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.
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.