Artist's concept of Mars opposition on December 24, 2007. The distances between the sun, the planets and the distant nebula are not to scale.
Image: NASA Mars Exploration Program
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By Geoff Brumfiel of Nature magazine
Without fanfare, astronomers have redefined one of the most important distances in the Solar System. The astronomical unit (au) — the rough distance from the Earth to the Sun — has been transformed from a confusing calculation into a single number. The new standard, adopted in August by unanimous vote at the International Astronomical Union's meeting in Beijing, China, is now 149,597,870,700 meters — no more, no less.
The effect on our planet’s inhabitants will be limited. The Earth will continue to twirl around the Sun, and in the Northern Hemisphere, autumn will soon arrive. But for astronomers, the change means more precise measurements and fewer headaches from explaining the au to their students.
The distance between the Earth and the Sun is one of the most long-standing values in astronomy. The first precise measurement was made in 1672 by the famed astronomer Giovanni Cassini, who observed Mars from Paris, France, while his colleague Jean Richer observed the planet from French Guiana in South America. Taking the parallax, or angular difference, between the two observations, the astronomers calculated the distance from Earth to Mars and used that to find the distance from the Earth to the Sun. Their answer was 140 million kilometers — not far off from today’s value.
Until the last half of the twentieth century, such parallax measurements were the only reliable way to derive distances in the Solar System, and so the au continued to be expressed as a combination of fundamental constants that could transform angular measurements into distance. Most recently, the au was defined as (take a deep breath): “the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.01720209895 radians per day (known as the Gaussian constant)”.
The definition cheered fans of German mathematician Carl Friedrich Gauss, whose constant sits at the heart of the whole affair, but it caused trouble for astronomers. For one thing, it left introductory astronomy students completely baffled, says Sergei Klioner, an astronomer at the Technical University of Dresden in Germany. But, more importantly, the old definition clashed with Einstein’s general theory of relativity.
As its name implies, general relativity makes space-time relative, depending on where an observer is located. The au, as formerly defined, changed as well. It shifted by a thousand meters or more between Earth’s reference frame and that of Jupiter’s, according to Klioner. That shift did not affect spacecraft, which measure distance directly, but it has been a pain for planetary scientists working on Solar System models.
The Sun posed another problem. The Gaussian constant is based on Solar mass, so the au was inextricably tied to the mass of the Sun. But the Sun is losing mass as it radiates energy, and this was causing the au to change slowly as well.
The revised definition wipes away the problems of the old au. A fixed distance has nothing to do with the Sun’s mass, and the meter is defined as the distance traveled by light in a vacuum in 1 / 299,792,458 of a second. Because the speed of light is constant in all reference frames, the au will no longer change depending on an observer’s location in the Solar System.
Redefining the au has been possible for decades — modern astronomers can use spacecraft, radars and lasers to make direct measurements of distance. But “some of them thought it was a little bit dangerous to change something,” says Nicole Capitaine, an astronomer at the Paris Observatory in France. Some feared the change might disrupt their computer programs, others held a sentimental attachment to the old standard. But after years of lobbying by Capitaine, Klioner and others, the revised unit has finally been adopted.
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33 Comments
Add CommentThe most stupid unit for measuring length. Nothing wrong with light years, minutes or seconds. Who needs a fixed length for this variable unit? Next you will be defining a fixed radius for every ellipse.
Reply | Report Abuse | Link to thisPut the Astronomical unit in the bin where it belongs with miles,feet,inches and the rest of the historic legacy of various cultures.
This news should make you happy, then. Since they defined the AU to be exactly 149,597,870,700 meters, I quickly jumped on Google and typed in "149,597,870,700 meters to light-seconds". In a split-second, it gave me this result: 149 597 870 700 meters = 499.004784 light-seconds. So, for you, just convert 1 AU to 499.004784 light-seconds in every context and you're golden.
Reply | Report Abuse | Link to thisI was unaware there was controversy surrounding this subject....
