Solar Term

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One of twenty-four fixed points in the solar year — these, not calendar months, decide your month pillar.

The year is divided into twenty-four terms by the sun's actual position, twelve of which act as month boundaries in Saju.

This is the single most common source of wrong charts. Your month pillar does not change on the first of the month; it changes when the sun reaches a specific ecliptic longitude, which lands a few days either side of the fourth of each month.