Matters can be improved if both observers have light sources which they cover with a screen. Time measurement begins when the first observer removes the screen sending light to the second. The second light source is uncovered when the second observer sees the first. Now when the first observer sees the second light source he again screens his source. The time between uncovering and covering the first light source is a measure of the time light takes to travel twice the distance between the two observers. The improvements are obvious. The distance is doubled and a single clock has replaced two supposedly synchronized clocks. Here was Galileo's proposed study of 1638; nearly 200 years would pass before it was improved sufficiently to produce results.
The necessary innovations were introduced by Hippolyte Fizeau (1819-1896). One innovation was to replace the second person by a fixed flat mirror whose surface is perpendicular to the beam of light from the source. When this was done, the light beam was reflected directly back at its origin and one human source of variation was completely removed from the system. The second innovation was to automate the covering and uncovering of the source, thereby further reducing the variation from the first human source.
Together, these advances allowed Fizeau to replace the direct measurement of time with an indirect measurement of speed. Rather than measure time between uncovering and covering, Fizeau could measure the minimum speed that the screen must travel in order to cover the source at the exact time the light returns. The trick was to use an accurately machined toothed wheel placed spinning in front of the source to act as the moving screen. The teeth screen the source while the gaps uncover it and so the wheel acted just as Galileo's observer. Any light returning to the source strikes either a tooth or a gap. If the wheel was set spinning fast enough that every beam sent out struck a tooth on its way back, no image of the source is observed. Twice this speed produces a full image as the beam sent out returns through the next available gap. Three times the speed produces no image, and so on. The speed of rotation, coupled with the distance travelled (twice 8,633 metres in Fizeau's setup), could be transformed into a measure of the speed of light. In this way, Fizeau produced the first terrestrial determination of the speed of light in 1849.
Others were quick to build on this monumental achievement. Only two years later Leon Foucault (1819-1868), a former collaborator of Fizeau, produced more accurate measurements based on a rotating mirror rather than a toothed wheel.