Solar Calendars

Solar calendars are based on the Earth's revolution around the Sun, typically using the tropical year as their reference. These calendars often drift from their starting positions due to the lack of divisibility of the tropical year, by as many as 5 days per year but typically less than 1 day every 4 years. Some solar calendars are locked to a solstice or equinox and do not drift with regards to the equinoxes, though the equinoxes themselves have drift.