We like marking time by the Sun. Its rising and setting marks a day, and its path along the ecliptic marks a year. The solar year seems to be our favorite marking of time. Its cycle follows the seasons, and so we have lots of annual celebrations, including our own special day. Of course there are lots of other ways we could mark our time.
We could note the seasons of the Moon, shifting from new moon to full and back to new over about 29.5 days. For young children such a clock is sometimes used, but as we get older a lunar month seems too short to take much notice. Of course the lunar cycle is a bit out of sync with the solar year, but every 19 years the Moon and Sun come back into sync. This Metonic cycle could be used as a basis of celebration, but nearly two decades seems too long for a personal celebration.
We could mark time by the Soros cycle, which follows the pattern of eclipses. While eclipses happen 4 times a year, the relative alignments of the Earth, Moon and Sun cycle through a pattern of about 18 years. But since each eclipse isn’t seen from everywhere on Earth, such a pattern isn’t particularly commemorative.
If we lived longer we might celebrate other cosmic cycles, such as the 1,400 year Sothic cycle of ancient Egypt, which marked the rising of Sirius just before sunrise on the first day of the year. Or the Great Year marking the 25,800 years it takes for the rotational axis of the Earth to make one complete precession. Or perhaps the galactic year marking the Sun’s 250 million year journey around the galaxy.
But our lives span an intermediate time between mayflies and immortals, and so the journey of the Earth around the Sun marks our cycles of time.