Just after Sirius appears in the July sky, the Nile River begins its annual flood, and so the ancient Egyptians connected the two. Consequently Sopdet was identified as a goddess of the fertility of the soil, which was brought to it by the Nile's flooding. This significance led the Egyptians to base their calendar on the heliacal rising of Sirius.
Sopdet was regarded as the consort of Sah, the constellation of Orion, by which Sirius appears, and the planet Venus was sometimes considered their child. The noticeably human figure of Orion was eventually identified as a form of Horus, the sky-god, and thus, together with her being a fertility deity, this led to her being identified as a manifestation of Isis.
After Sirius' appearance, the scorching heat of summer arrives, an aspect that was referred to as Sopdu, meaning (one who is) with Sopd, Sopd being simply the masculine form, and stem, of Sopdet. Since the heat arrived after Sirius' appearance, it was said that Sopdet had given birth to it, thus Sopdu was seen as being a child of Sopdet, and thus also of Sah.