PLANETARY ORBITS
The primary, standard, measuring scale of the solar system is based on the earth's average distance from the sun, which is known as the astronomical unit (1 astronomical unit is about 150 million kilometers).' In deriving the scale of the solar system, astronomers have employed several independent techniques, of which the most accurate is that of timing the round trip of a pulsed radio signal reflected from a planet. Combining this information with the planets' distances in astronomical units, from Kepler's third law, leads to the absolute size of the solar system in kilometers.
A German astronomer, Johann Bode (1747-1826), called attention in 1772 to a numerical scheme, or rule (originally discovered by Johann Titius (1729-1796) in 1766), that seemed to predict the mean distances of the then-known planets from the sun. Although not a physical law in the same sense as Newton's laws, it is known as Bode's law; both Uranus, discovered in 1781, and the first asteroid, Ceres, found in 1801, adhered fairly well to this rule, but it broke down later when Neptune and Pluto were discovered. Despite the rule's having no unique physical basis, a similar rule relating the separations between planets seems to be characteristic of the formation of bodies in the gravitational field of a star.
All the planets are much alike in orbital characteristics. They revolve around the sun in the same direction in roughly circular orbits that lie nearly in the same plane. Mercury, the innermost planet, and Pluto, the outermost planet, depart most from this regularity. Between the terrestrial planets (Mercury, Venus, the earth, and Mars) the average spacing is much smaller than that separating the Jovian planets, Jupiter, Saturn, Uranus, and Neptune. The planets orbit at mean distances ranging from 40 percent of the earth's distance from the sun to 40 times earth's distance, with orbital periods between a quarter of a year and 248 years.