Reply | Report Abuse | Link to thisto scientific earthling: There is nothing inherently wrong with using miles, feet, inches or any other unit of "historic legacy". We use the timing of light beams and translate that into linear feet or meters used in millions of deeds and boundaries around the world. We depend on atomic clocks in satellite to give us precise 'linear measurements' to accomplish similar expressions in feet or meters. Making such an all-encompassing, ridiculous statement as you did shows your lack of understanding of history's contribution of scientific pursuits. It's probably best to keep quiet about things you know little about.
Reply | Report Abuse | Link to thisDoes this change in definition affect the parsec?
Reply | Report Abuse | Link to thisAlso, Google needs to update their calculator. It says 1 Astronomical Unit = 149 598 000 000 meters.
The light year, being based on the Earth year, is no less arbitrarily linked to the dimensions of the Solar system than the au. From the reductionist standpoint which you have adopted ALL units are 'stupid' being entirely relative to Earthbound concepts and far from exact in any case. There is no absolute standard that makes any unit better or more complete than any other and if there were it's extremely unlikely that the time measurement you set such store by would pass that test! Base 10 for fractions of a second, base 60 for minutes and hours, base 24 for days, base 365 and a bit for years. What kind of madness is that?
Reply | Report Abuse | Link to thisFeet and inches is a breeze by comparison. You can even be exact about 1/3 inch as indeed you can an au since its meter measurement's prime factors include 2, 3 and 5, whilst a light year can boast only 2 and 5.
WolframAlpha's got it though.
Reply | Report Abuse | Link to thisNorth Carolina is passing a law that would make it illegal for the earth to move towards or way from the sun at a nonlinear rate so this value should remain constant.
Reply | Report Abuse | Link to thisIt's true that our units are necessarily arbitrary, unless of course if we defined a unit entirely in terms of natural constants. I.e. even though the Planck length is measured in terms of meters, we could dispense with the non-physically fundamental unit of the meter and define the Planck length h as a unit in itself.
Reply | Report Abuse | Link to thisI'm not sure if h is invariant in all relativistic reference frames though. (I've yet to study general relativity.) If someone could clarify this I'd appreciate it.
Like curmudgeon said, a lightyear is based on 365 days, which is Earth-specific - and therefore not physically fundamental.
Hmm, these are interesting questions...
Hmmm, North Carolina really has their act together. We need to get other states to adopt the same law though, or the sun will act differently there. We should really get the states to ratify it for a constitutional amendment. Once that is done we get the Fed to float a trillion dollar bond to pay off all the politicians and the great sun law will be the law of the land!
Reply | Report Abuse | Link to thisNo more no less? ... But we're in an elliptical orbit!
Reply | Report Abuse | Link to thisAny idea why 'planetary nebulae' have nothing to do with planets? It's history. Planetary nebulae were named before large telescopes and CCD cameras were invented.
Reply | Report Abuse | Link to thisThe AU was named before it's value had been figured out. From Kepler's orbital laws (about 1600), we knew the ratio of the sizes of the orbits in the solar system. But we didn't know the value of any of the distances very accurately until really quite recently. The AU was used in computing parallax distances to the stars (starting in about 1800). The idea was that once the AU was well known, we'd be able to plug it in a variety of places to know those distances in absolute terms (in inertial frames of reference that aren't near large masses).
I have a 1986 vintage HP calculator (that i still use) that thinks an AU is 149,597,900,000 meters. That's off the new official value by about 2%. Science moves on, and the old standards have to be updated from time to time.
Why not simply round it to 150,000,000,000? Is the size of an AU really all that important that it has to be precise to 10 places?
Reply | Report Abuse | Link to thisI think it would be best to use Light Years as the base unit. Its time to be more mature
Reply | Report Abuse | Link to this@commenters: Note that the story says that the change passed "by unanimous vote". If 10,000+ professional astronomers can all agree on this, it must be long overdue! Obviously, any realistic alternative was dismissed, and everybody went home happy. What's the problem here?
Reply | Report Abuse | Link to thisIs this that million/billion confusion again? I read 149,597,870,700,149 as 149 billion.
