Displaying Distances In Light Years

[Chuft-Captain]

Please remember that Lua/CelX is a general purpose programming language. It seems to me that providing a units conversion popup would be a reasonable project for someone who would like to learn more about Celx scripting.

Not a bad idea Selden, however this would only be useful if it could directly calculate conversion factors determined by constants somewhere in the core code, rather than by the CelX creator, and therefore still perform the correct conversions when these values change. For example, I think I saw a recent post (can't find it now) where Chris has modified the length of a light year. (number of Km's in a LY, that is)

[Chuft-Captain]

The actual, exact length of the light-year depends on the length of the reference year used in the calculation, and there is no wide consensus on the reference to be used.

Fridger, Is there wide consensus on the length of the reference AU used as the basis for the parsec calculation?
I assume there must be an accepted international agreement as to the size of an AU (in km) to be used to define a parsec.

[BlindedByTheLight]

Notwithstanding the parsec versus LY argument, I think it would be a useful feature to allow user control over ANY displayed units, at any time, within reason. (This could be a toggle thru the different distance scales from metres -> km -> AU -> LY -> parsecs -> kPc -> MPc ) Have I missed any???

BTW. Velocity as well as distance should be also expressed in the specified units. eg. m/s -> km/h or km/s, etc...

This would allow both the purists and the plebsto be happy, making the argument superfluous.

How about printing out radii of celestial objects in miles? After all this unit is still used a lot in American/British schools and daily life. Right?


Bye Fridger

Notwithstanding the parsec versus LY argument, I think it would be a useful feature to allow user control over ANY displayed units, at any time, within reason. (This could be a toggle thru the different distance scales from metres -> km -> AU -> LY -> parsecs -> kPc -> MPc ) Have I missed any???

BTW. Velocity as well as distance should be also expressed in the specified units. eg. m/s -> km/h or km/s, etc...

This would allow both the purists and the plebsto be happy, making the argument superfluous.

May I suggest that we are missing the essence of the problem. To me (and, potentially, speaking on behalf of students who wish to visualize distances) it doesn't matter so much WHAT the unit is that Celestia uses, so much as the fact the unit changes as the distance increases that makes it difficult for me to visualize the comparison of two distances.

As you move away from something, Celestia goes from kilometers to AU's to light-years to kilo-parsecs. I seem to recall a time when it just went from kilometers to light-years. That my feeble cortex could handle. I would marvel at something that was 1 light-year away, then continue to travel and marvel that I was now 500,000 times further and so on. With the unit change, it's hard to get a sense of comparative distances.

It seems we could please all parties by a new preference and a sub-preference. One toggles the "changing metrics" - so Celestia either stays in one distance scale or goes to progressively higher ones. Then, if you choose to stay in just one distance, there is a sub-preference that appears which gives you the option to choose which one (l/y's, parsecs, etc). I could go one step further to say that if you choose to stay in one, it always starts in kilometers then switches to the one you've chosen for the larger distances.

[t00fri]

the difference is only a factor of 3.26, anyhow. Yet 'parsec' is the unit that is uniformly used by professional astronomers/astrophysicists/cosmologists.

While i understand your need for accuracy, and so the preference for parsecs, to me it is just confusing... Because light-years gives me a sense of where i am (no matter how illusory the feeling is, since one really can??t grasp the scale of even a light-year), it still is a clue i can orient myself by. Parsecs i have no gut feeling about at all.

It seems to me the optimal solution would be the ability to toggle between the two. Maybe one could set the option in the config file, with parsecs as default to cater to the science purists like yourself

- rthorvald

Runar,

firstly, lightyears are also bad units as to intuition:
The name sounds like a time (year) but it measures a distance!

Secondly, as I emphasized earlier, there are various inequivalent and NON-standardized definitions of what one should take as the reference year during which the distance is measured which the light travels....

When you learned the notion of a lightyear in school, which definition of a year did your teacher use and how did he justify taking a particular one? . Or did you never learn that there are various different definitions of a "year" in astronomy? A lightyear is just conceptually ambiguous and that's BAD.

Thirdly, a good unit should be definable with a SINGLE measurement.

Indeed, the Parsec is defined through a single angular measurement (parallax) in the sky. That is quite unlike the ly unit. A parallax is a perfectly intuitive concept, except most teachers miss badly to introduce it early on. Hence later people develop a similar resistance against parsec as anglo-saxons do wrto metric units....

I think if our "celestial teachers" want lightyears or a unit switcher , they may easily use Lua and print out whatever they please. That's what Lua is there for.

Cheers,
Fridger

[Cham]

Fridger,

Firstly, Celestia uses LY units in its STC and DSC definitions. For consistency, it should also indicate distances in LY.

Second, the definition of the LY can be made rigorous. That's really not a problem.

