90 LUCY AMONG THE MOUNTAINS.

ought to be to see a star. We never see this
planet by day, although it goes with the sun,
sometimes a little before him, and sometimes a
little after him, but never a great way off.”

‘What makes it sometimes before him and
sometimes after him?” asked Lucy.

‘Why, that’s of course,” said Robert.

“No, not exactly of course,” said her mother.
“Jt might revolve around the sun in such a way
as always to appear to be at the same distance.
But, as it happens, it does not. It goes round in
such a way that sometimes it appears before the
sun, and sometimes behind it, and sometimes it is
directly between us and the sun. It passes for-
ward between us and the sun until it gets before
him ; then it turns and wheels away around on
the other side, and goes on until it gets behind the
sun. Then it comes round on this side again ;
and so it keeps going and coming.

« But, then,” she continued, “ we can very sel-
dom see it. ‘There are only three cases in which
we can see it. One is, that when it is before the
sun, we can see it in the morning ; because, then,
you see, it rises first, and so we can see it before it
becomes quite light.”

“ But Robert said i¢ was very light when he
saw it,” said Lucy.