Bode’s law, also called Titius-Bode law, empirical rule giving the approximate distances of planets from the Sun. It was first announced in 1766 by the German astronomer Johann Daniel Titius but was popularized only from 1772 by his countryman Johann Elert Bode. Once suspected to have some significance regarding the formation of the solar system, Bode’s law is now generally regarded as a numerological curiosity with no known justification.
One way to state Bode’s law begins with the sequence 0, 3, 6, 12, 24,…, in which each number after 3 is twice the previous one. To each number is added 4, and each result is divided by 10. Of the first seven answers—0.4, 0.7, 1.0, 1.6, 2.8, 5.2, 10.0—six of them (2.8 being the exception) closely approximate the distances from the Sun, expressed in astronomical units (AU; the mean Sun-Earth distance), of the six planets known when Titius devised the rule: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. At about 2.8 AU from the Sun, between Mars and Jupiter, the asteroids were later discovered, beginning with Ceres in 1801. The rule also was found to hold for the seventh planet, Uranus (discovered 1781), which lies at about 19 AU, but it failed to predict accurately the distance of the eighth planet, Neptune (1846), and that of Pluto, which was regarded as the ninth planet when it was discovered (1930). For a discussion of the roles that Bode’s law played in early asteroid discoveries and the search for planets in the outer solar system, see the articles asteroid and Neptune.