Thirdly, it is MUCH more intuitive to tell distances in LY, since it's just the distance made by light in one Earth-year (wathever the year you are using, be it 365 days, or 365.25 days, which is just a matter of convention). The comparison with inches is irrelevent.

[t00fri]

Fridger,

Second, the definition of the LY can be made rigorous. That's really not a problem.

Of course, one may calculate consistently and rigorously with ly units. Noone has ever doubted this. It's the conceptual ambiguity that makes ly's a bad unit in physics. That's also the reason why it's never used among astronomers/astrophysicists/cosmologists. There must be a reason for the exclusive use of parsecs, right?

Thirdly, it is MUCH more intuitive to tell distances in LY, since it's just the distance made by light in one Earth-year (wathever the year you are using, be it 365 days, or 365.25 days, which is just a matter of convention). The comparison with inches is irrelevent.

I disagree about intuition here. The name of a lightYEAR is bad and thus NOT intuitive. Moreover to measure an angular distance of 1 arc second in the sky is equally intuitive or unintuitive. I can't see any real difference.

Bye Fridger

[Reiko]

Well I like light year because that's what I'm used to. Just like I'm used to thinking in kilometers instead of miles.

[Cham]

Fridger,

do you think that the eV unit is bad too ? It's the same kind of definition as the LY. I don't find it ambiguous at all.

And I've very often seen published papers on astronomy/astrophysics/cosmology which are using the LY unit. There's no real consensus "out there" about the "right" or "wrong" units (LY vs psc), especially considering all the large uncertainties on astronomical distances...

[t00fri]

Fridger,

do you think that the eV unit is bad too ? It's the same kind of definition as the LY. I don't find it ambiguous at all.

And I've very often seen published papers on astronomy/astrophysics/cosmology which are using the LY unit. There's no real consensus "out there" about the "right" or "wrong" units (LY vs psc), especially considering all the large uncertainties on astronomical distances...

Well, Cham, the papers I am using in my daily professional work virtually never use lightyears for good reasons. I am working at a leading theoretical institute where we have about 3 (!) seminars/week on astroparticle physics /particle cosmology. I think I should simply know this don't you think so?

Even the Wiki says this about parsec/lightyear:

Since the parallax method is the fundamental calibration step for distance determination in astrophysics, its unit of choice, the parsec, is the most used unit of distance in scholarly astronomical publications. Newspapers and popular science magazines prefer a more intuitive unit, the light year.

The parsec is preferred because it can be more easily derived from, and compared with, observational data. However, outside scientific circles, the term light-year is more widely used.

Anyway, it's all I have to say about this issue.

Bye Fridger

[t00fri]

OK, one more citation:

Here is a quotation from our "bible": The Berkeley Particla Data group, a consortium of world renowned experts who set up the best and recommended values of measured constants for our community.


Here is the two lines about parsec and lightyears in the astrophysical constants section

http://pdg.lbl.gov/2006/reviews/astrorpp.pdf

parsec (1 AU/1 arc sec) pc 3.085 677 580 7(4)?— 1016 m = 3.262. . . ly [9]
light year (deprecated unit) ly 0.306 6 . . . pc = 0.946 1 . . .?— 1016 m

...and so on

Bye Fridger

[Cham]

So what ?

It doesn't make the LY obsolete. It's still in use (for a good reason). It's a usefull unit, and Celestia SHOULD show that unit too, especially since it's already using it in its DSC and STC definitions, and because IT'S USEFULL IN THE CLASSROOM and for the general public. Celestia isn't just for the scientific community (which is still using the LY anyway, despite your claims).

[Hungry4info]

Hmm...

Parallax, as I understand it, is a measure of how much an object moves in the sky as the Earth orbits Sol. Since Earth's orbit is eccentric, an object at one location in the sky will not appear to move as much as another object at equal distance at a different part of the sky (I would imagine that the natives of Sedna or HD 80606 b would not use parallax-based measurements).

But the point about the term "year" having multiple definitions is also a valid point. Hence, let's not use "year" anyway. "Year" is relative to your definition, and can vary due to differential definitions.

Lights speed is absolute. So is radioactive decay rates. A second will never be any longer or shorter than the conventional second, as cesium decays at a constant rate. So I believe "Light second" to be a valid term. We could perhaps finalize the term "year" accordingly.

[ajtribick]

How much do the various definitions of the term "year" compare to how accurately we know cosmological distances anyway? We already don't include light travel time effects outside the solar system because of the imprecision to which these distances are known.

Since the AU is defined in terms of the Gaussian year, maybe use that for the definition of "light-year"?

There is always an issue of amiguity here when working at all these different scales - at some point, the definitions of long length scales such as AU or parsecs come down to properties of our solar system such as the solar mass, the Earth's orbit, etc.

The standard unit of mass refers to a lump of metal in Paris.