The different forms of the first two decans that belong to the sign of Taurus in the Graeco-Roman zodiacs – a kneeling woman, a pig, the dead Osiris lying in a boat, and a pair of eyes – are shown to refer to the full moon in the month I Shemu, and the cultic and mythological connotations of this event. The basis of this connection was the fact that I Shemu overlapped with the sun’s annual stay in Taurus around the beginning of the Common Era. The Taurus decans showing Osiris and the pair of eyes in the barque derive from the New Kingdom astronomical diagrams which depicted a boat under the decan called ?r.j-jb-wj? “the middle of the boat”. In the later zodiacs the same decan belongs to the sign of Libra. The analysis of the symbols of this decan in the astronomical diagram of the Ramesseum and in the round zodiac of Dendera, unveiling the multiple layers of signification that have been attached to them, reveals that the Egyptian constellation of the boat was identical with the three brighest stars of the modern constellation of Libra (?, ?, and ? Librae, the ?r.j-jb-wj? decan being the star in the middle, ? Librae). This boat in the sky played a crucial role in Egyptian cosmographies and the astral myth of Osiris because of its closeness to the autumnal equinoctial point throughout much of ancient Egyptian history.