E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Cassiopeia [the lady in the chair].
The chief stars of this constellation form the outline of a chair. The lady referred to is the wife of Cepheus (2 syl.), King of Ethiopia. She boasted that the beauty of her daughter Andromda surpassed that of the sea-nymphs. The sea-nymphs complained to the sea god of this affront, and Andromeda, to appease their wrath, was chained to a rock to be devoured by sea-monsters. Perseus (2 syl.) delivered her, and made her his wife. The vain mother was taken to heaven out of the way, and placed among the stars.
1
That starred Ethiop queen that strove
To set her beautys praise above
The sea-nymphs and their powers offended.
Milton: Il Penseroso
N.B.Her beautys praise means that of her beautiful daughter. Andromèda was her mothers beauty.