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Description of the Planets

units, period, distance, miles, jupiters, inside and gap

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DESCRIPTION OF THE PLANETS The following description is confined to the first 90o members, as details of the remainder were not available when it was pre pared. There are two stragglers well on the inside of the family ; these are Eros (see separate article) whose mean distance is less than that of Mars, and Hungaria (which, curiously, was the very next discovery to Eros in order of time) at a mean distance of 1.9 astronomical units. The main body begins at mean distance 2.1 units, and continues without a break up to 3.5 units. Then follows a gap with a group of six (the Hilda group) at 3.9 units, and an isolated planet, Thule, at 4.3 units. Its name is from the classical "Ultima Thule," as it was thought to mark the outer boundary of the family till the Trojan group was found; that group consists of six planets, whose mean distance is 5.2 units, the same as that of Jupiter (see TROJAN PLANETS).

Although the main body has no absolute gaps there are several very sparse regions in it. These correspond to the distances where the period is a simple fraction of Jupiter's period, 11.86 years. The most notable gap is at 3.3 units, where the period is half Jupiter's; there is also a marked one at 2.5 units, period one-third of Jupiter's; the others are more feebly marked. The richest region of all is just inside the chief gap. Similarly in Saturn's ring the brightest part is just inside the great division, which is at a dis tance from Saturn corresponding to half the period of Mimas. In each case the reason is the same; repeated perturbations at the same region of the orbit cause a slight change in the period and distance ; inward shift has predominated, causing the rich region inside the gap. The influence of Jupiter on the motions of the asteroids is made manifest in another way: when the positions of their perihelia, or nearest approaches to the sun, are plotted on a diagram, it is seen that they congregate towards Jupiter's peri helion; the density of distribution is here three times as great as in the opposite region.

The eccentricities of the orbits are distributed as follows : 209 planets have eccentricity between o and .087, 375 between .087

and .174, 248 between .174 and .259, 49 between •259 and .342, 7 between .342 and .423, while four stragglers, Albert, Alinda, Ganymede and Hidalgo have eccentricity greater than one-half. The figures for inclination of orbits to the ecliptic are : 222 planets between o° and 5°, 297 between 5° and o°, 222 between o° and 15°, 98 between 15° and 35 between 20° and 25°, 14 between 25° and 3o°, and 3 above 3o°. There seems to be a tendency for high eccentricity and inclination to go together.

The diameters can be estimated only by the amount of light that we receive, since with few exceptions they are too small to measure even with the largest telescopes; the albedo has to be estimated, and some uncertainty is thus introduced. 195 planets have estimated diameters exceeding 61 miles, 502 between 61 and 25 miles, 193 between 25 and ro miles, and 22 less than 1 o miles, but these last very small ones can be seen only when they come fairly near to the earth, so there may be many of them undis covered in the outer part of the asteroid zone. Estimates of the total mass of the family have been made, assuming a density equal to that of the moon ; the following is taken from "Astron omy," by Drs. Russell, Dugan and Stewart p. 352. The total mass of the known asteroids is about 1/3000 of that of the earth, Ceres and Pallas accounting for half of the whole ; the undiscovered ones, though probably numerous would be mainly small, so that 1/50o of the earth is an extremely liberal estimate for the whole family. It thus appears that, even if united, they would form an insignificant planet.

There appears to be no reason to expect that the smaller planets would have a spherical form. In the case of large planets the force of cohesion is negligible compared with the gravitational forces, and any large departure from the figure of equilibrium is impos sible ; but with small bodies cohesion would be stronger than gravitation. Irregular forms are suggested by the fact that the light of many asteroids is variable ; this indicates either irregu larity of shape or unequal albedo of different parts of the surface.

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