Meridian from which astronomers reckon the Martian longitudes, is indicated by the apex of the small triangular light area just above the equator in Map I. It is marked on the map as "Fastigium Aryn," and is chosen as longitude "0," because from its general outline it cannot be mistaken by observers.

"Sirapion," the supposed landing-place of the travellers mentioned in the story, is shown on Map III., just above the central and lowest point of the dark area at the top of the map. This name will not be found upon any other map of Mars.

The chart showing the relative positions of the Earth and Mars during the years 1909–10 is reduced and modified from one prepared accurately to scale by the author for his own use in connection with the book. From it the reader will gain a clear idea of the shape of the two orbits and how they are placed with regard to each other.

It also shows the course supposed to be taken by the air-ship on its outward and homeward journeys, and the point reached when one of the travellers desired to turn back; together with the alternative routes which were then discussed.

This chart, and the other one showing the positions of the two planets at different oppositions of Mars, will enable the reader to understand how it is that Mars approaches so much nearer to the earth at some oppositions than it does at others. The positions of the oppositions from 1916 to 1922 are only approximations, as no exact 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 ray-streaks are only suggested in the picture, because, if shown complete, they would have the same effect as upon the moon, viz. they would entirely obliterate all the formations over which they passed.

The Key Map indicates the principal lunar formations, and includes the names of those mentioned in the book.

The last two plates in the book are from drawings made at the telescope (a 12-inch Calver reflector) by the Rev. T. E. R. Phillips. The opposition of 1909 was not favourable for the observation of Martian details from England; for although the planet was near to us, it was too low down in the sky; and many of the nights were either cloudy or misty.

[Pg xxii-xxiiv]

THE SUN, MOON, AND PLANETS