Diurnal Parallax
The most important fundamental distance measurements in astronomy come from trigonometric parallax, as applied in the ''stellar parallax method''. As the Earth orbits the Sun, the position of a nearby star will appear to shift slightly against the more distant background. This shift is the apex angle in an isosceles triangle, with 2 astronomical unit, AU (the distance between the extreme positions of Earth's orbit around the Sun) making the base leg of the triangle and the distance to the star being the long equal-length legs (because of a very long distance from the Earth orbit to the observed star). The amount of shift is quite small, even for the nearest stars, measuring 1 Minute and second of arc, arcsecond for an object at 1 parsec's distance (3.26 light-years), and thereafter decreasing in angular amount as the distance increases. Astronomers usually express distances in units of ''parsecs'' (parallax arcseconds); light-years are used in popular media. Because parallax bec ...
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