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Earth Space Science

Astronomers Fix the Astronomical Unit 182

Posted by Soulskill from the and-changed-the-oil dept.
gbrumfiel writes "The Astronomical Unit (AU) is known to most as the distance between the Earth and the Sun. In fact, the official definition was a much more complex mathematical calculation involving angular measurements, hypothetical bodies, and the Sun's mass. That old definition created problems: due to general relativity, the length of the AU changed depending on an observer's position in the solar system. And the mass of the Sun changes over time, so the AU was changing as well. At the International Astronomical Union's latest meeting, astronomers unanimously voted on a new simplified definition: exactly 149,597,870,700 meters. Nobody need panic, the earth's distance from the sun remains just as it was, regardless of whether it's in AUs, meters, or smoots."
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Astronomers Fix the Astronomical Unit More

Astronomers Fix the Astronomical Unit

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  • let's not waste significant digits! (Score:5, Funny)

    by markhahn ( 122033 ) on Friday September 14, 2012 @12:30PM (#41336921)

    you'd think they could have rounded up to 150 gigameters.
    if politicians can be SD-conservative, why can't astronomers? we all know that significance is precious and rare...

    • Re:let's not waste significant digits! (Score:5, Funny)

      by girlintraining ( 1395911 ) on Friday September 14, 2012 @12:34PM (#41336993)

      if politicians can be SD-conservative, why can't astronomers? we all know that significance is precious and rare...

      It was decided by committee. I'm sure it was a compromise of several possible values, with concessions on each side, a few attempts to filibuster it until Pluto was given recognition again, etc. No, I'm not trying to be funny.

      • Re: (Score:3)

        by jdray ( 645332 )

        I'm sure it was a compromise of several possible values, with concessions on each side, a few attempts to filibuster it until Pluto was given recognition again, etc. No, I'm not trying to be funny.

        Well, you succeeded anyway.

      • If they rounded it up to 150 million kilometers people would just remember it, much better if people have to look it up and be reminded who defined it. Yes, I'm being cynical.

    • Re: (Score:2)

      by cp5i6 ( 544080 )
      If they're goint o pick an arbitrary number, why even make it so complicated.

      they should just say 1 AU = 42 and be done with it.

    • Most of the time, 150 gigameters will probably be close enough, similar as to how 300,000 km/s is "close enough" to the speed of light for many things or 3.1415 is "close enough" to pi for many things.

      • Re: (Score:2)

        by icebike ( 68054 ) *

        Most of the time, 150 gigameters will probably be close enough, similar as to how 300,000 km/s is "close enough" to the speed of light for many things or 3.1415 is "close enough" to pi for many things.

        Well the speed of light can be measured fairly precisely and Pi is available to just about any random number of digits you want.
        You are free to choose the level of precision that fits the problem at hand.

        The distance to the sun, on the other hand, was always imprecise, and constantly changing. It depended on when you measured it.
        Apparently this drift in the AU constant started to matter in some calculations, and perhaps threatened interpretation of historical references and calculations.

        The difference of u

        • Re: (Score:3)

          by doshell ( 757915 )

          Well the speed of light can be measured fairly precisely

          Actually, the speed of light is not the result of a measurement, because the meter is defined in terms of the speed of light. The speed of light, by definition, is always 299,792,458 metres per second [wikipedia.org].

      • Screw your decimal, 22/7 is good enough for me.

    • Or rounded down to 137,438,953,472 meters - 2^37 meters.

    • Re:let's not waste significant digits! (Score:5, Interesting)

      by icebike ( 68054 ) * on Friday September 14, 2012 @12:54PM (#41337329)

      you'd think they could have rounded up to 150 gigameters.
      if politicians can be SD-conservative, why can't astronomers? we all know that significance is precious and rare...

      Interesting point.
      If you are going to pic arbitrary number, why not pick an easy one?

      I suspect there is a desire to keep all past references to AU meaningful within a small margin of error, so as to not have to translate any written works.
      The difference between the new arbitrary number and the prior imprecise one is probably infinitesimally small for the scale of reference AUs are use for.

      Rounding it up almost half a million kilometers (quarter million miles) maybe not so much.

