# Parsec or Light year

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Evening all,

I'm currently in the middle of writing (desperately trying to finish by September 1st!!!) a new astronomy course for my school, and I'm currently on the "methods of distance" bit. Describing what a parsec and light year are is all very well, but are their instances when one is favoured over the other, or is it just personal astronomical taste? As far as I'm concerned, the light year is the more "popular" choice for your standard pub discussions, but parsecs are favoured by professional astronomers.

Anyone got any idea?!?

Rich

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"A parsec is defined as the distance from the Sun which would result in a parallax of 1 second of arc as seen from Earth.

Since distances of nearby objects can be determined directly using parallax observations combined with elementary geometry, the parsec was historically used to express the distances of astronomical objects from the Earth."

Looks as if the sun and the earth are in the equation one could use parsec. It is a distance from the earth. If the distance is between other objects the light year might be a better unit of measure.

But I could be wrong.

Bill

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I've been wondering when this question would crop up!

I've never liked the parsec - because it simply doesn't relate to something we can grasp in a straightforward way.

The lightyear makes sense to most people - once you explain that it gives a quick indication of how long the light has taken to travel from an object. If I point out a star in the sky, e.g. Arcturus, and remark that it's 36 light years away, I can immediately explain that the light we're seeing it by, started on its journey, say, when Harold Wilson was just stepping down as PM. People can relate to something like that. But to say that it's 11 parsecs away, and then have to explain what parallax means and what an arcsecond is - well you'll lose most non-technical people.

So I go for the lightyear because it makes astronomical distances more accessible to non-astronomers.

Anyway neither distance is more scientifically appropriate than the other. What it boils down to, is, both units are derived from the Earth's orbit around the sun, so there's a strong 'anthropic' principle in both. If we'd evolved on a different planet: well the length of our year would probably be different, hence so would the definition of 'light year'. And, if the radius of our orbit around our primary star were different, so would our AU, and hence so would our 'parsec'. Not to mention the fact that we might have come up with a totally different definition of an arc-second.

So neither of these units is 'fundamental' in any way. Take your pick.

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I'd say use light years because of their success in the afore mentioned 'pub test' unless you really want to add emphasis to a lesson on parallax. What level do you teach?

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Cheers all - Pete, you've pretty much summed up my opinion and thoughts there. I just wanted to be sure that there wasn't an inter-stellar etiquette I was unaware of really! I'm happiest with light years, so after letting my little darlings know what a parsec is, I'll think I'll stick to that.

Thanks again,

Rich

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I'd say use light years because of their success in the afore mentioned 'pub test' unless you really want to add emphasis to a lesson on parallax. What level do you teach?

They're KS4/Year 11 BTEC students. They do need to be able to use parallax as a method in theory, but they're going to find that VERY hard - only the keenest kiddies will be completing that part of the assignment I think!

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"she made the Kessle run in less than 12 parsecs"....Han Solo to Obi Wan in replying to the question is the Millenium Falcon fast?

kind of like the name though... also really helps when you try and get a grip on the sheer distance of things..ie Parsec = 3.26 light years, Kilo Parsec = 1,000 Parsecs, Mega Parsec = 1,000 Kilo Parsecs

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"she made the Kessle run in less than 12 parsecs"....Han Solo to Obi Wan in replying to the question is the Millenium Falcon fast?

kind of like the name though... also really helps when you try and get a grip on the sheer distance of things..ie Parsec = 3.26 light years, Kilo Parsec = 1,000 Parsecs, Mega Parsec = 1,000 Kilo Parsecs

Damn you!! I was hoping to get in first with that one and castigate the dumbo's here for not realising that a parsec was a unit of speed and not distance and that I had that on authority from Harrison Ford himself

The Family Guy Star Wars special took the **** out of that mistake too

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"she made the Kessle run in less than 12 parsecs"....Han Solo to Obi Wan in replying to the question is the Millenium Falcon fast?
I think Star Wars fans have come up with some sort of explanation for Solo's apparently meaningless remark there: how can a parsec, a unit of distance, be used to represent a period of time? Something to do with the shortest 'distance' through hyperspace, or between black holes, or whatever....

Most SF authors have stuck with light years, but Isaac Asimov obstinately referred to parsecs in his Foundation trilogy. I've often wondered why.

When Friedrich Bessel became, in 1838, the first person to measure the distance of a star other than the Sun, although he made the measurement using parallax, and 'parsecs' would have been more natural, he still published his result in light years. Indeed the term 'parsec' had not yet been coined in those days.

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That was just George Lucas' excuse for making such a glaring error, but it is pretty poor.

