Era is a segment of time based on Milankovitch cycles. The Earth's axis completes one full cycle of precession approximately every 26,000 years. An Era represents one eighth (1/8th) of this cycle of precession, which is 3,210 years every seven cycles, and 3,211 years every eighth cycle (Great Precession). An extra year is added during a Great Precession which is called year zero.