Inspection shows at once why the oppositions of Mars which occur in August and September are the most favourable because Mars being then near the perihelion point of its elongated orbit is comparatively near the earth, while oppositions which occur in February and March are very unfavourable because then Mars is near the aphelion point of its orbit, and its distance from the earth is much greater. The oppositions occur along the more favourable part of the orbit about two years and two months apart. Thus the figure shows that the opposition of September 24, 1909 was followed by one on November 25, 1911. ]