But with all his variations in brilliancy and beauty Mars remains ever
a charming, rosy-hued planet, shining always with a steady, clear
light, and when once we have come to know him is not easily mistaken
for any other planet, or for any of the brilliant stars that may more
or less resemble him in color. Red in varying degrees of intensity is,
perhaps, the most obviously distinguishing mark of Mars; but his own
characteristics are never more distinct than when his path takes him
into the region of the two best-known red stars in the heavens. These
are Antares, the glowing star in the constellation Scorpio, which we
see in the southern sky during the summer, and ruddy Aldebaran, which
shines in the head of Taurus and under the Pleiades through the bright
wintry nights. On every journey around the skies Mars passes near these
two stars. They are both in the constellations of the zodiac, and are
often quite near to Mars, as well as to the other planets and the moon.
The stars, though of the same color as Mars, are much more jewel-like
than the planet. Mars is less sparkling. When it is small, it shows
a placid, rosy little disc, without much gaiety, and not in any way
suggesting anything martial; but at its largest, it has a distinctly
flame-like aspect, which easily suggests why it was named for the god
of war.
[Illustration: THE TWO PHASES OF MARS
We see its full face when it is opposite the sun. When half-way
between opposition and conjunction it becomes gibbous, as shown in the
photograph on the right. These photographs were made at the Mt. Wilson
Observatory.]
Mercury is the only planet that in color even suggests Mars, and for
Mercury it can never be mistaken after one has once seen the two
planets. Mercury, we know, is always very near the sun; but when
visible at all is, even in that unfavorable situation, always as
bright as a first-magnitude star. Mars is near the sun, to our view,
only when it is approaching conjunction, and it is then so far from
us that it always appears as a rather small star, and, while never
insignificant, is, in this situation, quite inconspicuous even as
compared with the rarely visible Mercury.
On seeing a planet, then, sufficiently high above the horizon to
attract one’s attention, one may be sure that it is Mars if it is red,
and equally sure that it is not Mars if it does not show this color.
Under certain atmospheric conditions the sun, moon, and all the planets
sometimes appear red when they are very near the horizon; but in this
situation there is always something other than color that marks them.
If its color is not a sufficient mark by which to identify Mars, a
still further difference between it and the stars is its markedly rapid
movement. A single night will make a sufficient change in its position
to show the planet as a wanderer. On an average, it travels over about
four-tenths of a degree in the heavens in one day. This equals more
than half the diameter of the moon, a change of position sufficiently
great to be easily detected.