New Scientist

I love this kind of science. It's a reminder, to me, that everything is all interrelated.

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The views expressed are the febrile product of an overactive imagination of a person who in shadows sees the gyrating Elvis-like ghost of Leonid Brezhnev.

I am philosophically opposed to the astronomical unit anyway. It is, in its way, as archaic as the Imperial units. It is based on something that has meaning only in this one speck of the whole universe and even that is variable - it must be averaged even over a year and, as the New Scientist article points out, it is truly only valid at this moment in time. Go too far back or forward in time and you would arrive at a different value.

Keep the AU if we must (it can be a useful way of discussing distances within the solar system), but base it on something less malleable, like the distance traveled by light in 8(ish) minutes. That would allow it to retain its usefulness without becoming a source of confusion for our descendants born on worlds orbiting other stars.

Another redefinition? I don't think they've recovered from the last one.

I just ignore AUs. I'm working on a story and I just use mega-klicks (1,000,000km) as a unit of distance measurment. While, I'm here, I'll say that I use Exaton as a measure of mass for planetary comparisons.

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Agreed. What I like about it is that it's an "in-your-face" reminder that the solar system is a dynamic place and there's a lot of really nifty things always going on...and it just another way to appreciate the beauty of E=mc^2.

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It would be nice if astronomers would simply use metric units. There is enough useful information that we should not forever be learning different distance systems and converting parsecs, lightyears, AUs, miles, rods, fingers, cupits, hands, feet and knots.

Simplify.

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It's ok not to know. We should try harder to find out.

Unfortunately I believe a minute, or any temporal measurement, is just as 'provincial' and unreliable as that which the AU is based on. Especially the quantity 8, again based on something as abitrary are our planet's current location.

As we as a species broaden our horizons, and as a conseqence, our possibilities, there are many such measurements, values and designations that will change. That must change. In fact, this process has already begun - the reclassification of Pluto, seen by many as a non-event at best and a farce at worst, is an example of the requirements posed on us all by our advancement and that of our knowledge.

This requirement will increase as we move forward, through time and certainly through space. Much has been considered about what calendar will be used for example, even what division of a 'day' will be adopted, as we establish colonies on planets in our solar system, where day/night cycles and seasonal periods will be vastly different from that of earth. While our circadian rhythm is a powerful dictator, it is - even now - somewhat flexible, and will no doubt be even more so as we go on, and outward, from our current home.

So I do not see any temporal foundation as being useful too far into the future - and hopefully, we humans have a very long future ahead of us. Some spatial constant perhaps may be a better choice. Something with a chance of being udnerstood, and perhaps even shared, by others, would probably be best.

Luckily a light year is approximately 10 to 16 th meters = ten million times a billion. It would get awkward for most of us to think about 7 trillion terra meters = 7 times ten to the 24 meters to a galaxy that is 700 million light years away. Neil

Awkward for those of us not used to that terminology, sure. But for those who grow up with that, converting "light-years" to real numbers is awkward.

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Well, while I agree about AUs, there's definitely a place for the standard Solar mass and Earth mass. After all, the only way we have to measure planetary masses is by their gravitational effects on other bodies. So, what we really end up knowing is the product of their mass and the gravitational constant. Since we can get remarkable precision in tracking the orbits, and hence in that product, but we can only nail down the gravitational constant to about four decimals, any orbit-based claim that a planet or star weighs X number of kg/tonnes/whatever else inherently assumes a certain value of G, which you don't know unless it's explicitly stated, in which case you might as well be giving that product of mass and G (also known by mu, the Greek letter).

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But the definition and units would be precise, even if the measurements are approximated.

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I don't see how the meter, originally based on the polar circumference of the Earth, is any less provincial than the AU?

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How common is the usage of AU in the astronomical circles?
Is a parsec directly based on AU? Is parsec very common in relation to LY?

I've always thought that it was more of a popular measurement rather than a scientific one which would mean precision is not necessary.
In other words, I always thought the experts used whatever units are better for thier usage, while relating it to the general populus is just converted to something they can understand.

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I agree.

Besides, it's a handy distance scale coincidence that there are similar numbers of AUs in a light year as inches in a mile.

Dave Mitsky

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Sure. At a distance of one parsec, an AU subtends an angle of one second of arc (arbitrary, blame the ancient Mesopotamians). That means that one parsec equals, by definition, 3600*180/pi = 206264.8 AU. For some reason, parsecs (and their larger cousins Mpc and Gpc) are much more commonly used in the research literature than light-travel-time distances. As if the definition of parsec were any less Earth-based than light-years.

Trivia question I seem to bring up every couple of years - without doing the numerical calculation, for which major planet in our system would the locally-based definitions of parsec and light-year be most nearly equal?

That's why I asked. Has parsec been scientifically defined in that way, or has it been locked into a more stable definition?

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Quite common in Solar System studies.
"Parsec" is short for "parallax (arc) second" and is the distance at which an object will have an annual parallax on Earth of one second of arc. This distance turns out to be:

(1 AU)/(1 parsec) = arctan(1 arcsecond) or pi/648000 approximately, or

1 parsec = 206,265 AU.

Actually, it is a professionally-used unit where high precision is unattainable.
Actually, in the Solar System the AU is a much better choice than millions of kilometers. 5.2 AU means 5.2 times the semimajor axis of the Earth's orbit, it means Jupiter (in this example) is 5.2 times farther away from the Sun than Earth. Not something easily gleaned from saying Jupiter is 780 million km from the Sun.

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The bar has been lowered for the promotion of ATM ideas; the bar for the acceptance of ATM ideas must remain high.

So; If I'm consolidating things in my mind correctly.
The units used are usually based on the magnitude of objects being described and are nearly equally "scientific".
AU for solar system objects.
Parsec for outside of the solar system for observational purposes.
LY for non-observational purposes.

Yes; but is that the official designation?

If so, then redefining AU makes sense to lock it down, but from observational perspectives, it makes sense to keep it fluctuating so the angular measurements are consistant.

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How different is AU from Light Year?

Isn't a light year dependent on how long it takes the earth to revolve around the sun? Doesn't that have meaning only in this one speck of the universe at this time?

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