Quote:
Originally Posted by Sil_liS
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You are confusing the definition of the unit with the use of the unit.
From Wiki:
"The parsec is equal to the length of the adjacent side of an imaginary right triangle in space. The two dimensions on which this triangle is based are the angle (which is defined as 1 arcsecond), and the opposite side (which is defined as 1 astronomical unit, which is the distance from the Earth to the Sun). Using these two measurements, along with the rules of trigonometry, the length of the adjacent side (the parsec) can be found."
The only parallax measurement required to calculate this is to the closest star of all, the sun.
That gives a fixed definion of the parsec as a distance.
You can then describe any distance in parsecs, just as you could in feet or metres or miles.
You don't calculate a distance in light-years by shining a light and then waiting to see how long it takes to arrive, do you?
You can also use parallax as a measuring tools, but any errors involved in that measurement have nothing to do with the units you choose to express the answer in. There would still be the same error regardless of whether you quoted the results in parsecs or light-years.
And as far as measuring parallax only for close stars:
"The parallax method is the fundamental calibration step for distance determination in astrophysics; however, the accuracy of ground-based telescope measurements of parallax angle is limited to about 0.01 arcseconds, and thus to stars no more than 100 pc distant.[5] This is because the Earth’s atmosphere limits the sharpness of a star's image.[6] Space-based telescopes are not limited by this effect and can accurately measure distances to objects beyond the limit of ground-based observations. Between 1989 and 1993, the Hipparcos satellite, launched by the European Space Agency (ESA), measured parallaxes for about 100,000 stars with an astrometric precision of about 0.97 milliarcseconds, and obtained accurate measurements for stellar distances of stars up to 1,000 pc away.[7][8] NASA's FAME satellite was to have been launched in 2004, to measure parallaxes for about 40 million stars with sufficient precision to measure stellar distances of up to 2,000 pc. However, the mission's funding was withdrawn by NASA in January 2002.[9] ESA's Gaia satellite, due to be launched in late 2012, is intended to measure one billion stellar distances to within 20 microarcseconds, producing errors of 10% in measurements as far as the Galactic Center, about 8,000 pc away in the constellation of Sagittarius.[10]"
And from the site you posted:
"The light year is used primarily by writers of popular science books and science fiction writers. It is rarely used in research astronomy."
That sounds quite a lot like saying that light-years are used more often in science-fiction than they are in reality.