I know how a parsec is defined, and it is about 4 ly (don't recall the exact number..I'll check Wiki later).

If it is so similar to a lightyear, why do we need this unit? Is it used in calculations for distant objects?

A lightyear as a unit means more to me then a parsec does, as I think it is an easier definition to comprehend

Any thoughts or insight?

Pete

__________________
PJE

There's so much I don't know about astrophysics. I wish I had read that book by that wheelchair guy.

3.2 (ish) light years. It's based on some archaic measurement of Parallax, or something (not that the measurement isn't still valid, it's just not needed...).

I don't see the parsec as needed, personally. But it is interesting, if only because of that archaic parallax info. More trivia than needed, to me at least.

It's been pointed out on another thread that we don't need AU (astronomical unit) or even lightyear. The distance to the Sun is about 150 gigameters, a fairly round number. The lightyear is 10 petameters (no animals were harmed in the creation of this post), the parsec maybe 10pi petameters--throw the pi in there to remind us we're talking arcseconds.

Professional astronomers have some good reasons for using parsecs. I listed a few in this little document written more than ten years ago.

http://stupendous.rit.edu/richmond/answers/parsecs.txt

It's likely that the answers will be just as good ten years from now.

If _you_ don't want to use them, fine. No one is forcing you to do so. If you want to read the technical literature, though, you'll have to understand them.

Why throw away a unit of measurement that sounds really good in sci-fi movies???

...I'd be behind getting rid of rods and furlongs, tho.....

It's pretty much like saying to people who has been using the metrical system (centimeters, meters, kilometers, gigameters, terameters, etc) to use the imperial system (inch, foot, yard). They have been using parsecs for a while, as well as AU and LY. Why change it now?

Just my 0.02.

I would agree that the word parsec is a cool sounding unit, even if a particular sci-fi movie didn't quite understand the difference between time and distance

Pete

__________________
PJE

There's so much I don't know about astrophysics. I wish I had read that book by that wheelchair guy.

Astronomers like to use units which are tied to observables as directly as possible. So, for instance, when astronomers talk to each other about cosmologically distant objects, they don't use distances at all, they use redshifts. They do this because the distance is derived from a cosmological model, so it depends on assumptions in the model. But the redshift is measured. So a cosmological object may have any number of model dependent distances associated with it, but only one measured redshift.

But in the case of nearby objects, we can measure the parallax, and derive the distance from geometry, the mathematics of which is not model dependent in the way of other, physical models. Distances in parsecs refer directly to the measured parallax, so the parsec is a measurement based unit, not a model based unit. So it makes more sense to use the parsec than it does to use an SI or metric unit system.

__________________
Don't try this at home - We're what you call "professionals" - MythBusters.

All measurement systems are arbitrary, metric included. Makes no difference to the actual distances being measured.

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

if you get rid of the parsec, then how do you know who got thru the Kessel run the fastest?

__________________
"blacker than the blackest black... times infinity."- Nathan Explosion
The.. Best.. Thread..Ever...

Is is comparable to using radians rather than degrees for Calculus?

Radians are a ratio, so have no unit at all. Is it the same for a parsec? This has no bearing on my life whatsoever, I'm just curious!

Pete

__________________
PJE

There's so much I don't know about astrophysics. I wish I had read that book by that wheelchair guy.

No, parsec is a real distance unit, like meters and furlongs.

We could just start measuring stuff in non-arbitrary units.


Three hundred and thirty five trillion Plancks. Or, about about a tenth of a micrometer. :P

I'm partial to lightnanoseconds

but then, that's what I grew up with

One profession's unit-of-convenience is the rest of the world's conversion error. Astronomy is saturated with conversion nightmares.

__________________
jwj

If you always believe what you already know, you can't learn anything - Liz

Parsec is a fine example of the type of jargon that professionals in an academic field dream up to set themselves apart from the masses. It is only half an order of magnitude longer than the older term light year, making it unnecessary. The term light year is a marvelous scientific unit that actually weds the concepts of distance and time. Its use is far more meaningful to anyone, whether a professional astronomer or not. I suggest that astronomers stick to the term light year for stellar distances, even when writing a scientific paper. However, the term astronomical unit (Earth-Sun distance) is the best one for getting readers to picture distances in the solar system. Stating those distances as millions or billions of kilometers of miles is meaningless to most people on Earth.

__________________
Curt Renz - "Centaur"
For monthly astronomical calendar visit:
www.CurtRenz.com/astronomical.html

First you insult us astronomers, then you make claims which are demonstrably false, and finally you tell us what to do.

Has this approach been effective for you in other spheres of activity?

I found that unusually harsh, even for me. It appears that this:
was what set you off. I also disagree with Centaur about this. While the light-year makes for a good colloquial unit of measurement that is immediately understood, it is not a practical one for measurement. We can't roll out lightyears-long tape measures nor can we bounce radar off the stars.

The parsec is related to parallax measure and that is how we measure the distances to the nearest stars. I will also point out that the distance to the Moon used to be given, not in km or AU but as a parallax (based on the Earth's equatorial radius), again because that is what could be measured and is also most relevant to observing the Moon against the background of stars. (The Moon's parallax is about a degree and thus quite noticeable.)

Another reason for the survival of the parsec is that all absolute magnitudes for stars, clusters, galaxies, etc. are defined using the distance of 10 parsecs as the reference distance.

Finally, one quick question: how many AUs in a light-year? Umm, umm, no number comes readily to mind. Now AUs in a parsec is another matter because of the definition: there are 648,000/pi or approximately 206,264.8 AUs in a parsec. Something to think about.

Edited to add: The number of AUs in a parsec had a typo. It is now correct.

__________________
Microsoft is over if you want it.

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

[sarcasm]well, that really clears that up. that's a nice, simple conversion factor that even a 9th grade dropout can do in their head.[/sarcasm]

__________________
"blacker than the blackest black... times infinity."- Nathan Explosion
The.. Best.. Thread..Ever...

I am reminded of the science class in which the students were asked to invent the most bizarre, convoluted, counterintuitive system of units they could think of. The unit for electric charge they came up with was the charge on one hogshead of electrons. (How densely do electrons pack if you put them in a barrel, anyway?)


When I first heard the term "parsec" as a unit of astronomical distance, I thought it was named for the French astronomer Jean Parsec. No, really!

__________________
The truth, as always, is more complicated than that.

[sarcasm]If a 9th-