The Maya of Copan, Honduras used Stela D, its altar, and the surrounding structures as a sundial to record time. Archaeological investigations show that wooden posts and stelae could have been used to measure time and to perform associated rites in the northern sector of the Main Plaza of the Copan Archaeological Park. We constructed a digital model of Stela D to study the shadows cast at different times of day and on different dates of the year, such as solstices, equinoxes, and solar zenith passages. The size and orientation of the shadows may have served as a time marker that ancient residents of Copan used to accurately track the tropical year. We also found evidence that supports the iconographic interpretation of an analogy between serpents’ bodies that adorn the figure of the ruler on Stela D and shadows and sun positions on dates of major solar events that form a solar calendar that counts years from winter solstice day.