Original: The moon was discovered by British astronomer William Lassell on October 10, 1846, just 17 days after Neptune itself was discovered by German astronomers Johann Gottfried Galle and Heinrich Louis d'Arrest.
Simplified: It was discovered by British astronomer William Lassell on October 10 1846, just 17 days after Neptune itself was discovered by German astronomer Johann Gottfried Galle.