Chinese astrology is based on the Sexagenary cycle of 60 years that has been documented since the time of the Shang Dynasty. This cycle has been constructed from two cycles. One is the 10 heavenly stems and the other the 12 earthly branches, or the 12-year cycle of the animals, referred to as the Chinese zodiac. Not only this, the zodiac also operates on a cycle of months, or "moons", and the hours of the day. It refers to a pure cylindrical cycle, with no equivalents like those of the occidental zodiac. In the imperial times, there were astrologers who watched for heavenly omens.