A lunar mansion is a segment of the ecliptic (often called a station, or house) through which the moon moves in its orbit around the earth, often used by ancient cultures as part of their calendar system. In general, though not always, the zodiac is divided into 27 or 28 segments relative to fixed stars - one for each day of the lunar month, which is 27.3 days long - and the position of the moon is charted with respect to those fixed segments. Since the position of the moon at given stage will vary according to the position of the earth in its own orbit, the mansions of the moon are an effective system for keeping track of the passage of seasons.