna and Makemake as "Planets". Since there was no major distinction between Pluto and these other objects, they changed their classification. "Language" is fluid and defined only by common use. They have no power over the language, only their own organization's taxonomies.

Also, to say that a "dwarf planet" isn't a planet, while a "giant planet" is, is illogical.

Actually you are just confused here. Dwarf Planet is a specific classification, like "Sea Lion". "Giant Planet" is actually nothing - you made that one up. But I'll assume you don't have a good grasp of the english language(ha!), and meant "Gas Giant Planet". Gas Giant Planets are like "White Lions" which are just lions that are white.

Q-Star
2.8 / 5 (4) Sep 19, 2012
I voted to demote Pluto. Planet needed a more definite meaning. But the AU? Why does anyone need a specific constant for that? NO one uses the AU for anything more than a relative description. NOT one person I know who does science uses it for any critical quantification.

What would the exact number be useful for? You can't use to it do mechanics or predict motions, because the "official" AU would not be the reality at anytime other than at the instant it was codified, if even then.

Dwarf as applied to Pluto is not an adjective. "Dwarf Planet" is a noun. "Dwarf planets" are not little planets, they are not planets, they are "dwarf planets".
nkalanaga
1 / 5 (3) Sep 20, 2012
No, I meant "Giant Planet". We've discovered a lot of very large extra-solar planets that are ASSUMED to be "gas giants", but there's no firm evidence as to what many of them are made of.

As for the dwarf planets, that is an adjective and a noun. If a dwarf tree is a tree, and a dwarf human is a human, then a dwarf planet is a planet.
gwrede
1.3 / 5 (3) Sep 20, 2012
Defining an exact value for the Astronomical Unit is about as smart as defining a Geographical Unit as the exact distance between New York and London to the nearest micrometer.

The value is bound to be arbitrary. But much worse, it is a changing value. Just like the continents are moving and the distance between the two cities is growing, the Sun-Earth distance is getting smaller.

The next thing I see them doing is something even more stupid: an exact length for a Parsec. Now, there's a unit that should be shot on sight.
fmfbrestel
not rated yet Sep 20, 2012
No, I meant "Giant Planet". We've discovered a lot of very large extra-solar planets that are ASSUMED to be "gas giants", but there's no firm evidence as to what many of them are made of.


1 - "giant planet" is not a term that the IAU uses, which was my point. Gas Giants are commonly referred to, but again, is merely descriptive.
2 - you mean besides the rather firm measurements of mass and diameter, which allow us to calculate their density? I guess they might be made out of swiss cheese, or maybe marshmallow fluff?

As for the dwarf planets, that is an adjective and a noun. If a dwarf tree is a tree, and a dwarf human is a human, then a dwarf planet is a planet.


So the Sea Lions MUST be Lions by the same logic. Except that they obviously are not lions. New York isn't literally a new version of York, it is just New York. Proper names (of a taxonomic system or otherwise) do not follow the standard rules of Language. They did no harm to the English Language.
fmfbrestel
5 / 5 (1) Sep 20, 2012

I find it difficult to understand why a scientist would use a unit of measurement at the core of his research that would make his work "less accurate".

They wouldn't. Thats the problem. The AU is a useful measuring stick for communicating research to the public because it is easy to understand. Well, with the rigid definition, they dont have to sacrifice precision when writing for a lay audience.
antialias_physorg
3 / 5 (1) Sep 20, 2012
I thought the public were complaining because of a misconception.

The public was complainig because someone redefined something that they took (great) pains to learn at school.
The AU is not taught in school. And if it is, then you don't get taught (or at least don't remember) the actual value (range) but remember that it's the average distance between the Earth and teh Sun - which it still is.

So for the public: no change at all (hence no outcry)

What would the exact number be useful for?

So that when talking about distances expressed in AUs two people don't come to different conclusions.
(Example: Person 1 says "the asteroid belt is x AU out - barely enough for solar powered craft", while Peron 2 claims that it's not (taking the larger value of the AU range))
Also many ratios in Astronomy are expressed as AUs which benefit from a more stringent definition.
nkalanaga
1 / 5 (1) Sep 21, 2012
"2 - you mean besides the rather firm measurements of mass and diameter, which allow us to calculate their density? I guess they might be made out of swiss cheese, or maybe marshmallow fluff?"

