Transcript: The first direct estimate of stellar distances used geometry. In 1838, Friedrich Bessel measured the parallax of the bright star 61-Cygni. This is the seasonal shift in the apparent position of the star on the sky relative to more distant stars as the Earth travels its orbit of the Sun. The shift was only 0.6 seconds of arc, a very small effect, which is in part why it took two hundred years of telescopic observations before parallax to any star was measured. Here, however, was finally a direct measure of the distance to the stars showing that the stars were indeed hundreds of thousands of times further away than the Sun itself. The formal equation that gives the distance to the stars in terms of parallax is that the distance in astronomical units is roughly two hundred thousand divided by the parallax angle in arcseconds, or the distance in parsecs equals one over the parallax angle.