NOTES ON THE MAPS AND CHARTS
The maps included in this work have been photographed from a globe of Mars specially made for the purpose from various charts. In all the maps the south is at the top and the north at the bottom; and the series shows the general surface configuration all round the planet, together with the principal canal lines which have been observed; but many other canal lines exist, especially on the dark areas near the south pole. These lines are usually straight and uniform in width throughout their whole length: indeed it is difficult to mark them upon a globe so that they shall appear as regular and uniform as they are actually seen on the planet.
The names on the maps are those now generally accepted and used by astronomers.
The "Greenwich" of Mars, i.e. the point on the 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.