The ideal point situated midway between the two foci is called _the centre of the ellipse_, or _orbit_; while the imaginary straight line which passes through both foci and the centre, with its ends at opposite points of the ellipse, is called "_the major axis_" of the orbit. It is also known as "_the line of the apsides_." The ideal straight line which, in passing through the centre of the orbit, cuts the major axis at right angles, and is prolonged on either side to opposite points on the ellipse, is called "_the minor axis_" of the orbit. When a planet reaches that extremity of the major axis of its orbit which is the nearest to the Sun, it is said to be in its "_perihelion_;" while, when it arrives at the other extremity, which is farthest from this body, it is said to be in its "_aphelion_." When a planet reaches either of the two opposite points of its orbit situated at the extremities of its minor axis, it is said to be at its _mean distance_ from the Sun. The rapidity with which the planets move on their orbits varies with their distance from the Sun; the farther they are from this body, the more slowly they move. The rapidity of their motion is greatest when they are in perihelion, and least when they are in aphelion, having its mean rate when these bodies are crossing either of the extremities of the minor axes of their orbits. The imaginary line which joins the Sun to a planet at any point of its orbit, and moves with this planet around the Sun, is called "_the radius vector_." According to Kepler's second law, whatever may be the distance of a planet from the Sun, the radius vector sweeps over equal areas of the plane of the planet's orbit in equal times. There is a remarkable relation between the distance of the planets from the Sun and their period of revolution, in consequence of which the squares of their periodic times are respectively equal to the cubes of their mean distances from the Sun. From this third law of Kepler, it results that the mere knowledge of the mean distance of a planet from the Sun enables one to know its period of revolution, and _vice versa_. The orbit described by the Earth around the Sun in a year, or the apparent path of the Sun in the sky, is called "_the ecliptic_." Like that of all the planetary orbits, the plane of the ecliptic passes through the Sun's centre. The ecliptic has a great importance in astronomy, inasmuch as it is the fundamental plane to which the orbits and motions of all planets are referred.