Neptune was then discovered that very night on September 23 , 1846 , within 1 Â of where Le Verrier had predicted it to be , and about 10 Â from Adams ' prediction . The very evening of the day of receipt of Le Verrier 's letter on September 23 , 1846 , Neptune was discovered within 1 Â of where Le Verrier had predicted it to be , and about 12 Â from Adams ' prediction .