gives the nail does not
coincide with the bang that reaches you, for light gets to you
practically at once, and the sound comes after it. No sound can travel
without air, as we have heard, therefore no sound reaches us across
space. If the moon were to blow up into a million pieces we should see
the amazing spectacle, but should hear nothing of it. Light travels
everywhere throughout the universe, and by the use of this universal
carrier we have learnt all that we know about the stars and planets.
When the time that light takes to travel had been ascertained by means
of Jupiter's satellites, a still more important problem could be
solved--that was our own distance from the sun, which before had only
been known approximately, and this was calculated to be ninety-two
millions seven hundred thousand miles, though sometimes we are a little
nearer and sometimes a little further away.

Jupiter is marvellous, but beyond him lies the most wonderful body in
the whole solar system. We have found curiosities on our way out: we
have studied the problem of the asteroids, of the little moon that goes
round Mars in less time than Mars himself rotates; we have considered
the 'great red spot' on Jupiter, which apparently moves independently
of the planet; but nothing have we found as yet to compare with the
rings of Saturn. May you see this amazing sight through a telescope one
day!

Look at the picture of this wonderful system, and think what it would be
like if the earth were surrounded with similar rings! The first question
which occurs to all of us is what must the sky look like from Saturn?
What must it be to look up overhead and see several great hoops or
arches extending from one horizon to another, reflecting light in
different degrees of intensity? It would be as if we saw several immense
rainbows, far larger than any earthly rainbow, and of pure light, not
split into colours, extending permanently across the sky, and now and
then broken by the black shadow of the planet itself as it came between
them and the sun. However, we must begin at the beginning, and find out
about Saturn himself before we puzzle ourselves over his rings. Saturn
is not a very great deal less than Jupiter, though, so small are the
other planets in comparison, that if Saturn and all the rest were rolled
together, they would not make one mass so bulky as Jupiter! Saturn is
so light--in other words, his density is so small--that he is actually
lighter tha