![]() One parsec is defined geometrically as the distance from the Sun where the motion of the earth around the Sun causes a parallax of one arcsecond. ![]() Venus, the bright morning or evening star, tends to be a few tens of arcseconds in the sky. To give you some scale, the full moon is half a degree across - thirty arcminutes. They are a unit of size on the sky - sixty arcseconds go into an arcminute, sixty arcminutes go into a degree, and 360 degrees makes a full circle (no one has ever said that astronomers use sensible units). Arcseconds tell us how far these stars appear to move. Applying this idea to the stars, stars which are quite close to us will appear to shift ever so slightly relative to other stars in the sky which are at much greater distances, which will appear fixed. The most common example is looking out the window of a moving vehicle - the shrubs, trees, and telephone poles near the road move very quickly past your window, but things further away - any mountains or distant buildings will appear to move at a much slower pace from your perspective. But more generally, parallax is the term for the relative motions of any two objects at different distances. Parallax, as we just mentioned, is describing the apparent motion of a star. Arcseconds are frequently abbreviated as “arcsec”, so parallax-arcsec got shortened into parsec. The actual term “parsec” has a somewhat fun origin - it’s actually a mashup of the words “parallax” (the apparent motion of the stars) and “arcsecond” (how far they move). Unlike the light year, which is based in the physics of light, a parsec was determined geometrically using the size of the orbit of the Earth around the sun, and the apparent motion of nearby stars. The distance from us to the center of our galaxy is measured in tens of thousands of light years. A light year, as we’ve talked about before, is quite a long distance, but once you get away from the immediate group of stars surrounding our solar system, it’s less useful as a ruler, since everything is so far away. The parsec (regardless of what you remember from Star Wars’ infamous line re: the Kessel Run) is a unit of distance that’s equivalent to 3.26 light years. In this case, the base unit is a parsec, instead of a gram or a meter. ![]() If that was not your question, please feel free to ask again!) Kiloparsecs are another unit used to measure huge distances between objects in space, and if the ‘kilo’ part seems familiar from having seen words like kilogram and kilometer, you’d be exactly right - the prefix here just means 1000 of the part that follows. Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow on Twitter for your daily dose of fun science facts.(Note: I am assuming this submission means “What is a kiloparsec, and what do we use it for?”. The nearest star, Proxima Centauri, is about 1.3 parsecs away, while the Milky Way is more than 31,000 parsecs across. One parsec is defined as the distance at which one astronomical unit (the average distance between the Earth and Sun) subtends an angle of one arc-second. This only works for relatively nearby stars. Using the principle of parallax, some stellar distances can be found by measuring their tiny shift in position as the Earth undergoes its yearly orbit around the Sun. The unit has its origins in one of the first methods of determining the distance to the stars. Unfortunately, like the similarly misused ?light-year?, the parsec is a unit of length, not of time.Ī parsec is equal to about 3.26 light-years or roughly 31 trillion kilometres (19 trillion miles). Han Solo tells Luke Skywalker and Obi-Wan Kenobi that the Millennium Falcon ?made the Kessel Run in less than 12 parsecs?.
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