Neptune revolves around the Sun in its distant orbit once in about 165 years. Uranus completes its orbital revolution in 84 years, Jupiter in 12 years, Mars in about 15 months, Venus in 11 months, and Mercury in 18 weeks. If you imagine these bodies to be revolving in a plane around the Sun and yourself to be standing within the Sun, the motions of these bodies will appear almost uniform and always in one direction. Were the orbits of the planets circular and the Sun holding the centre of the circle, their motions would be constant, that is to say, always in the same direction and at the same rate. But the orbits are elliptical, and the Sun holds a position in one of the foci of each ellipse. Consequently the planets are at times further from the Sun than at others, and they are then said to be in their aphelion, the opposite point of the orbit where they are nearest to the Sun being called the perihelion. When at aphelion the planets move slower, and when at perihelion they move quicker than at the mean distance. Astronomers employ an imaginary circular orbit for the planets, in which they move at an uniform rate of velocity, which is called the mean motion. This is subject to an equation depending on the position of the planet in its orbit, and it determines the difference between the imaginary planet and the true planet. The equation itself depends on the eccentricity of the orbit, that is to say, its relation to a circle drawn around the same focal centre. The Earth follows the same laws as all other bodies of the same system. But if we imagine the Earth to be stationary in space and the centre around which the planets revolve, their motions present several irregularities. Mercury and Venus will then appear to revolve around the Sun while the Sun revolves around the Earth, sometimes being between the Earth and the Sun, which is called an Inferior conjunction, sometimes on the further side of the Sun away from the Earth, as at their Superior conjunction; and again, at other times to the right or left of the Sun, in East or West elongation. The other planets, having orbits greater than that of the Earth, will appear to revolve around it at constantly varying distances and velocities. At certain points in their orbits they will appear to remain stationary in the same part of the Zodiac. The annexed illustration will assist the lay reader perhaps. The body M is Mercury when at Inferior conjunction with the Sun, as seen from the Earth. The letter V is the planet Venus at Superior conjunction with the Sun. The points W and E are the points of greatest elongation West and East, and the letter S shows the points in the orbit at which those bodies appear to be stationary when viewed from the Earth, at G. As seen from the Earth, Venus would appear to be direct and Mercury retrograde. [Illustration]