Planet

PLANET (Gr. 7-Xavirns, a wanderer). The term was applied in the ancient Ptolemaic or geocentric astronomy to the seven heavenly bodies which were observed to change their places as pro jected against the background of the so-called fixed stars. These bodies were the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn, all of which were supposed to revolve round the earth. In the modern or heliocentric astronomy (taught by Aristarchus of Samos in the third century B.C. but forgotten or neglected till revived by Copernicus in the 16th century of the Christian era) it is applied to all dark and opaque bodies in revolution round the sun, and the number of such bodies known has been greatly in creased by successive discoveries. In 1781 William Herschel dis covered Uranus. In 1801, on the first night of the new century, Piazzi discovered Ceres, a small planet revolving between the or bits of Mars and Jupiter and the first of a great host of small bod ies known as minor planets or asteroids; while in 1846 Neptune was added to the list of major planets following on the mathemati cal work on the unexplained irregularities in the motion of Uranus by Le Verrier and, independently, by Adams (see notes on Nep tune). Recently (193o, Feb.) a yet more remote planet has been found at the Flagstaff Observatory near the position predicted by the late Professor Lowell. It has been named Pluto. Most of the major planets possess moons or satellites. So far as at present known, Neptune possesses one, Uranus four, Saturn nine, Jupiter nine, Mars two and the Earth one.

Distances of the Planets.

An empirical law expressing very closely the relative distances from the sun of the primary planets known to the ancients was discovered about the middle of the seventeenth century by Titius of Wittenburg, but it was later brought into notice by Bode and is generally known as Bode's law. If a series of 4's be written down, and 3 be added to the second, 6 to the third, 12 to the fourth and so on, each of the added num bers being double its predecessor, the sums of the first seven num bers are 4, 7, 1o, 16, 28, 52, 100. These numbers divided by ro were found to give with a surprising closeness the relative dis tances of the planets as far as Saturn, except that no planet was known to revolve in the orbit correspondng to 2.8 between the

orbits of Mars arid Jupiter. When Uranus was discovered and found to fit in well with the next number in the series (19.6) the conviction was strengthened that the gap between Mars and Jupi ter must be occupied by some planet and a search for the missing body was organized. But before systematic work was actually be gun the little planet Ceres was found. The discovery was soon fol lowed by others, and at the present time considerably more than 1,000 of these minor planets are known. They are not, however, actually confined to the gap above mentioned. The little planet Eros discovered by Witt in 1898 comes at perihelion within 13, 000,000 miles of the earth's orbit and hence it is of special value to astronomers for the determination of the solar parallax which fixes the scale of the planetary system. Hidalgo, discovered by Baade in 1922, in the most distant part of its orbit goes out as far as the orbit of Saturn and has a mean distance slightly greater than that of Jupiter. The actual distances of the greater planets from the sun in miles will be found in Table I.

Orbits and Phases of the Planets.

From the observations of Tycho Brahe, the great Danish observer, Kepler deduced the following three great laws of planetary motion:— (r) The planets move in ellipses with the sun at one focus. Hitherto it had been believed that the circle being the most per fect geometrical figure celestial motions took place in circles.

(2) The straight line from the sun to a planet (radius vector) passes over equal areas in equal times. The importance of this law was that it involves a continually varying rate of motion, which was also contrary to accepted beliefs at the time. To maintain the equal description of areas in unit time a planet must travel faster in the neighbourhood of perihelion where the radius vector is at a minimum than near aphelion where it attains its maximum value.