In his Astronomiae Elementa (‘The Elements of Astronomy’) in 1702 David Gregory (1659-1708) notes the distances of the planets from the Sun are roughly proportional to the numbers 4, 7, 10, 15, 52, 95. Johann Daniel Titius (1729-96) put the sequence into the form 4, 4+3, 4+6, 4+12, 4+48, 4+96. None of the planets corresponded with the missing term (4+24) and Johann Elert Bode (1747-1826), who publicised the law (1768), became convinced that there was an undiscovered planet in the Mars-Jupiter gap.
Friedrich Wilhelm Herschel (1738-1822) left Hanover for England in 1757. At the age of nineteen he changed his name to Frederick William Herschel. He settled in Bath in 1766 and as an organist, composer and music teacher he built up an income sufficient to enable him to pursue his love of astronomy. In 1781, while scanning the heavens with his younger sister Caroline Lucretia Herschel (1750-1848), he discovered the planet Uranus. It was found to have an orbit that corresponded with the next term in the sequence, 4+192.
In 1801 Giuseppe Piazzi (1746-1826) discovered the first asteroid Ceres and it was found to correspond with the term (4+24) in Bode’s law. Then, in 1802, Heinrich William Mathaus Olbers discovered a second asteroid, Pallas. To comply with the law, Olbers suggested that the asteroids were fragments of a full-sized planet that had once occupied the gap.
In 1937 Karl Wilhelm Reinmuth (1892-1979) noticed an odd streak of light in a picture he had just taken of the night sky. It was an asteroid, close to Earth and moving fast – so fast that he named it Hermes. Hermes is a member of the Apollo group, a group of asteroids that cross Earth’s orbit, but whose average distances from the Sun are greater than that of Earth. They are therefore known as Earth-crossing or Earth-grazing asteroids.
In 1891 Maximilian (‘Max’) Franz Joseph Cornelius Wolf (1863-1932) instituted a programme of wide-field photography for the discovery of asteroids, which appeared on the developed plates as trails among the stars. Over 200 asteroids are larger than 100 km, and about a million have diameters of one km or more. The total mass of the asteroid belt is estimated to be about 3×1021 kg, which is just 4% of the mass of the Moon. They are believed to be the survivors of a myriad of small objects that failed to coalesce into a planet because of the attractive pull of the newly-formed Jupiter.
John Couch Adams (1819-92) calculated the orbit of a new planet whose gravitational effects would explain why Uranus did not follow its predicted path. In 1846, from independent calculations by Urbain Jean Joseph Le Verrier (1811-1877), Johann Gottfried Galle (1812-1910) sighted the new planet, which was subsequently named Neptune.
Percival Lowell (1855-1916) calculated the position of another unknown planet that he believed was the cause of irregularities in the orbits of Uranus and Neptune. Pluto was discovered in 1930 by Clyde William Tombaugh (1906-97) at Lowell Observatory (est.1894). Neptune and Pluto do not conform to Bode’s law.
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