64 



SCENERY OF THE HEAVENS. 



magnitude rather than to situation, and in that sense is erroneous. Venus, an inferior 

 planet, is far larger than Mars, one of the superior ; and more resplendent than any of the 

 rest. The words interior and exterior will exactly express what is intended, prevent 

 confusion, and had better be adopted. The planets are denoted in astronomical writings 

 by their respective symbols, which were in use at a very early date, derived from the 

 ancient mythology. 



The paths of the planets are elliptical, as previously described ; but the ellipses vary 

 to some extent in their eccentricity, or as to the distance of the sun at s, from the centre 

 of the oval, which here represents a planetary orbit. This 

 distance in the case of Mercury is about seven millions of miles ; 

 in that of Venus it is less than half a million ; in that of the Earth 

 it is somewhat more than a million and a half; in that of Mars it 

 is upwards of thirteen millions ; while in that of Juno and Pallas 

 it is sixty-four millions and a half. The orbits of the planets 

 are more or less inclined to each other, instead of lying in the 

 same plane. It may be desirable here to explain the meaning of 

 this oft-recurring phrase. Referring to the earth, an imaginary smooth and thin surface, 

 cutting through the centre of the sun, and reaching out to the fixed stars, is the plane in 



which the earth moves in revolution round 

 the sun. This is represented by the shaded 

 part of the diagram. The stars to which it 

 extends form the constellations of the 

 zodiac, the circle A, B, c, D, being the 

 ecliptic, which shows the sun's place in the 

 heavens as seen from the earth, or the 

 earth's as seen from the sun. The orbits 

 of the other planets are in different 

 degrees inclined to this plane, one half 

 being below and the other half above it. If 

 suppose the shaded part of the diagram 

 to be the surface of a basin of water, a ring 

 held inclined so as to dip into it half way, 

 will describe the relation between the plane 

 of the earth's orbit and that of another 

 planet. The mean velocity of the planets 

 in their orbits exhibits great diversity, those which are nearer to the sun being far more 

 rapid than those which are remote. Saturn prosecutes his circuit at the sober pace of 

 little more than twenty thousand miles an hour, while Mercury rushes on at a rate which 

 is more than five times that speed. The extraordinary swiftness of the interior planets 

 is obviously necessary to counterbalance the powerful attraction of the solar mass, which 

 acts with diminished force upon the bodies that are more distant from him. 



MERCURY, the nearest planet to the sun, is the smallest primary in the system, with the 

 exception of the asteroids. It is the fastest traveller also, having a velocity in space 

 which is nearly twice the rate of the earth's orbital motion. It is the densest celestial 

 body with which we are acquainted, supposed to be fourteen times that of water. A 

 globe of lead therefore of the same volume, if weighed in the balances against its mass, 

 would be found wanting. Yet notwithstanding this remarkable density, if loosened from 

 the centrifugal force, it would require more than a fortnight for the planet to accomplish 

 its dash headlong to the sun. The days and nights are about the same length as our 

 own ; but a whole cycle of seasons has been four times gone through, before the earth's 