Reply | Report Abuse | Link to thisThey have gone metric. OMG the solar system became French.
Reply | Report Abuse | Link to thisIn so far as possible, units of measurement in the International System are based on physical units. Thus the meter is defined as the wavelength related to (if I recall correctly) an isotope of cesium. The second is defined in terms of pulse time of hydrogen. If we take the year as 365.2425 days, admittedly an approximation, but consistent with the Gregorian calendar, the year is then 31556952 seconds, and a light year can be defined as accurately as the speed of light can be defined.
Reply | Report Abuse | Link to thisThe International System, for convenience, uses powers of ten to expand from basic units, thus avoiding the tangle of 12's, 60's and 14's of the English system. However, if the speed of light is given in terms of furlongs per fortnight, then a light-fortnight can be defined in terms of furlongs. (As if anybody cares.)
someone better check with God and see if she agrees.....
Reply | Report Abuse | Link to thisYou must have no idea what "expressing numbers to six significant digits" means.
Reply | Report Abuse | Link to thisSix significant digits is most reasonable in this case.
D.A.W.
"astronomical unit (au)"
Reply | Report Abuse | Link to thisThe abbreviation for astronomical unit is "A.U.", just as it is for most S.I. units, such as these:
ampere (A), coulomb (C), farad (F), henry (H), joule (J), kelvin (K), liter (L), newton (N), pascal (Pa), roentgen (R), siemens (S), tesla (T), volt (V), watt (W), and weber (Wb). Even the omega for ohm is a capital Greek letter.
The whole idea is that every ellipse has an average distance from its primary focus, and that is what they are talking about that. Also, the eccentricity of the orbit of the Earth is tiny. If you drew a "circle" on a chalkboard with a standard piece of chalk, the "ellipse" of the Earth's orbit would be contained within the thickness of the chalk line. You need to learn to pay attention to that which is important, and not "run off on tangents" about that which is inconsequential.
Reply | Report Abuse | Link to thisThe orbit if Venus is elliptical, yes, but its eccentrictity is even less than that of the Earth. Just count Venus and the Earth as being in circular orbits.
In contrast, the eccentricity of the orbits of Mars, the Moon, Saturn, and Pluto are significant and they must be taken into account at all times.
D.A.W.
We use the French and American definitions of "trillion" and "billion" in the S.I. and most other places.
Reply | Report Abuse | Link to thisIn case you haven't noticed, other definitions of "billion" and "trillion" are just about dead. They are definitely archaic. Please enter the 21st Century.
D.A.W.
Why don't you look it up before you write anything?
Reply | Report Abuse | Link to thisYou have the resources of the Internet at your fingertips.
The length of the meter used to be defined in terms of the wavelength of orange-red light emitted by the isotope krypton-86, and this is still a very good practical standard. This is not hard to remember.
The length of a second is now defined in terms of the vibrations of an isotope of cesium in a maser.
These are two different kinds of "beasts": krypton for distance and cesium for time.
D.A.W.
"So, for you, just convert 1 AU to 499.004784 light-seconds in every context."
Reply | Report Abuse | Link to thisWhy not just define the A.U. to be EXACTLY 500.000 light-seconds by definition, and cut out all of the cockeyed decimal places that don't really do anything?
We're talking about the Solar System, not splitting the atom.
Two things:
Reply | Report Abuse | Link to this1. I taught introductory astronomy for twenty years. Nobody was ever confused by the definition of an A.U. It's the average distance from the Earth to the Sun. Not confusing. It would only be confusing to an advanced student who understood relativity-affected ellipses.
2. I disagree with those saying the unit is quaint and should be abolished. It is very useful for understanding solar system relationships. But it could easily be made exacty 150 million km. That makes sense, and further simplifies a simple idea.
