xact data are yet
available. The earth is closest to the orbit of Mars about the 27th of
August each year, and if Mars comes into opposition about that date it
is then only about thirty-five million miles away. If, however, the
opposition occurs near the 22nd February, the earth is then at its
greatest possible distance from the orbit of Mars, and that planet will
then be over sixty million miles away: appearing very much smaller than
when at its most favourable point of approach.

On the 18th of September, 1909, Mars was only slightly over thirty-six
million miles from the earth, and it will be fifteen years before such a
favourable situation again occurs.

The nearest point of approach does not necessarily occur on the actual
date of the opposition. In 1907 Mars was in perigee, as it is termed,
seven days after the opposition; while in 1909, perigee was before
opposition.

The diagram showing the positions and movements of the planets during
the period covered by the outward voyage of the _Areonal_ is
sufficiently explained by the notes printed thereon. It may, however, be
pointed out that though the orbits of the planets are all elliptical,
especially those of Mercury and Mars, they are so nearly true circles
that, when reduced to the scale of these diagrams, they practically
become circles. The exaggerated ellipses so often found in astronomical
books are very misleading. The orbits of Mercury and Mars have an
appearance of ellipticity because the sun does not occupy the central
point in the diagram.

The view of the moon is photographed from a large coloured drawing by
the author, which occupied many months in preparation and execution. It
shows all the principal formations seen through the telescope as the
moon passes through its various phases, but it must be understood that
the formations can never all be seen at one view as shown in this
picture. As the sun rises on any particular formation the details are
gradually revealed by the long shadows cast by the more elevated
portions when the sun is low down in the lunar sky. As the sun rises
higher and higher the shadows grow shorter and shorter, and when the sun
is vertically over the formation the shadows entirely disappear; all
details are thus rendered invisible.

When the moon is full the sun is practically vertical over the whole
lunar surface, so the only details then seen are those which are vaguely
brought out by differences in tint.

The bright ra