![]() ![]() However, it’s often important in modern astrophysics to know the light-crossing time of a given distance - and if the distance is given in light-years, there you are. sec pär-sek : a unit of measure for interstellar space that is equal to 3. ![]() Moreover, as a practical matter, few distances in modern astronomy are directly measured by the parallax method anymore. ![]() The light-year, by contrast, is based on only one arbitrary value (Earth’s orbital period) and on a fundamental constant of nature: c, the speed of light. This means it is based on two arbitrary quantities: the radius of Earth’s orbit, which was a random accident of how the solar system fell together, and the definition of the arcsecond - an even more arbitrary unit that stems from the ancient Babylonians’ base-60 style of arithmetic, along with their notion that the circle should be divided into 360 degrees because there “ought to be” 360 days in a year. But before we define the parsec, lets look at some more familiar units of astronomical distance: the light year (ly), and the astronomical unit (AU). The parsec (which equals 3.26 light-years) is defined as the distance at which a star will show an annual parallax of one arcsecond. parsec: A unit of astronomical length based on the distance from Earth at which stellar parallax is one second of arc and equal to 3.258 light-years. The nearest star, Proxima Centauri, is about 1.3 parsecs (4.2 light-years) from the Sun. Light-years, no question! Here’s how I see it. This corresponds to 648000/ astronomical units, i.e. Which is preferable? Trigonometric parallax determines the distance to a star by measuring its slight shift in its apparent position as seen from opposite ends of Earth's orbit. You give astronomical distances beyond the solar system in light-years, but professional astronomy papers use parsecs. ![]()
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