The epoch is roughly synchronized with the northern winter solstice, and the exact instant of this calendar's epoch is officially defined as 221788790 SI seconds (measured at the geoid) before 1977-01-01 00:00:00 TAI. Though for most practical purposes, it may be reasonable to use 1969-12-22 00:00:00 UTC or -864000 seconds of (10 days before) the UNIX Epoch as the terran computational epoch. For the rationale, see the epoch section on the homepage