Sothis is the Greek name of Sopdet, who in Egyptian mythology was the goddess of fertility, associated with the star Sirius. When Sirius appeared in the sky in July, the Nile River began flooding and so the ancient Egyptians naturally connected the two. Sopdet was regarded as the consort of Sah, who was the constellation of Orion, and their child the planet Venus. She was later merged with Isis.
I'm sad to inform, but it seems this explanation is not quite accurate.
The information I have is that solstice is derived from Latin solstitium (from sol: "sun" and sistere: "stand still"). During the year, the position of the sun seen from earth moves North and South. When it changes direction it stands still momentarily...
