26 ASTRONOMY IN ANCIENT NATIONS surface in one point." The axis of the excentric sphere is incli- ned to that of the mumaṭṭal sphere, which causes the motion in latitude. The lunar system comprises an additional sphere outside the others, the centre of which coincides with the centre of the world, and which is called al-gauzahar, signifying the cosntella- 10 8 J 9 • II Fig. 2-Spheres of Mercury 1. Upper Apsis, 2. Lower Apsis, Upper Apsie of deferent sphere, 5. Deferent sphere. 4 Lower Apsis of deferent sphere. 6. Epicycle. 7. Mercury, 8, Surrounding complement. 9. Surrounded part of Mumattal sphere. 10. Mudir sphere. 11. Centre of the world. 12, Centre of Mudir, 13. Centre of deferent sphere. tion Draco, as this sphere provides for the revolution of the lunar nodes ("the head and tail of the dragon") round the zodiac. The inner one of the two concentric spherical surfaces, between which the excentric sphere lies, surrounds immediately the fire sphere of the Earth. The system of Mercury is more complicated, as a space had to be provided for the revolution of the centre of the (Continued from previous page) one is that of the concentric oblique intersphere (called the mail sphere or the sphæra deflectens) round the centre of the world, 11°9' per day, by which amount the lunar apogee moves towards the west. The third motion is that of the excentric, carrying the centre of the epicycle 24°22' towards the gast. The fourth is the motion on the epicycle, Abu 'l Faraj, p. 27,
