The process has gone further and a number of distinct tails can now be
counted.]
But there are many things that point to absence of air on the moon. Even
the photographs we reproduce tell the same story. The edges of the
shadows are all hard and black. If there had been an appreciable
atmosphere it would have scattered the sun's light on to the edges and
produced a gradual shading off such as we see on the earth. This
relative absence of air must give rise to some surprising effects. There
will be no sounds on the moon, because sounds are merely air waves. Even
a meteor shattering itself to a violent end against the surface of the
moon would make no noise. Nor would it herald its coming by glowing into
a "shooting star," as it would on entering the earth's atmosphere. There
will be no floating dust, no scent, no twilight, no blue sky, no
twinkling of the stars. The sky will be always black and the stars will
be clearly visible by day as by night. The sun's wonderful corona, which
no man on earth, even by seizing every opportunity during eclipses, can
hope to see for more than two hours in all in a long lifetime, will be
visible all day. So will the great red flames of the sun. Of course,
there will be no life, and no landscape effects and scenery effects due
to vegetation.
The moon takes approximately twenty-seven of our days to turn once on
its axis. So for fourteen days there is continuous night, when the
temperature must sink away down towards the absolute cold of space. This
will be followed without an instant of twilight by full daylight. For
another fourteen days the sun's rays will bear straight down, with no
diffusion or absorption of their heat, or light, on the way. It does not
follow, however, that the temperature of the moon's surface must rise
enormously. It may not even rise to the temperature of melting ice.
Seeing there is no air there can be no check on radiation. The heat that
the moon gets will radiate away immediately. We know that amongst the
coldest places on the earth are the tops of very high mountains, the
points that have reared themselves nearest to the sun but farthest out
of the sheltering blanket of the earth's atmosphere. The actual
temperature of the moon's surface by day is a moot point. It may be
below the freezing-point or above the boiling-point of water.