As one by one, at dread Medea's strain,
The sick'ning stars fade off th' ethereal plain.
It's the use of the word strain and its multiple meanings that suggests there could have been another aspect of Medea known only to the poet, or there was another meaning of the word back in 1728.
Is strain her willing, or is she singing some dread dirge? Her herbal medicines were mostly of a "beneficial" nature, but could she be straining the stars (her enemies?) into some kind of poisonous concoction?