medio . . . erat, ‘when the sun was in the full heat of his mid course,’ was at his strongest with his noon-tide heat, orbis being the circular path of the sun's apparent motion as in G. IV. 426, medium sol igneus orbem hauserat. The same expression occurs I. 592, XI. 353.
plurimus, ‘in fullest presence’ and so ‘most powerful.’ Cf. Her.IV. 167, per Venerem parcas oro, quae plurima mecum est, Virg. Aen.III. 372, multo suspensum numine, and the corresponding use of “πολύς”, for which see Palmer on Her.l.c.