      I suspect that since it was imprecise in the first place, and used for almost nothing except astronomical reference, preserving existing references in the literature was more important than the ease of writing it down.

      • Re:let's not waste significant digits! (Score:5, Informative)

        by hde226868 ( 906048 ) on Friday September 14, 2012 @01:01PM (#41337435) Homepage
        This is correct. originally the AU was defined as the average distance between the Earth and the Sun. The problem then was to convert this distance to meters. The way to do this conversion in the end involves the product of the mass of the Sun and the Gravitational constant G. Both quantities are not well known (e.g., G is known to 4 or 5 digits only). But their product can be determined from modeling the motions in the Solar system to much higher precision. So by that time the AU was then redefined by defining the product GM (often called k^2, where k is called the "Gaussian gravitational constant"). It is my understanding that this has now been simplified. The difference between both is only a few meters.
    • You'd think we would at least use gm instead of million km. What's with the bizarre preference for km for long distances? (Not just here, but darned near everywhere.)

      • Re: (Score:2)

        by doshell ( 757915 )

        It becomes particularly not-so-funny when you have to constantly make the distinction between American and European billions (as is the case e.g. with money). If everyone agreed on using the giga and tera prefixes, that would never be a problem.

    • I'm sure it's 150 gigameters when you use PI=3 in the equations.

  • No, panic. (Score:5, Funny)

    by girlintraining ( 1395911 ) on Friday September 14, 2012 @12:31PM (#41336933)

    Nobody need panic, the earth's distance from the sun remains just as it was, regardless of whether it's in AUs, meters, or smoots."

    I'm more concerned about the fact that the distance changes depending on where we are. That means that the Earth is moving, and I don't believe in that. It's more heliocentric non-sense by the astronomical community. What next; astronomical bodies that aren't perfectly spherical? The madness of the commoners, I tell you.

    • Relax, would you? The equant [wikipedia.org] lets it all fit back together nicely. Ptolemy's Standard Model still fits the data; there's no need to bring pseudoscience like heliocentricity into this.

      • Relax, would you? The equant lets it all fit back together nicely. Ptolemy's Standard Model still fits the data; there's no need to bring pseudoscience like heliocentricity into this.

        You are further proof that astronomers have absolutely no sense of humor.

        • You might want to read my post again. I was playing along with a joke that you yourself had started, so, basically, you just whooshed yourself.
          • In space you can't hear you "Whoosh".

          • You might want to read my post again. I was playing along with a joke that you yourself had started, so, basically, you just whooshed yourself.

            No, I just played along with your joke of my joke, which apparently resulted in a black hole.

    • Re: (Score:2)

      by keytoe ( 91531 )

      I'm more concerned about the fact that the distance changes depending on where we are. That means that the Earth is moving, and I don't believe in that. It's more heliocentric non-sense by the astronomical community. What next; astronomical bodies that aren't perfectly spherical? The madness of the commoners, I tell you.

      Spheres? Heathen!

  • Distance remains the same? (Score:3, Informative)

    by wierzpio ( 570121 ) <piowieNO@SPAMgmail.com> on Friday September 14, 2012 @12:34PM (#41336991)
    Since the Earth's orbit around the Sun is eliptical it's _never_ the same, is it?

    • Re:Distance remains the same? (Score:5, Funny)

      by mblase ( 200735 ) on Friday September 14, 2012 @12:37PM (#41337035)

      Since the Earth's orbit around the Sun is eliptical it's _never_ the same, is it?

      Even an elliptical orbit is right twice a year.

      • Assuming the sun as at the center of the ellipse - which I believe it isn't in this case. So it's right roughly once a year.

        • Re: (Score:2)

          by jstave ( 734089 )

          Assuming the sun as at the center of the ellipse - which I believe it isn't in this case. So it's right roughly once a year.

          Actually, depending on how big the ellipse is compared with the circle with a 1 AU radius, it could be right from 0 times (circle way too big, or way too small), to as many as 4. Play around with an ellipse and a circle centered at one focus and you'll see what I mean. As the relative sizes change, the number of times it's right changes too. Thank god we got *that* straightened out.

