of . Along this path the Sun pursues his course w.* at a rate which has been estimated at about 150 millions of miles, or five-sixths of the diameter of the Earth's orbit in each year. Hence, since the Earth's orbit-plane is carried along at this rate, while the Earth circles around that orbit once in each year, it follows that the actual path pursued by the Earth in

Illustrating the motion of the Earth's orbit through space.

space is such as is indicated in fig. 89. It is in fact a skew spiral or helicoidal path.‡

thereabouts.

Or rather within the sidereal system, which itself doubtless has some motion-perhaps an inconceivably rapid motion.

Some persons have expressed great anxiety lest, if the Sun is really travelling so swiftly through space, he should leave the Earth behind. There is not the least fear of this, any more than there is that the Earth should leave the Moon behind. Whatever forces have caused the Sun to follow his present career have acted upon the Earth and all the planets with equal effect. And as the Sun's course through

The other members of the planetary system pursue paths of different figures. The coils of the helicoids traversed by Mercury and Venus lie relatively close together; those of Mars are not so near as the coils of the Earth's path. The asteroidal helicoid pathsamazingly complicated they must be-are yet more drawn out (so to speak). But it is when we pass beyond the asteroidal orbits that we get the most ex

tended helicoids. Jupiter is carried some 1,700,000,000 miles onward with the advancing Sun, while he circuits once around his orbit of less than 1,000,000,000 miles in diameter; Saturn sweeps on through some 4,400,000,000 miles, while circuiting his orbit, less than 1,850,000,000 miles in diameter; and the paths

space becomes modified by the varying attractions to which he may be subjected, every planet in the solar system, every satellite, meteor, planetary comet, and asteroid, will experience the same influences, and accompany the Sun just as faithfully as at present.

traversed by Uranus and Neptune amid the depths of sidereal space are even more remarkably drawn out, regarding them in their helicoidal character.

As the Sun travels through space the planets sweep onward with him. But has he, besides his planetary dependants, any companions on his voyage? Do any of his brother suns travel along with him? As yet we have no means of knowing, for a strange difficulty arises. If the Sun has companions, these must, of course, be relatively near to him. It does not follow that they will appear brighter than other stars, because if they are no larger than the Sun, we know that other orbs (as Sirius and Arcturus) must largely exceed them in real size, and so their relative proximity may not be rendered apparent. But this is not all. The stars which astronomers select as most likely to afford measurable indications of proximity are those whose apparent motions on the heavens are exceptionally large. Now the companions of our Sun on his voyage through the sidereal system doubtless travel on a nearly parallel course; and therefore, setting aside their orbital motions around each other, or around the common centre of gravity of the family, they must appear, as viewed by us, to be almost at rest. They would of course indicate in a more marked manner than any other stars the effect of the Earth's annual

*It is worthy of notice that the Sun's northern hemisphere travels forwards. Is it not conceivable that in this peculiarity we may find some explanation of the greater heat which has been said to be emitted from the northern solar hemisphere? See also p. 210, footnote.

motion (or, technically, they would have a large annual parallactic displacement); but then astronomers would not be led to look for such effects, since we know as a matter of fact that the stars hitherto examined for signs of annual parallax are those which, either

through exceptional brilliancy or through exceptionally large proper motion, seem likely to be near

to us.

We find signs in the heavens leading us to regard the existence of such companions of the Sun' as at least not wholly improbable. Here, for example, is a picture (fig. 90) borrowed from my Other Worlds,'

in which the chief stars of the constellation Ursa Major are depicted, and some few others belonging to Draco and Bootes. To each star is attached a small arrow indicating the direction of its motion, and the amount of such motion in 36,000 years. We see here decided signs of star-drift. We can scarcely doubt that the five principal stars of Ursa Major included within the dotted line are travelling together though space; while the four stars above, belonging to Draco,

α

FIG. 91.

Observed proper motion of Stars in head of Aries.

seem similarly to be companion suns. The remaining stars may also not improbably form a single family.

The group of stars shown in fig. 91 seem to form a system within which probably there are orbital motions of considerable magnitude.

It is worthy of notice that in the two instances here referred to there are evidences of association apart from the observed proper motions. For in the second the stars forming the system seem segregated in a somewhat marked manner from neighbouring star