"According to Plutarch, Osiris was slain-suffocated in the box-on the seventeenth day of the month of Athyr, when the sun was in Scorpius, in the twenty-eighth year of his reign.
The numbers are significant. Although the moon completes its phases in 29½ days, the number 28 was used symbolically for this interval. And an intricate logic links the moon with the Nile through the number 28. At Aswan, the traditional southern limit of Egypt, a nilometer measured the height of the river's maximum rise as 28 cubits. This peak occurred in the fall, in the month of Athyr, just as the Nile was about to decline. Osiris was slain on the seventeenth day of Athyr. Full moon was counted as the month's fifteenth day, but the moon looks full the day before and the day after. By day 17, it is obvious the moon has started to wane. So, the death of Osiris marked the "death," or fall, of the Nile from its annual flood height and coincided with the day of the moon's monthly "death." Osiris, the god of the Nile and Egypt's life, was a moon god because the river, the land, and the moon wax and wane, cycle after cycle.
Other lunar symbolism also threads through the Osiris myth. His casket floated downstream, out of the Nile, into the Mediterranean. Isis followed it, found his body at Byblos, in Syria, and carried it back to Egypt. Although unable to revive him, she managed through magic to conceive a child by him. She hid his body in the thickets of the Delta marshes and nursed, in secret, the newborn son of Osiris. One night, while hunting by moonlight, Set chanced upon the body of Osiris. He tore it into 14 parts and scattered them up and down the Nile. Once again Isis set out to retrieve the body of her husband, now dismembered and dispersed throughout the land. With patience and perseverance she sought the far-flung parts of Osiris; all but his sexual member were retrieved. The symbol of his vitality remained lost in the Nile.
The 14 pieces of the body of Osiris sound like the 14 days of the waning, or "dying" moon, and on the main ceiling of the Dendera temple are inscriptions and pictorial reliefs that leave no doubt. In one panel, an eye, installed in a disk, is transported in a boat. The eye, we know, was a symbol of the sun or moon. Thoth, the ibis-headed scribe god of wisdom and knowledge, pilots the barge. Thoth was closely associated with the moon and counted the days and seasons. The text for this panel refers to the period after full moon, and 14 gods accompany the eye in the disk.
Next to the portrayal of the waning moon, another carved panel represents the 14 days of the waxing moon. A staircase with 14 steps, a god on each, leads up to the same eye and disk, and hieroglyphics verify the god's association with days of the growing moon. Osiris, it is written, is "luminous," as the god of the moon.
Finally, a third, adjacent panel shows Osiris in a boat with Isis and her sister Nephthys. Goddesses of the four cardinal directions support the sign of heaven, on which the boat floats, and the inscription says Osiris is the moon."