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>Astronomers have discovered a world only a little bit bigger than Earth, whirling around a bright star about 31 parsecs from our planet. Parsec is one of thos
by jcims 7y ago
>Astronomers have discovered a world only a little bit bigger than Earth, whirling around a bright star about 31 parsecs from our planet.
Parsec is one of those units that is difficult to remember if you just look at the conversion (1 parsec = 3.26 light years). If you remember how it's defined, however, it somehow becomes easier (for me anyway). One parsec is the distance at which one astronomical unit (AU, mean distance between earth and sun) spans one arcsecond in the sky. One arcsecond, in turn, is 1/3600th of a degree, and is approximately the width of a human hair from 20 meters away.
On a side note, if you think that's small, many stars in the night sky have apparent diameters measured in thousandths of an arcsecond.
- saagarjha 7y agoI’m not sure if the definition helped.
- jcims 7y agoHaha fair point. I don't know why it helped me, but it seemed to.
- hcrisp 7y agoHow about a picture? https://socratic.org/questions/how-do-you-convert-parsecs-to-arcseconds https://socratic.org/questions/how-do-you-convert-parsecs-to...
- saagarjha 7y agoI mean, that helps explain the definition. But in practical use I just know to multiply by three to get lightyears and I couldn't really care less about the projection of an arc subtended by a certain angle at a certain distance.
- dcminter 7y agoThe point of a parsec is that if you're stuck on earth a very long way from the stars and you're trying to work out where they are relative to each other, you can take advantage of the fact that the earth itself moves around the sun. So if you take a photo of a star in summer and then take another photo of it in winter, your vantage point has conveniently moved 2AU because during that time you've moved to the other side of the sun (and 1AU is by definition your distance from the sun). So then if the star appears to move against the stars so far away that they didn't appear to move, you can do a spot of geometry and work out how much the subject star has moved in "arcseconds". A star that is exactly far enough away to "move" 2 arcseconds over this 2AU distance (or, if you prefer, one which has "moved" 1 arcsecond when we move 1AU) is one parsec away. By definition. When discussing relative distances from earth for stars, it's therefore a useful unit (we can't just compare arcseconds because further stars move less against the background, not more). As with so many puzzling units the key is to find out why they're used. This one was used by astronomers who didn't have a better way to determine how far away stars were. The wikipedia article, incidentally, is much better than that old Encyclopaedia Britannica article that I read in high school. I envy kids today. https://en.wikipedia.org/wiki/Parsec https://en.wikipedia.org/wiki/Parsec Oh and the actual distance is kind of irrelevant and I don't even try to remember it :)
- sukilot 7y agoThat's a great way to compute the distance to a star from yourself stuck on Earth for half a year. It's not a great way to understand the magnitude of that distance when planning a trip.
- dcminter 7y agoBut if you're planning a trip, why would you plan it in Parsecs rather than Light Years (or some other more convenient unit) when you're not stuck on earth?
- kempbellt 7y agoThat was a lot of fascinating information in one comment. Also fun to note that you mentioned "mean distance" and not just "distance" from the Sun because the Earth's orbit isn't perfectly circular. Thanks for the explanation.
- skykooler 7y agoOf course, that definition is confusing, because it's an inverse thing - from the name you'd expect something 31 parsecs away to have a parallax of 31 arcseconds, but instead it's 1/31 of an arcsecond.
- dmurray 7y agoI just remember that a parsec is the same as a light year. By astronomical standards of precision, that's about right.
- lxe 7y agoIf you're measuring galactic distances (nearest galaxy is 2 million light years away) then that's fair, but for "nearby" objects, a 3x the distance means a lot.
- fastbeef 7y agoI remember it as 1/12th of a Kessel Run, easy peasy
- basilgohar 7y agoThe Kessel Run as performed by the Millennium Falcon; not just any ship can do that!
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- thekingofh 7y agoIs that the same as discarding constants in big O notation?
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- dotancohen 7y ago> I just remember that a parsec is the same as a light > year. By astronomical standards of precision, that's about right. I just multiply them by π. Close enough, and probably on par with our measuring precision anyway.
- Diederich 7y ago> 1 parsec = 3.26 light years I mentally just estimate 1 parsec as equal to π light years.
- nxpnsv 7y agoOr that 1 apc (attoparsec) is ~3cm...
- XaspR8d 7y agoPretty close to 1.2in, hence why I just prefer to remember a parsec as quite close to 0.1 eft (exafoot).
- kirubakaran 7y agoI just imagine 337453805774280 football fields end-to-end when I want to visualize a parsec
- nkrisc 7y agoFinally, a measure that speaks to me, as an American.
- clSTophEjUdRanu 7y agoI imagine 18131846280409076 George Smoots standing on top one another.
- blaser-waffle 7y agoThey still painting those on the bridge? Smoots, I mean. Been outta the US for a while...
- omarhaneef 7y agoI think of a parsec as 1/12th of a Falcon Kessel Run
- the8472 7y agoPerhaps it is difficult to remember, but the light years are more useful because it directly tells you the communication delay or lower bound for travel time. Arcsecond parallax in the sky doesn't really help much.
- commandlinefan 7y ago> 1 parsec = 3.26 light years So that means that even if we could travel at the speed of light, it would still take 100 years to get there, so probably not a candidate for near-term colonization (if something bad happens to this planet, for example).