          • It couldn't be zero (circle way to big/small) if 1 AU is based on the average distance. But you're right that it could be 4 (or 3!) in addition to 2.

        • Re:Distance remains the same? (Score:5, Interesting)

          by Baloroth ( 2370816 ) on Friday September 14, 2012 @01:13PM (#41337605)

          The center is actually the center of mass for the Earth-Sun. Actually, I believe it's the center of mass for the whole solar system, but if we treat it as a 2-body problem it's just the Earth-Sun. If only the Earth was affecting the Sun with it's gravity, the distance would be right twice a year (assuming the major or minor axis) or 4 times (if you use some other axis), since the Sun would be traveling in an ellipse identical to the Earth's but proportionately smaller, so it would be on the fall on the axis at the same time as the Earth would every single year.

          In reality the Sun is also moved by the other planets, so the distance will never be correct, since it isn't moving on a pure ellipse at all. Also the Earth isn't either. That's why we use the average distance over a few years, since that will always be the average. Except for the fact that the Sun is losing mass, and therefore gravity, so Earth gets further away every year, so the average is itself changing.

      • Unless it's a min or a max, it's right four times a year!

    • The Astronomical Unit (AU) is known to most as the mean distance between the Earth and the Sun

      The summary omitted the word "mean". The linked article has the correct description.

    • The semi-major axis is usually called the "distance" because it's equal to the averaged mean distance between the planet and the foci (one of which is the sun). Though that depends what you average over. I think if you average over time it's approximately equal if eccentricity is small.

  • The new AU ... (Score:5, Funny)

    by PPH ( 736903 ) on Friday September 14, 2012 @12:38PM (#41337055)

    ... will be established by machining a bar of pure platinum to a length of exactly 1 AU. It will be stored in a vault in Paris.

  • Eve nav problems (Score:2, Funny)

    by Anonymous Coward

    Great, everyone in Eve is going to be missing jump gates, plowing through asteroid fields at warp. Going to be chaos.

    • But without these precise calculations, you could fly right through a star, or bounce too close to a supernova! That'd end your trip real quick, wouldn't it?

  • ... because it's not an SI unit.

  • I'd have gone for 149,896,229,000m (Score:5, Interesting)

    by wonkey_monkey ( 2592601 ) on Friday September 14, 2012 @12:52PM (#41337311) Homepage
    I know it's a bit out, but I'd go for 149,896,229,000m - exactly 500 light-seconds.

  • Good news! (Score:3)

    by GodfatherofSoul ( 174979 ) on Friday September 14, 2012 @12:55PM (#41337345)

    Now when I read an article about an Oort cloud object 10,000 AU from the Earth, I'll know to scrub off that extra 2000 km from my mental model.

  • I mean, how many Libraries of Congress is this new measurement?

  • Thank you, Bob Barker. (Score:5, Funny)

    by Minwee ( 522556 ) <dcr@neverwhen.org> on Friday September 14, 2012 @01:26PM (#41337851) Homepage
    Now that it's "Fixed", it's technically an Astronomical Eunuch.
  • ... lengths of football fields. Or school buses lined up end to end. Or number of King Georges standing with arms out stretched touching finger tips to finger tips stretching all the way from the center of Earth to center of Sun. That is the kind of units that makes sense. Not this convoluted French thingies that we don't even agree on the right way to spell, meter? metre? what the hell?

  • How much is that in Cesium atom wavelengths? (Score:4, Funny)

    by jfengel ( 409917 ) on Friday September 14, 2012 @01:29PM (#41337909) Homepage Journal

    Or more correctly, units of c times the period of "radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom". Let's get this down to fundamentals and not muck about with intermediate convenience units like "meters".

  • We can rest easy now.
  • If the distance is going to be bouncing around for various reasons it sounds like it isn't a good measurement. Personally I've always liked using light second as a measurement.

  • There's no point in a unit (AU) being a large multiplier of another unit. We have an entire metric system for that (well, some of us do). The nice part about AU was precisely that it represented something dynamic. I don't always care how far away some asteroid is to the metre. I want to know how far it is relative to the sun.

  • ...Now fix the kilogram.

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