It is not a great news as the distance from Sun to Earth as well as the diameters of all planets and the Sun as changing over a cyclic period of 4.32 million years. Our calculation shows that about 2.17 million years ago, the distance between the Sun and the planets and Moon and Earth as well as their respective sizes (rounded up)were as follows:
Reply | Report Abuse | Link to thisPresent Previous present distance Previous
diameter diameter from Sun distance
SUN 1392000 1261942 NIL NIL
MERCURY 4878 2104 57900000 15149700
VENUS 12104 7166 108200000 38696418
EARTH 12756 23236 149600000 62902953
MARS 6796 10955 227900000 118311123
JUPITER 143000 120894 778300000 746089634
SATURN 120000 214586 1427000000 1854025209
Hence we should be careful while using the AU.
About the A.U." "It's the average distance from the Earth to the Sun. Not confusing."
Reply | Report Abuse | Link to thisI agree - it is not confusing at all. I understood it when I was in elementary school. The A.U. was quite clearly explained in (nonfiction) books by such writers as Isaac Asimov and Sir Arthur C. Clarke.
People wait until college to learn about the A.U.?
This is a big problem when students never read anything or learn anything outside of school. Wow, there are libraries, and I guess that nobody ever heard of reading scientific tutorials on the Internet.
The main goal is to use a measurement unit that works for making astronomical measurements. In earlier times a foot was the average size of feet. It worked to some degree, as long as there were not any challenges to the precise measurement. So we are in the same situation here.
Reply | Report Abuse | Link to thisThe Earth is on an elliptical orbit so its precise average measurement is subject to disagreement. The reality is it matters little about what the perfect answer is. All that matters is we that we have a usable measurement tool or unit. Earlier people did not have to base "the foot" on human feet etc.
To put gravity in its pitiful context the sheer scale of our local system can be based on the AU, as presented by Robert Burnham Jr's "Celestial Handbook". The Model uses Old Money measurements:
Reply | Report Abuse | Link to this1)AU = 93 million miles
2)Number of inches in 1 mile = number of AU in 1 Light Year so 1 light yr = 1 mile
3)Earth orbit = 2 inches diameter
4)Sun = 880,000 miles diameter = 0.009inch = very fine pencil point, at the centre of a 1 inch radius circle
5)Pluto is an invisibly small spec about 3.5 feet from Sun
6)Extend your arms and you can just reach across the solar system (planets), if you are tall enough
7)The nearest star....4.5 MILES away!
8)Feel that isolation, a speck of dust, miles from its nearest neighbour
9)The collection of stars in our Milky Way galaxy is about 100,000 miles in diameter.
10)The next galaxy is separated by hundreds of thousands of miles of 'empty' space.
11)Galaxies exist in clusters and super cluster tubes are thought to exist
How can gravity have any effect over these distances???
Reply | Report Abuse | Link to thisINTRODUCTION OF A NEW UNIT TO MEASURE THE HEAVENLY BODIES
There are few units to measure the distances of planets, stars, galaxies etc. The new unit has been drawn by using the ratio of atom & electron with the value of Pi & Avogadro’s number which has taken as centimeter in the form of 2 x Pi^2 x Avogadro number x R & it shows the radius of the universe..
Astronomical Units
The well known astronomical units are normally used to measure the distance of planets, stars, galaxies etc at the present scientific world. These units are given below:
1. One astronomical unit = 1.49597892 x 10^13cm,
2. One light year = 9.4605 x 10^17cm
3. One parsec = 3.086 x 10^18 cm
One parsec (one parallax – second or parsec or pc) is 2.062863 x 10^5 & 3.2619840 (≈Pi=3.141592654) times larger than one astronomical unit and one light year respectively.
The ratio of 3.261984039 / Pi = 1.038321768 ≈ Pi^1/5= 1.03642035
Again we see that the ratio of one parsec & one astronomical unit = 207674.5476
But, Root of 207674.5476 = 455.7132296 ---- ( a )
Almost equal to ¼ x mass of atom / mass of electron = 455.7221325 --- ( b )
The ratio R = 1822.8885 which is known to us as the mass of atom is divided by the mass of an electron & one forth of this value is 455.7221325 tallies with the above value
The above numbers like Pi, Avo. Number, R has no units. If we consider the value of Avogadro No. as a length in centimeter, the length for calculating the distance of planets, stars etc, then we will able to measure the distance of celestials bodies and this will prove that this new unit is more active, sensitive and useful.
