William Matheson (nova_one) wrote,
William Matheson


The parsec is a convenient unit of measure in astronomy because it's the distance at which the average Earth - Sun distance (roughly equal to one astronomical unit, another convenient unit of measure) causes a parallax (a measured change in the sky of a star's position relative to relatively static "background" stars that are farther away) of one arcsecond. Okay, what the hell is an arcsecond? It's a sixtieth of an arcminute, which is in turn a sixtieth of a degree. The width of your little finger at arm's length covers about one degree of sky.

If you're curious, a parsec is about 3.26 light-years or 206,260 astronomical units, but those units are better for getting a mental picture of an star's distance rather than actually calculating it (or using modica of algebra, calculus, and physics to determine the intrinsic† properties of the star). But when you need to make that conversion you can just look it up or use something like WolframAlpha. Knowing those conversions is far, far less important than understanding where the parsec comes from (parallax of one arcsecond) and what it's good for. As always, a picture diagram is worth 1*10³ words.

† - An intrinsic property: This is a 100 watt light-bulb. (How "powerful" it is. This property is about the light-bulb itself.)

An extrinsic property: From 50 feet away, it looks about as bright as a candle would be at 4 feet. (How bright it looks. This property is dependent on the light bulb too, but it is also dependent on where the observer is located relative to the light bulb. If you know one property, you can determine the other.)

So what was Han Solo on about when he said the Millennium Falcon could make the Kessel Run in less fewer than twelve parsecs? It seems to have started out as obvious misinformation but in the world of Star Wars beyond the movies he's taken literally. I prefer the simple former explanation (it also fits his grammar), but I've read (and enjoyed) several expanded universe stories dependent on the latter.

But I actually wrote all that to give context to this: In an assignment question, I was asked to take a 1672 parallax measurement for the planet Mars and determine its distance from Earth at the time. Only this measurement was based on simultaneous observations taken from both sides of the Earth, so the arcseconds were subtending one Earth diameter, not the Earth-Sun distance! After calculating that Mars was about 6139 AU away and wondering what was up (it was really more like 0.5 AU), I discovered my error. But I was only off by about a factor of 11,800. I've done worse. ;-p
Tags: astronomy, math, science

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