Sopdet (Sepdet, Sothis) personified the 'dog star' Sirius. This star was the most important of the stars to the ancient Egyptians, and the heliacal rising of this star came at the time of inundation and the start of the Egyptian New Year. As a goddess of the inundation, she was a goddess of fertility. She also was linked to the pharaoh and his journey in the afterlife.

She was represented as a woman with a star on top of her headdress, or as a seated cow with a plant between her horns (just as Seshat's hieroglyph might have been a flower or a star) as depicted on an ivory tablet of King Djer. The plant may have been symbolic of the year, and thus linking her to the yearly rising of Sirius and the New Year. She was very occasionally depicted as a large dog, or in Roman times, as the goddess Isis-Sopdet, she was shown riding side-saddle on a large dog.

Sirius was both the most important star of ancient Egyptian astronomy, and one of the Decans (star groups into which the night sky was divided, with each group appearing for ten days annually). The heliacal rising (the first night that Sirius is seen, just before dawn) was noticed every year during July, and the Egyptians used this to mark the start of the New Year (wp rnpt, 'The Opening of the Year'). It was celebrated with a festival known as 'The Coming of Sopdet'.

The time period between Sothic risings is called the Sothic Cycle and it is one of the tools Egyptologists use to create a chronology of Egyptian history.

-- Sopdet, April McDevitt

Even as early as the 1st Dynasty, she was known as 'the bringer of the new year and the Nile flood'. When Sirius appeared in the sky each year, the Nile generally started to flood and bring fertility to the land. The ancient Egyptians connected the two events, and so Sopdet took on the aspects of a goddess of not only the star and of the inundation, but of the fertility that came to the land of Egypt with the flood. The flood and the rising of Sirius also marked the ancient Egyptian New Year, and so she also was thought of as a goddess of the New Year.

Her aspect of being a fertility goddess was not just linked to the Nile. By the Middle Kingdom, she was believed to be a mother goddess, and a nurse goddess, changing her from a goddess of agriculture to a goddess of motherhood. This probably was due to her strong connection with the mother-goddess Isis.

Not just a goddess of the waters of the inundation, Sopdet had another link with water - she was believed to cleanse the pharaoh in the afterlife. It is interesting to note that the embalming of the dead took seventy days - the same amount of time that Sirius was not seen in the sky, before it's yearly rising. She was a goddess of fertility to both the living and the dead.

In the Pyramid Texts, she is the goddess who prepares yearly sustenance for the pharaoh, 'in this her name of "Year"'. She is also thought to be a guide in the afterlife for the pharaoh, letting him fly into the sky to join the gods, showing him 'goodly roads' in the Field of Reeds and helping him become one of the imperishable stars. She was thought to be living on the horizon, encircled by the Duat.

In the Pyramid Texts, paralleling the story of Osiris and Isis, the pharaoh was believed to have had a child with Sopdet:

Your sister Isis comes to you rejoicing for love of you. You have placed her on your phallus and your seed issues into her, she being ready as Sopdet, and Horus-Soped has come forth from you as Horus who is in Sopdet.

-- Sopdet in the Pyramid Texts

Sirius happens to travel the sky just ahead of the large constellation of Orion. (His belt of three stars serves as an easy pointer towards Sirius, the unmistakable bright star that is one of the few visible even in city lights' glare). Orion was identified with the dying-and-resurrected god Osiris, in Egyptian mythology, who was one of the most well-known gods of the pantheon. His wife and sister Isis was Lady of Magic, who brought her husband back to life, and the bright star his constellation followed naturally came to be associated with her.

-- Inventing the Solar System: Early Greek Scientists Struggle to Explain How the Heavens Move, Ellen N. Brundige

She was also given a masculine aspect, and linked with Horus as Sopdet-Horus during the Middle Kingdom. She was also linked with Anubis during Greek Times as Sopdet-Anubis, probably because of the iconography of her as a god, or riding on the back of a dog. She was also linked with other goddesses such as Hathor, Bast and Anqet.

She was worshiped through Egyptian history, from predynastic times, through to the Graeco-Roman period. She was venerated in Per-Soped (Saft al Hinna), in the 20th Nome of Lower Egypt.