No-one ever says 'I went to Peckham in 6 miles.' They say 'It took me hours to get to Peckham!'

---

- TheThing.

Sent from my iPhone using Tapatalk

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"she made the Kessle run in less than 12 parsecs"....Han Solo to Obi Wan in replying to the question is the Millenium Falcon fast?

kind of like the name though... also really helps when you try and get a grip on the sheer distance of things..ie Parsec = 3.26 light years, Kilo Parsec = 1,000 Parsecs, Mega Parsec = 1,000 Kilo Parsecs

Alan, you're right. If it's good enough for Han Solo, it's good enough for me!

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The parsec is a virtuous unit because it is recreatable. It is the distance to a star which shows a parallax of one second of arc due to the orbit of the Earth.

If (or when) some catastrophe wipes out most of humanity and culture has to begin again the parsec can again be derived from observation. An example of the opposite situation would be the Stade, a lost unit of measurement derived from the length of a human stride. This is a shame because it is the unit in whch Eratosthenes first derived the circumference of the Earth and it would be nice to know just how close he got it! It was very close indeed, that we do know.

But, yes, since the LY and the Parsec are of the same order of magnitude, and since we now know the speed of light to a high level of precision, I don't suppose the parsec is really needed. It may one day joint the rod and the cone and the furlong as a pleasant reminder of the past!

Olly

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I still use the stade - only I used to call it a yard till europe made us go metric - now I call it a meter.... (well.... it's near enough lol).

I like "parsec" on University Challenge and Master Mind, but on Weakest Link it's gotta be Light Year hehe

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That was just George Lucas' excuse for making such a glaring error, but it is pretty poor.

No-one ever says 'I went to Peckham in 6 miles.' They say 'It took me hours to get to Peckham!'

Here's the definitive explanation, from Wikipedia:

The Falcon is often connected to the Kessel Run, a pathway from Kessel past the Maw Black Hole Cluster used by smugglers to transport precious Glitterstim spice.[9] Solo, in A New Hope, brags that the Falcon made the Kessel Run in "less than twelve parsecs". As this is a unit of distance, not time, different explanations have been provided. In the fourth draft of the script, Kenobi "reacts to Solo’s stupid attempt to impress them with obvious misinformation."[10] In the Expanded Universe, it refers to his ability to move the ship closer to the Maw's black holes and therefore cut the distance traveled.[9] On the A New Hope DVD audio commentary, Lucas comments that, in the Star Wars universe, traveling through hyperspace requires careful navigation to avoid stars, planets, asteroids, and other obstacles,[11] and that since no long-distance journey can be made in a straight line, the "fastest" ship is the one that can plot the "most direct course", thereby traveling the least distance.[11] The novelization backs away and changes the line to "twelve Standard Time Units."
Simples!
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As far as I'm concerned, the light year is the more "popular" choice for your standard pub discussions, but parsecs are favoured by professional astronomers.

I agree with this. A few years ago, while teaching physics at a small university, I had an office directly across the hall from a professional astronomer for a year. We became fairly good friends, and we often had interesting, wide-ranging discussions. When we talked about astronomy and cosmology, he used parsecs and I used light-years. Once, he replied to a comment of mine with "There you go using those damned light-years again!"

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I'd agree that light years are conceptually more simple, but a parsec is an entirely self contained and *observable* distance measurement -- which makes it far more important than a light year. The distance to something in light years is not directly measureable, you have to convert from some other measurement to get it (and you need to know the speed of light)

To measure parsecs you don't need to know anything other than that the Earth goes round the Sun in a reasonably circular orbit. You only need to know the size of the Earth's orbit if you want to convert it to other units (i.e. meters or light years). You only even need a telescope because the angles turn out to be so small.

So, I'd argue that, as far as 'measures of distance' in astronomy go, the parsec is a lot more important than the light year is.

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Both would be meaningless to ET's though.

How far away is your planet ET?....before you answer, could you please convert your answer into one of either of these two arbitrary formats, the arc subtended by the diametre of the orbit of our little backwater planet here or the distance light travels in the amount of time it takes this little planet of ours to orbit our star once.

Surely the most universal measurement would be the distance light travels during the half life period of radioactive element X etc.

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hello, i am a physics teacher and we use the parsec for magnitude calculations at ks5. and not much else really. I dont remember it coming up in any great detail lower down in the school other than "playing" with units.

I can send through our a-level notes etc if it will help at all.

cheers

welshdai

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I find that the light year is a more comprehensible measurement for galactic distances and the "au", astronomical unit, works really well for smaller distances ie; solar system objects. But this doesn't help you it just throws in another measuement to think about. sorry.

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