More refinements of measuring time occurred later. In order to correct for the Sun’s changing path over the sky throughout the year, the gnomon—or object that creates the shadow on the sundial—had to be set at the correct angle (what we call latitude). Eventually, the sundial was perfected using multiple designs. For example, shortly before 27 B.C.E., the Roman architect Marcus Vitruvius Pollio’s (c. 90-20 B.C.E.) De architectura described thirteen different designs of sundials.