Chinese astronomers developed "Sun Calendars" similar to those developed by the Maya, Egyptians and others. The pole at the center is known as GNOMON, a word pointing to KNow. In 1276, the Chinese astronomer Guo Shoujing calculated that one year is equivalent to 365.2425 days. Such precise calculations were based on the shortest and longest shadows cast by the Gnomon.

The ancient Roman festivity of Saturnalia or todays Christmas coincides with the winter solstice (shortest gnomon shadow). Easter, on the other hand, is celebrated to coincide with the vernal equinox (when daytime and nighttime are equal in duration).