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invisible to us.

"Now, we are out in space where there is no atmosphere at all, so the
sky appears a very dense black; and the stars, having nothing to obscure
their light, shine out more brilliantly than they do on the earth. They
appear as bright points of light, and even the sun does not shed a
general light over the sky, there being no atmosphere to diffuse it."

"Yes," he persisted, "but you said we should have no more night until we
got to Mars!"

"Certainly," I answered. "Surely, M'Allister, you must have forgotten
that night is brought about by the earth's rotation on its axis, and
that the part which is turned away from the sun is in darkness because
its light is hidden by the solid body of the earth, while the earth's
shadow darkens all the sky. When, by the earth's rotation, that part is
again turned to the sun then it becomes daylight. Remember we are not
now on the earth, but out in space!"

"Of course I did know all that, Professor," he exclaimed, "but, just for
the time, I had forgotten."

"Never mind, M'Allister, we all forget such matters sometimes, and this
is quite a new experience for you. But just take a good look at the
sun--have you noticed any difference in its appearance?"

"Yes, Professor, it doesn't look the same colour as when we saw it from
the earth; it seems to have a violet tinge, like some of the electric
lights in our streets. There are also long streamers of light around it,
and coloured fringes close to the sun!"

"Yes, that is so," I said; "and we can see all those things now because
there is no atmosphere. No doubt you have noticed that on the earth the
sun appeared red when low down in the sky, and during a fog it appeared
redder and duskier still."

"Oh yes, I've often noticed that," he answered.

"That was caused by our atmosphere which, when thick, absorbs all but
the red rays of light. On a clear day the sun appears an extremely pale
yellow, or very nearly white; still the atmosphere absorbs some of the
light rays, so we cannot see its true colour as we do now. Those
coloured fringes round the edges can only be seen from the earth by the
aid of a special instrument, and then they do not show all their true
colours.

"That pearly light all round the sun, and the long streamers that give
it the appearance of an enormous star with six long points, form what is
termed the solar corona, and this can only be seen from our earth during
the very few minutes when an ecli