THE INNER PLANETS
VULCAN
About the middle of the last century, Le Verrier, a great French
astronomer, having added the planet Neptune beyond the outside confines
of the solar system, sought evidence of a lesser planet traveling round
the sun within the orbit of Mercury. For many years close watch was kept
on the sun in the hope of discovering such a body in the act of passing
across the disk, or in transit, as it is technically termed.
Lescarbault, a French physician, announced that he had actually seen
such a planet, Vulcan it was called, passing over the sun in 1859. Total
eclipses of the sun would afford the best opportunity for seeing such a
body, and on several such occasions astronomers thought they had found
it. But the signal advantages of photography have been applied so often
to this search, and always unsuccessfully, that the existence of Vulcan,
or the intramercurian planet, is now regarded as mythical.
MERCURY
This planet is an elusive body that very few, even astronomers, have
ever seen. It is not very bright, has a rapid motion and never retreats
far from the sun, so that it was a puzzle to the ancients who saw it,
sometimes in the twilight after sunset and again in the twilight of
dawn. When following the sun down in the west, in March or April,
Mercury is likely to be best seen; twinkling rather violently and nearly
as bright as a star of the first magnitude.
Very little is to be seen on the minute disk of this planet, except that
it goes through all the phases of the moon--crescent, gibbous, full,
gibbous, crescent. Whether Mercury turns round on its axis or not,
cannot be said to be known, because the markings that are suspected on
its surface are too indefinite to permit exact observation. More than
likely the planet presents always the same side or face to the sun, so
that it turns round on its axis once, while traveling once around the
sun in its orbit. Mercury's day and year would therefore be equal in
length. Nor have we much evidence on the question of an atmosphere
surrounding Mercury; probably it is very thin, if indeed there is any at
all. When Mercury comes directly between us and the sun, crossing in
transit, the edge of the planet as projected against the sun is very
sharply defined, and this would indicate an absence of atmosphere on
Mercury.
Transits of Mercury can occur in May and November only: there was one on
November 7, 1914, and there will be one on May 7, 1924. The latter will
be nearly eight hours in length, which is almost the limit. Mercury's
distance from the sun averages 36 million miles, the diameter of the
planet is 3,000 miles, and his orbital speed is 30 miles per second, the
swiftest of all the planets. No moon of Mercury is known to exist,
although many times diligently searched for, especially during transits
of the planet.
VENUS
Brightest of all the planets, and the most beautiful of all is Venus.
Its path is next outside the orbit of Mercury, but within that of the
earth, so that it partakes of all the phases of the moon. Like Mercury
it sometimes passes exactly between us and the sun, a rare phenomenon
which is known as a transit of Venus.
Being without telescopes, the ancients knew nothing about these
occurrences, but they were puzzled for centuries over the appearance of
the planet in the west after sunset, when they called it Hesperus, and
in early dawn in the east when they gave it the name Phosphorus.
Venus is known to be girdled with an atmosphere denser than ours, and it
seems to be always filled with dense clouds. It is the reflection of
sunlight from this perpetually cloudy exterior which gives Venus her
singular radiance. So brilliant is she that even full daylight is not
strong enough to overpower her rays; and she may often be seen
glistening in the clear blue daytime sky, if one knows pretty nearly in
what direction to look for her.
Venus is 67 million miles from the sun, and as our own distance is 93
million miles, this planet can come within 26 million miles of the
earth. It is therefore at times our nearest known neighbor in space,
excepting only the Moon and Eros, one of the erratic little planets that
travel round the sun between Mars and Jupiter. Also possibly a comet
might come much nearer.
Astronomers always take advantage of this nearness of Venus to us, if a
transit across the sun takes place; because it affords an excellent
method of finding out what the distance of the sun is from the Earth. A
pair of these transits happens about once a century, there were transits
in 1874 and 1882, and the next pair occur in 2004 and 2012. In actual
size, Venus is almost as large a planet as our own, being 7,700 miles
in diameter, as compared with 7,920 for the earth. Her velocity in her
orbit is twenty-two miles per second, and she travels all the way round
the sun in seven and one half months or 225 days.
Venus from her striking brilliancy always leads the novice to expect to
see great things on applying the telescope. But aside from a brilliant
disk, now a slender crescent, now half full like the moon at quarter,
and again gibbous as the moon is between quarter and full, the telescope
reveals but little. There is pretty good evidence that the markings
thought to have been seen on the planet's surface are illusory, and so
it is wholly uncertain in what direction the planet's axis lies; also
there is great uncertainty about the length of the day on Venus, or the
period of turning round on its axis. Probably it is the same in length
as the planet's year.
Once when Venus passed very close to the sun, just barely escaping a
transit, Lyman of Yale University caught sight of it by hiding the sun
behind a tall building or church spire. The dark side of Venus was
turned toward us and he could not of course see that. But the planet was
clearly there, completely encircled by a narrow delicate luminous ring,
which was due to sunlight shining through the atmosphere that surrounds
the planet. Similar ring effects were seen by observers of the transits
of Venus in 1874 and 1882; and from all their observations it is
concluded that Venus has an atmosphere probably at least twice as dense
and extensive as that which encircles the earth. Spurious satellites of
Venus are many, but no real moon is known to attend this planet.
[Illustration: THE SURFACE OF THE MOON IN THE REGION OF COPERNICUS.
Photograph made with the Hooker 100-inch reflecting telescope.
(_Photo, Mt. Wilson Solar Observatory._)]
[Illustration: A VIEW OF THE SOUTH CENTRAL PORTION OF THE MOON AT
LAST QUARTER. (_Photo, Mt. Wilson Solar Observatory._)]