Mars is, on an average, about one and a half times farther from the sun
than we are. Its mean distance is, in round numbers, one hundred and
forty-one million miles; but, since its orbit is very eccentric--more
eccentric than that of any other of the planets except Mercury--its
distance from the sun varies as much as twenty-six million miles.
At its nearest the planet is a little more than one hundred and
twenty-eight million miles from the sun. Its greatest distance from
that luminary is one hundred and fifty-four million miles. At its mean
distance something more than twelve and a half minutes are required for
light to travel from the sun to the planet.
The sun becomes quite a medium-sized object as viewed from Mars, and
must lose some of the majesty of aspect that it has to us. Its apparent
diameter is about twenty-one minutes, which would make it less than
two-thirds as large as we see it. The average amount of light and heat
that it furnishes to that poor, lightly clad little planet is less than
half as much as we receive, though when the planet is at perihelion the
sun’s radiance is forty per cent. more powerful than when it is at its
greatest distance from the source of these life-giving forces.
The eccentricity of the orbit of Mars is the cause also of his great
variations in distance from us, and hence of his extreme changes in
brilliancy. These changes are many times greater with reference to
the earth than to the sun. At the planet’s nearest approach to us it
comes a little nearer than thirty-five millions of miles. This is when
it is in opposition in August. When opposition occurs in February, it
is as much as sixty-two millions of miles from us; and when it is in
conjunction, and on the other side of the sun from us, it is sometimes
two hundred and forty-eight million miles distant. At his nearest
approach light leaps over to us from Mars in about four minutes and
eighteen seconds; at his greatest distance it cannot reach us in less
than twenty-two minutes. The apparent mean diameter of Mars is about
nine and fifty-six hundredths seconds, but varies from three and
six-tenths seconds, when the planet is farthest away, to twenty-five
seconds when it is nearest to us.
While Mars does not exhibit the phases of the inner planets Venus
and Mercury, by showing a disc sometimes at half-full and sometimes
at crescent it is sufficiently near us to be, in certain positions,
gibbous, or to show a little less than a full face. When this occurs
Mars is about half-way between opposition and conjunction, and the
earth and the sun are so situated that we are slightly to one side
of the fully illuminated face of Mars. This phase, however, is not
sufficiently marked to make any material difference in the brilliancy
of the planet. It is not apparent without the aid of a telescope.
From Mars the earth shows all the phases that Venus shows to us. When
Mars is flaming down upon us in his position of greatest brilliancy we
present to him a thin crescent. When he sees our full face we are on
the opposite side of the sun from him. It would be necessary to have
a more brilliant electrical illumination than any we have yet seen to
lighten the dark side of the earth and exchange signals with Mars when
we are nearest to him--if, indeed, our atmosphere would permit from
Mars any view at all of the surface of the earth, which is not at all
certain. In spite of its phases, the earth must shine on Mars at times
in a very attractive way. It is not so bright, perhaps, as Venus is
to us, nor as we are to Venus; but with our moon circling about us we
may well be, when in a favorable situation, a very interesting double
star, the distance between earth and moon appearing on Mars about equal
to one-fourth of the apparent diameter of the moon.
[Illustration: MARS: DIFFERENCE IN ITS APPARENT SIZE AT ITS NEAREST,
MIDDLE, AND FARTHEST DISTANCE FROM THE EARTH