We can get radius of the universe as: 2 Pi^2 x Avo. No. x R = 2.1669 x 10^28 cm, Ref: 2x10^28 cm
Determination of different distance of planets of our solar system:
We can use this number as root of 2 Pi^2 x Avo. No. x R = 1.47204 x 10^14 and may treated as 1.47204x 10^14 cm ( Let = P) to measure the distances. For examples:
a) 1x P = 1.472x10^14 cm = mean distance of SATURN from SUN, Ref: 1.43x10^14cm,
b) 2x P = 2.944x10^14 cm = mean distance of URANUS from SUN, Ref: = 2.87x10^14 cm,
c) 3x P = 4.416x10^14 cm. = mean distance of NEPTUNE from SUN, Ref: = 4.5x10^14 cm,
d) 4x P = 5.881x10^14 cm = mean distance of PLUTO from SUN, Ref: = 5.9x10^14 cm,
Similarly we can measure the distances of other planets in the following way as:
e) The mean distance of JUPITER = P /2 = 7.36021x10^13 cm, Ref: = 7.78x10^13 cm.
f) The mean distance of Mars = P / 2Pi = 2.34x10^13 cm, Ref:= 2.28x10^13 cm
g) The mean distance of Earth = P / Pi^2 = 1.49x10^13 cm, Ref:= 1.5x10^13 cm
h) But the distance of moon from the Earth = root of Avo. No. / 2Pi^2 = 3.931384x10^10 cm, Ref: 3.85x10^10 cm
g) The mean distance of Venus = P / 4 Pi = 1.17x10^13 cm, Ref: = 1.08x10^13 cm
[Ref: http://photojournal.jpl.nasa.gov/cgibin/PIAGenCatalogPage.pl?PIA00559 )]
The calculated results are nearly equal to the experimental results. So, we may consider the distance P. The matter was responsible to create the distance as a hole. Nirmalendu Das.
Email: nirmalgopa@gmail.com
I haven't read all of the comments on this subject. The earth's orbit around the sun is elliptical so the AU is an intrinsically imprecise unit however you define it.
Reply | Report Abuse | Link to thisJustification of the equation root of (2 Pi^2 x Avo. No. x R).to measures the heavenly bodies without using the Astronomical Unit.
Reply | Report Abuse | Link to thisDistance of Earth from the Sun, Let P = 1.491490469x10^13 cm (Calculated Result)
149,597,870,700 meters = 1.495978707x10^13 cm (As per Scientific American, September, 2012)
Ratio between two units = 1.00300923
But 1 / 1.00300923 = 0.996999798 which is near value of:
Maximum value of Pi (3.152970491) – normal value of Pi (3.141592654) = 0.11377837.
Now 1 – 0.011377837 = 0.988622163.
(Ref: Complete Unified Theory, page- 424, 1998 written by me).
Then, 1.491490469x10^13 cm (Calculated Result) / 0.988622163 = 1.508655708x10^13 cm.
But, 1.495978707x10^13 cm x 0.988622163 = 1.478957705x10^13 cm.
According to report collected from Internet as “Why Earth is closest to Sun in Dead of Winter by Mary Lou Whitehorne , Date 2, January, 2007, time 09.00 AM ET”
Earth is Closest to Sun in Dead of Winter
“Earth reaches perihelion on January 3, 2007 (Figure C). The Earth-Sun distance will be 147,093,602 km. Aphelion, the greatest distance from the Sun, occurs on July 7, 2007, when the Earth-Sun distance will be 152,097,053 km”
.
The calculated result is supporting this distance within some period in a year. This P is obtained from equation as: root of (2 Pi^2 x Avo. No. x R). about this equation is discussed in this page. So, the New Unit may use to determine the distances of planets, stars, galaxies etc.
Nirmalendu Das. Dated: 26-09-2012.