that in its atmosphere are the vapours of elements which our fiercest furnaces can only liquefy. If, then, we could approach that self-luminous orb, we should find long before we reached the confines of its system that it is a true sun. And within its system—as seen, in fact, from a distance which, though enormous, is reduced to absolute nothingness when viewed from our enormous distance— it is certain that the star is a sun in this sense, that it is capable of dispelling night, that when it is above the horizon of any world having airs like ours there must be a glowing sky like that which, during our own day, hides the stars from our view.

Thus every one of the worlds, in systems belonging to a double star, has a quadruple alternation, in place of that double alternation which we call day and night. There is, first, double day,' when both suns 6 are above the horizon; next, single day with one sun; then, single day with the other sun; and, lastly, true night when both suns are below the horizon.

In my next I shall consider some of the results which must follow from these singular vicissitudes as well as the peculiarities of scenery, &c., which must prevail in worlds circling around coloured double stars.

229

WORLDS LIT BY COLOURED SUNS.

I WILL consider, first, the case of a world circling as our earth does in her orbit, but around a sun of a rich orange colour, while a companion sun of a blue colour travels around the same sun 1 on a path resembling that of the planet Jupiter. The blue sun would be a large and brilliant orb, as seen from the world whose condition I propose to describe; but the orange sun would necessarily be far more brilliant and look far larger, being in reality the larger sun, and also the nearer. We will assume that the world we are considering has a moon somewhat like our own, and we may reasonably imagine that several other planets travel around the orange sun, others around both suns (that is, outside the path of the blue sun), and that, again, the blue sun has several planets travelling in immediate dependence upon it.

Now, in the first place, let us take the case where the world is between the orange sun and the blue one, and let us suppose that the season corresponds to our spring. Then it is manifest that since one sun illumines one side

1 Speaking exactly, we should say that the two suns circle around their common centre of gravity; but here I deem it sufficient to use such expressions as accord best with ordinary modes of speaking.

of the globe, and the other illumines the other, there can be no night; it is orange day to one half of the world, and blue day to the other. Moreover, since the season corresponds to our spring time, it follows that orange day lasts exactly as long as blue day, and using for convenience the division of the day into twenty-four hours (which may or may not be nearly the same as our terrestrial hours), there are, all over the world, twelve hours of orange day and twelve hours of blue day. This, however, would not last very long, any more than on our own earth we have Jupiter visible all night for any length of time. The blue sun would gradually take up the position which Jupiter has when he is an evening star.1

Now, we can easily see what would follow from this. The blue sun would, in fact, rise before the orange sun had set. Thus there would be orange day as before, but towards orange sunset there would be two suns, the orange sun nearing the west, the blue sun passing over the eastern horizon. Then would come orange sunset and blue day; but the blue sun would set before the orange sun rose, and there would be therefore a short night, though no doubt not a dark night, since there would be blue twilight in the west and orange twilight in the east. Gradually the length of this night would increase, the length of the double day also increasing, but the orange and blue hours gradually shortening. At

1 This would happen at least if the blue sun were going the same way round the orange sun that the planet was going.

length the blue sun would have drawn quite near to the place of the orange sun in the heavens, and there would be double day and night, but neither orange day nor blue day alone. The double day would probably be white, since the colours of the two suns are supposed to be complementary. After this the blue sun would pass to the other side (the west) of the orange sun, and would be placed like Jupiter when he is a morning sun. There would then be blue morning, white day, orange evening, and night, the night gradually growing shorter and shorter, until at length the blue sun would be opposite the orange sun, and there would be no night, but simple alternation of blue day and orange day, as at first.

I have not, in following these changes, taken any account of the varying seasons, because except when the two suns are together or opposite to each other, the considerations involved become rather more complicated than is desirable for these pages. But I will now, without following the blue sun round again, consider the effect of seasonable peculiarities when the two suns are on opposite sides of the earth. (When the two suns are together, the effects are of course the same as those recognised in our ordinary seasons.) Now, first, be it noticed that whatever be the pose of the earth, if the two suns are on opposite sides of her there cannot be any night, since one sun must illumine one half, the other sun illumining the other. But, whereas when the earth is posed as our earth is in spring, or autumn, there is

everywhere equal orange day and equal blue day, this is not the case at other times.

Thus suppose the northern pole bowed towards the orange sun, as the northern pole of our earth is bowed towards our sun in summer, then in northern regions there is a long orange day, and a short blue day, and the reverse in southern regions. All round the northern pole that is, within the regions corresponding to our Arctic regions—the orange sun does not set and the blue sun does not rise throughout the twenty-four hours; while in the corresponding southern regions the blue sun does not rise, and the orange sun does not set. At the equator, however, orange day and blue day are equal. Of course all is reversed when the southern pole is bowed towards the orange sun.

But now let us consider how curiously the moon of our imagined earth must vary in aspect. I will consider just a few cases to show how wonderfully complex and beautiful must be the variations of a moon belonging to a double sun system.

Suppose the two suns on opposite sides of the earth. Then it is clear that the moon's globe must (precisely like the earth's globe) have one half lit up by orange light, and the other half by blue light. Now the orange half will pass through all the phases that our own white moon exhibits. It will be in turn round, half-full, gibbous, full, gibbous again, half-full, round, and new. But the part of the moon which with us appears dark, or