The results of celestial observation in antiquity seem extraordinary when we consider that they were obtained with the naked eye. Eratosthenes estimated the Earth's circumference from the different lengths of the shadows cast by the Sun in Alexandria and Cyrene. Hipparchus of Nicaea, comparing the apparent dimensions of the Moon with the dimensions of the Earth's shadow, arrived at a reasonable approximation of the distance of the Moon. Ptolemy catalogued the precise positions of 1,025 fixed stars. With the naked eye and the help of large mural quadrants or the triquetrum, astronomers made observations to record the changes in the Sun's altitude during the year. Using armillary instruments, large sextants, and altazimuth quadrants, they measured the coordinates of the planets to describe their movements along the zodiac and, later, their orbits in space. Thanks to geometric methods for resolving spherical triangles, astronomers used the angular distances between stars to identify their relative positions with ever greater accuracy. This allowed the production of superb maps and celestial planispheres.