Chauntlucet is an ancient spell which, though by 1817 it had not worked for centuries, may again have become functional since magic has returned to England.
It is one of the several spells John Childermass refers to when his agitated master demands the name of any which ceased to take effect after the Raven King's departure. Reports of 'wild magic' happening in different parts of the country - that is, spells worked spontaneously by persons without magical training - have alerted Mr Norrell to the success of his rival Strange in restoring true English magic. The purpose of Chauntlucet was to "[encourage] the moon to sing". The song of the moon is supposedly "very beautiful and can cure leprosy or madness", if any of its listeners happen to be so afflicted .
It is perhaps worth observing that an ancient spell used by Jonathan Strange to protect himself and King George III from the effects of an enchantment intended to deceive their senses also includes an invocation to the moon:
"Place the moon at my eyes and her whiteness shall devour the false sights the deceiver has placed there."
It may be that the moon is in some way a corrective to false visions and delusions.