We have mass estimates for all of them, but diameter only for the transiting planets. I will admit that all of them seem to be Jovians, but that's no guarantee that there aren't nontransiting non-gaseous planets of similar masses.

In fact, we know how to make such a world: Start with a Jovian and blow the gas off. At least one planet in the process of losing it's atmosphere has been found. Since measured densities give apparent masses for Jovian cores from less than one to over 100 Earth masses, a "hot rocky giant planet" is practical.
fmfbrestel
not rated yet Sep 21, 2012
but that's no guarantee that there aren't nontransiting non-gaseous planets of similar masses.


Yes, yes. Of course. I get caught up with the spitzer data too much and discount the other planetary observations.

Still though, the IAU has no category called "Giant Planet". You made that up in order to provide some sort of syllogistic evidence against the dwarf planet decision.
nkalanaga
3 / 5 (2) Sep 21, 2012
My objection is that the IAU has claimed to be the sole authority on anything dealing with extraterrestrial objects. They don't represent all astronomers, much less all apace-related scientists. The Pluto debate showed that, with many planetary scientists still considering Pluto a variety of planet, whether the IAU agrees or not.

No, I did not make up the term "giant planet", even if the IAU doesn't accept it.
fmfbrestel
4.5 / 5 (2) Sep 21, 2012
My objection is that the IAU has claimed to be the sole authority on anything dealing with extraterrestrial objects.

that might be your objection, but that is nothing like your original rant against the IAU destroying the english language. They changed their own taxonomic system, thats all.

Of course you didnt invent the phrase, the point is that it is not an analogue for dwarf planets. And you better hope those "many planetary scientists" dont write the textbooks because then kids will have to memorize at least 15 different "planets".
nkalanaga
3 / 5 (2) Sep 22, 2012
They have to memorize 50 states, if they live in the US, so why would 15 planets be so bad? If that's the reason for deciding dwarf planets aren't planets it seems a little flimsy. By the IAU's reasoning, and I have seen this in print, an object the size of the Earth, in the Kuiper Belt, would still not be a planet.

Besides, most kids today wouldn't memorize the planets anyway. They'd just look them upon their phone.
Q-Star
2.3 / 5 (3) Sep 22, 2012
By the IAU's reasoning, and I have seen this in print, an object the size of the Earth, in the Kuiper Belt, would still not be a planet.


You seem not to know what the IAU's reasoning is. Or the actual definition they use. Google it up, there are only three simple requirements that must be met.

By the by,,, if you don't like the IAU and their definitions, then just fore-go them. Your other option is to pay your dues, join and then move to have the body change it's definition of planet. Do you also rail against the American Medical Association for their internal decisions? The American Bar Association?

It's a private & non-governmental organization, made up of professionals who have been accepted on their credentials, paid their dues. As such they are not required to accept the wishes and whims of amateurs or the lay public at large.
T2Nav
not rated yet Sep 22, 2012
I halfway agree with Aidoru. While I want the astronomers to use the most accurate numbers possible, since I'm not an astronomer or doing anything important with the figures, whenever I see AU I immediately convert it to 100 million miles. :)
nkalanaga
not rated yet Sep 24, 2012
Yes, I do know their definition. Roughly, it says "any nonstellar object, in orbit around a star, which has not gravitationally cleared its neighborhood, is a dwarf planet." The "classical" planets all orbit in cleared spaces, except for temporary intruders, or Trojan asteroids, which in turn are gravitationally controlled by the planet.

Dwarf planets, on the other hand, share their orbital space with objects not dominated by them, such as the main belt asteroids, and the objects in the Kuiper Belt. Even an object as massive as the Earth couldn't clear an entire orbital zone of the Kuiper Belt, thus it would be a dwarf planet. As an example, there are numerous bodies in orbits similar to Pluto, classed as "plutinos", this Pluto is not a planet by the IAU's definition.

And, yes, BY THEIR DEFINITION, it is not a planet, I will agree. However, many other planetary scientists prefer a definition based on mass rather than orbits.
fmfbrestel
not rated yet Sep 24, 2012
And, yes, BY THEIR DEFINITION, it is not a planet, I will agree. However, many other planetary scientists prefer a definition based on mass rather than orbits.


good then we finally agree that this:

In effect, they were telling people that they, not the speakers of English, would define the word "planet".


Is wrong. Some people may have mistakenly taken offense, but the IAU changed "THEIR DEFINITION", not THE ENGLISH LANGUAGE.