Transcript: In the eighteenth century astronomers began using larger and larger telescopes to map the galaxy to begin to learn the distribution of stars far from the Sun. The astronomer William Herschel was prominent in this effort. The discoverer of Uranus used his telescope to sweep the sky in parallel strips night after night. This was in the days before telescopes had motorized drives so Herschel just used the rotation of the Earth to have stars scan across his visual field, and he counted them and kept careful notes. In this way he was able to see the different distributions and densities of stars on the plane of the sky. He also developed a technique to compare the brightness of two stars. Pointing two identical telescopes at stars of different brightness he covered the aperture of one telescope partially until the apparent brightness of the images was the same. The ratio of the covered aperture to the uncovered aperture then gave the relative brightness of the stars. By assuming stars had the same intrinsic luminosity, the relative brightness could be used to give the relative distances. This is a crude assumption because stars do not all have the same luminosity, but it allowed Herschel to combine his counts of stars in areas of the sky with the sense of depth so he could map out the third dimension. In this way Herschel was like a cartographer mapping out the galaxy for the first time.