4. The orbits of the moon and planets.
All the planets move round the sun in orbits whose planes are slightly inclined to that of the earth, the plane of Mercury having the greatest inclination of 6°. The plane of the moon’s orbit round the earth is also inclined at 5° 9′ to the ecliptic. The orbits of the moon and all the planets must necessarily intersect the plane of the earth’s orbit on the ecliptic at two points, and these are called the nodes of the moon and each planet respectively. In consequence of the inclination being so slight, though the course of the moon and planets is not actually on the ecliptic, they are all so close to it that they are included in the belt of the zodiac. Thus the moon and all the planets follow almost the same apparent course on the zodiac or belt round the ecliptic in the changes of position resulting from their own and the earth’s orbital movements with reference to what are called the fixed stars.