The Star That Brought the Flood

Spring evenings over Cairo, a day at a time. Sirius — Sopdet to the Egyptians — stands a little lower in the western glow each night, as the Sun catches up with it. By the end of May it is gone into the twilight, and for weeks it cannot be seen at all.

Now the dawns. The Sun has passed Sirius, and each morning the star comes up a few minutes earlier, until one dawn it shows low in the east before the sky turns blue: its heliacal rising. In Old Kingdom Egypt it opened the year, near the Nile flood.

Each dawn, Sirius's height at the moment the sky is just dark enough for it. The dotted curve is the evenings. Between its last evening and the morning the solid curve clears the haze is the band: the weeks Sirius is lost in the Sun.

The date belongs to a latitude. Upriver at Aswan Sirius returns days before it reaches Cairo; London waits weeks longer. And Egypt's civil year had 365 days, a quarter-day short, so the star's return slipped through its calendar a day every four years: the Sothic cycle.

A fainter star needs a darker dawn. In Aotearoa the cluster that matters is the Pleiades, Matariki, gone for weeks and back before dawn in June. Its return opens the Māori new year — since 2022 a public holiday, set by the Tangaroa nights of the Moon after it.