ust be influenced solely by the changes in his distance from the sun,
which are much greater than in our own ease. There is some reason to
believe, too, that Mercury's day and year are the same length. This
means that as the planet circles round the sun he turns once. If this is
so the sun will shine on one half of the planet, producing an
accumulated heat terrific to think of; while the other side is plunged
in blackness. The side which faces the sun must be heated to a pitch
inconceivable to us during the nearer half of the orbit--a pitch at
which every substance must be at boiling-point, and which no life as we
know it could possibly endure. Seen from our point of view, Mercury goes
through all the phases of the moon, as he shines by the reflected light
of the sun; but this point we shall consider more particularly in regard
to Venus, as Venus is nearer to us and easier to study. For a long time
astronomers had a fancy that there might be another planet even nearer
to the sun than Mercury, perhaps hidden from us by the great glare of
the sun. They even named this imaginary planet Vulcan, and some thought
they had seen it, but it is tolerably certain that Vulcan existed only
in imagination. Mercury is the nearest planet to the sun, and also the
smallest, of course excepting the asteroids. It is about three thousand
miles in diameter, and as our moon is two thousand miles, it is not so
much bigger than that. So far as we are concerned, it is improbable we
shall ever know very much more about this little planet.

But next we come to Venus, our beautiful bright neighbour, who
approaches nearer to us than any other heavenly body except the moon.
Alas! when she is nearest, she like Mercury, turns her dark side toward
us, coming in between us and the sun, so that we cannot observe her at
all.

Everyone must have noticed Venus, however carelessly they have looked at
the sky; but it is likely that far more people have seen her as an
evening than a morning star, for most people are in bed when the sun
rises, and it is only before sunrise or after sunset we can see Venus
well. She is at her best from our point of view when she seems to us to
be furthest from the sun, for then we can study her best, and at these
times she appears like a half or three-quarter moon, as we only see a
part of the side from which the sunlight is reflected. She shines like a
little silver lamp, excelling every other planet, even Jupiter, the
largest