![]() Or, in other words, an arcseconds spans a larger and larger physical size (as described by James K) out to ~15 Gly (where it spans roughly 28,000 lightyears), after which it spans a smaller and smaller size. km pc 0.000000000000032408 Kilometers The kilometre is unit of length in the metric system equivalent to one thousand metres. 0.000000000000032408 Kilometers The kilometre is unit of length in the metric system equivalent to one thousand metres. The result - which is obtained by integrating the Friedmann equation - is that galaxies become smaller and smaller out to a certain distance (roughly 15 billion lightyears), after which the second effect starts to dominate and they start to grow in size. On the other hand, we look further and further into the past, and hence see galaxies at a time when they looked larger and larger. An arcsecond is a unit for small angles, such as the parallax one. Some important points about the previous relationship: The distance to stars is usually a huge number, so the parallax angle is really tiny. Thus, we have two competing effects: On the one hand, galaxies become smaller and smaller with distance, as expected and as decribed in James K's answer. D 1/P, where: D Distance between the star and the Earth, in parsecs ( pcs) units and P Parallax angle, in arcseconds ( arcsec) units. astronomers use a set of units that are appropriate to measure quantities The astronomical unit is used to determine planetary distances, which are the same as the mean earth-sun distance. And because the Universe expands, everything was closer together in the past, so we see distant galaxies as they were when they were closer and hence spanned a larger angle on the sky. A star with a one-second arc must have a 1 parsec distance. Therefore, a star that appears to move 1 arcsec while the Earth moves laterally by its orbital radius is at a distance of 1 parsec. I just wanted to mention an effect which comes into play if you look really, really far away:īecause light moves at a finite speed, we see galaxies (and other things) as they were in the past. ![]() James K's answer is right on, and probably what you're after. ![]()
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