Great Syzygy

The Great Syzygy was an astronomical event which marks the beginning of the AGS period, in which the Sun, the planet Winter and its two moons were in alignment on Winter's winter solstice. The moons were both on the opposite side of the planet to the sun, meaning both were full and in conjunction in Winter's night sky.

Universal time
The exact moment of conjunction was chosen as the zero point for the Universal calendar, since this rare coincidence of important astronomical cycles makes an ideal reference point when comparing and converting between other calendars. The line of longitude which was pointing away from the sun and towards the moons at the moment of conjunction was taken to be the line of zero longitude, therefore the Great Syzygy is defined as occurring at midnight on day zero in universal time. Note that the period of 24 hours following the syzygy is day zero in universal time, but more commonly referred to as day 1 of year 0.

Frequency of Conjunction
Winter's moons align in a double full conjunction approximately every 192 days. Since this is independent of Winter's orbit around the sun, the double full conjunction is equally likely to fall on any day of the year. It follows that an event like the Great Syzygy should occur every 192 years on average. In fact, similar events tend to occur with gaps of 172, 185, 330 or 515 years, but are not as closely aligned. For example, a double full conjunction occurred on the final day of 184 AGS, the day before the 185 AGS winter solstice, while in 515 the events fell on the same night, but Bigmoon had technically reached its true full phase the night before. Another close alignment, a double full conjunction on the morning of the second day of 687 AGS, was shortly followed by the only other true realignment in recorded history, on the solstice of 700 AGS. Note that while such an event may still be called a great syzygy, the name "Great Syzygy" properly refers only the 0 AGS alignment.

Some years hosting similar astronomical events are listed below:


 * 607, 1994: Double full moon conjunction on the summer solstice.
 * 653, 1340: Double new moon conjunction on the autumnal equinox.
 * 693, 1393: Double new moon conjunction on the summer solstice.
 * 39, 739, 1439: Double full moon conjunction on the autumnal equinox.
 * 786, 1486: Double new moon conjunction on the winter solstice.

Astronomical observation
The primary calendar used by the Xiyeru is based around the cycle of the double full moon conjunction, called ūžāwiʼivāń "whole-eye-moon" in Xiri. The Great Syzygy of 0 AGS was recorded by contemporary Xiyeru astronomers, who noted that the ūžāwiʼivāń had unusually fallen on the night of the winter solstice, but since the solar calendar was not in common use, this alignment was not considered of as much importance as other events in the lunar calendar.

The coincidental alignment of various events on the solar and lunar calendars in the 7th century was noticed by Xiyeru astronomers, who, in the 690s, subsequently predicted the 700 AGS syzygy. Its arrival was heralded by some as a good omen marking a new time of prosperity in the wake of the famine of 656-664 AGS. This likely influenced the formation of the Azodo government, which is thought to have first assembled in 700 or 701 AGS.