Astronomy of To-day

f important bodies known as planets. Certain of these planets, in their courses, carry along in company still smaller bodies called satellites, which revolve around them. With reg

l devote our attention merely to the

alone which appears to characterise all solid objects in the cel

dle held pointing in one fixed direction while the ball is turned round and round. Well, it is the same thing with the earth. As it journeys about the sun, it keeps turning round and round continually as if pivoted upon a mighty knitting needle transfixing it from North Pole to South Pole. In reality,

ed by astronomers to signify the motion which a celestial body has upon an axis; the term "revolution," on the other hand, is used for the movement

the system rolled together. Next comes Jupiter and afterwards the other planets in the following order of size:-Saturn, Uranus, Neptune, the Earth, Venus, Mars, and Mercury. Very much smaller than any of these are the asteroids, of which Ceres, the largest, is less than 500 miles in diameter. It is, by t

66,54

ry 2,

s 7,

h 7,

4,3

OF AS

er 87

n 73,

[3] 34

e[3] 3

th, and a similar decrease in size from Jupiter outwards. Were Mars greater than the earth, the planets could then with truth be said to increase in size up to Jupiter,

s nearly four times. After this, just as we found a sudden increase in size, so do we meet with a sudden increase in distance. Jupiter, for instance, is a

Neptune made it more than half as broad again. Nothing indeed can better show the import of these great discoveries than to take a pair of compasses and roughly set out the above relati

s cannot be regarded as true circles. They are ovals, or, to speak in technical language, "ellipses." Their ovalness or "ellipticity" is, however, in each case not by any means of the same degree. Some orbits-for instance, that of the earth-differ only

e various rates at which they do so will, however, be best apprecia

ground we have, if any, for assert

immense distance of the celestial bodies, and in the absence of any knowledge of the kind they were inclined to imagine them comparatively near. It was indeed only after the lapse of many centuries, when men had at last realised the enormous gulf which separated them from even the nearest object in the sky, that a more reasonable opinion began to prevail. It was then seen that this revolution of the heavens about the earth could be more easily and more satisfactorily explained by supposing a mere rotation of the solid earth about a fixed axis, pointed in the direction of

egard, however, to Uranus and Neptune there is much more uncertainty, as these planets are at such great distances that even our best telescopes give but a confused view of the markings which they display; still a period of rotation of from ten to twelve hours appears to be accepted for them. On the other hand the constant blaze of sunlight in the neighbourhood of Mercury and Venus equally hampers astronomers in this quest. The older telescopic observers considered that the rotation periods of these two planets were about the same as that of the ea

h has to be considered. Here, too, it is well to start with the earth's period of revolut

known to us as a "year." The following table shows in days and years the periods

about

s "

year and

11 years a

9 years an

84 years

64 years an

nt fact, namely, that the nearer a plan

t the middle of the reign of George II., that "year" would be only just coming to a close; for the planet is but now arriving back to the position,

the sun, the next point to be inquired into is:-What are the

sizes, placed one within the other, and the sun by a small ball in the middle of the whole; in what

h regard to the others, so that one half of a hoop rises out of the water and the other half consequently sinks beneath the surface. This indeed is the actual case with regard to the planetary orbits. They do not by any means lie all exactly in the same plane. Each one of them is tilted, or inclined, a little with respect to the plane of the earth's orbit, which astronomers, for convenience, regard as the level of the solar syst

luminous, whereas the planetary body around which a satellite revolves is not. True, planets shine, and shine very brightly too; as, for instance, Venus and Jupiter. But they do not give forth any light of their own, as the sun does; they merely reflect the sunlight which they receive from him. Putting this one fact aside, the analogy between the planetary system and a satellite system is remarkable. The satellites are spherical in form, and differ markedly in size; t

here be mentioned that the above figures are taken from Professor F.R. Moulton's Introduction to Astronomy (