VENUS AND JUPITER
THE BRIGHT PLANET OF VENUS, THE GODDESS OF LOVE
_Diameter--7,700 miles_
ALMOST every one knows Venus, the beautiful Evening Star,
which is also, by the way, an equally beautiful Morning Star. In
ancient times the Greeks believed these to be two different stars,
calling the Morning Star "Phosphorus," the torchbearer who
lights the way for the Dawn, and the Evening Star "Hesperus,"
leader of the stars of night. These names were translated into
Latin as Lucifer and Vesper. The Morning Star was the son of
Astræus and Aurora and the Evening Star a brother of the Titan
Atlas. Pythagoras is said to have been the first to identify Hesperus
with Phosphorus.
"Now glowed the firmament
With living sapphires: Hesperus, that led
The starry host, rode brightest."
--_Milton._
The dates that Venus may be seen are announced in the Almanac. Always
look in the west after sunset for this lovely star, or in the east
before sunrise, never in either case more than 48 degrees away from the
sun. It follows the sun so closely that it is never seen more than four
hours after sunset.
"The Sun was sunk, and after him the Star of Hesperus, whose Office is
to bring the Twilight upon the Earth."
--_Milton._
Every 584 days Venus appears in the west as an evening star, first
appearing about 20 minutes after sunset, near the horizon.
[Figure 147: VENUS. Showing crescent phase. Photographed with the 40-inch
refractor at the Yerkes Observatory.]
After being visible but a short time, it disappears from view not far
from the point where the sun went down.
Every succeeding day this star appears at a higher elevation and glows
brighter and brighter; this continues for several months until it
appears like a great lamp, outshining every other object in the heavens,
except the moon. It then begins to appear at a lower elevation each
evening and to set a little earlier, until in the course of two or three
weeks it disappears in the early twilight. About two weeks after her
departure from the evening sky, Venus appears in the east shining
brightly in the cool, gray hours just before Aurora opens the gates for
the Sun-god.
Day by day, the Morning Star increases in brilliance just as did the
Evening Star until it appears at the highest point above the horizon;
then its light gradually declines as each day its stay becomes shorter
and shorter, until finally it pales in the morning light and is immersed
in the rays of the sun.
Venus passes through its period of greatest brilliance every eighth
year. This last occurred in 1921 and will next occur in 1929. At such a
time the planet is so bright that it may be seen in daylight, if one
knows where to look for it.
The discovery through a telescope that Venus had phases firmly
established the theories of Copernicus.
[Figure 148]
When this Polish astronomer, in 1543, announced the astounding theory
that the sun was the center of our solar system, he said that if the
planets between the earth and the sun could be clearly seen, they
would show phases like the moon. When in 1610, Galileo turned his
telescope on Venus and beheld the beautiful little crescent, the
followers of the ancient system of Ptolemy--who claimed that the _earth_
was the central body around which the sun and planets revolved--were,
for the first time, silenced, for they saw that the orbit of the earth
must enclose the orbit of Venus, and that they must both _revolve around
the sun_. Even a small telescope will show the phases of Venus, the
crescent being far more beautiful than one would imagine.
"Calm star! who was it named thee Lucifer,
From him who drew the third of Heaven down with him?
Oh! it was but the tradition of thy beauty!
For if the sun hath one part, and the moon one,
Thou hast the third part of the host of Heaven--
Which is its power--which is its beauty!"
--_Bailey._
When first seen through a telescope, in the evening twilight, Venus is
gibbous, or nearly round. It has then just emerged on the farther side
of the sun from us. After first becoming visible as a disk, it gradually
assumes the figure of a half moon (this is at the point of its greatest
elongation when it is at its highest in the sky); then as it again
approaches the sun, it gradually becomes more and more like a slender
crescent of light. When Venus is showing nearly a full face, it is the
whole diameter of its orbit farther away than when it shows a slender
crescent. In this position it is 160 millions of miles distant from the
earth; when nearest the earth it is only 22 millions of miles distant.
This is the reason why it appears much smaller when its full hemisphere
is turned toward us than when it is seen as a small crescent. The
proportion of this disk to the crescent is as 10 to 65. After becoming a
"silver bow," Venus disappears from the evening sky, later to appear on
the other side of the sun in the same guise, and gradually, as the
days go by, changing to a half moon and on again through all its phases.
The orbit of Venus is tipped slightly from the plane of the earth's
orbit; thus only on very rare occasions does it pass directly in front
of the sun. Such a passage is called a transit. These transits occur in
pairs--two transits being 8 years apart and the successive pairs being
separated by 122 years. The last transit occurred on December 9th, 1874,
and December 6th, 1882. They will not occur again until June 8th, 2004,
and June 6th, 2012. There is only a slight possibility that some young
reader may see this.
When passing before the face of the sun, Venus appears as a round, black
spot surrounded by an illuminated ring of atmosphere. The spectroscope
has shown that this atmosphere is about the same as the earth's,
possibly a little more dense, and, strange to say, there is now strong
evidence that water vapor and oxygen are conspicuous by their absence.
The dense atmosphere on Venus causes it to reflect light most
beautifully but it also obscures its surface so that it is very
difficult to see any markings. From time to time, when Venus passes
between the sun and ourselves, it approaches to within 25 million miles
of the earth, and even less--which is about 10 million miles closer than
Mars comes at its nearest approach--yet we can clearly see the surface
of Mars for its atmosphere is very tenuous, while the surface of Venus
is almost completely hidden under thick white clouds. At such a time,
Venus is also a slender crescent while Mars shows a full round face. The
spectroscope combined with observations made upon such surface markings
as may be seen in full daylight has, however, enabled us to determine
that Venus rotates so slowly upon her axis that one face is always
turned toward the sun. Thus the day on Venus must last forever and its
night-side has only one long night. It has been conjectured that this
condition may cause storms of frightful intensity upon its
surface, for high air currents on the light torrid side probably flow
rapidly to the dark, cold side, with the cold air near the surface of
the dark, cold side flowing with equal rapidity to the torrid side. This
state of affairs may tend to equalize the intensity of the two extremes
of climate. Being almost 30 million miles closer to the sun than the
earth, Venus may have a higher temperature than the earth, although the
heat may be somewhat modified by the clouds which are always in the sky.
Sometimes a faint, grayish light has been observed on the dark side of
the planet when it is nearest the earth. This was at one time thought to
be a reflection from a cold and frozen hemisphere, but a more likely
suggestion hints that it may be an electrical manifestation in the
planet's atmosphere, similar to our Aurora.
Venus completes its orbit in 7½ months, its year being rather shorter
than ours.
The question of the existence of life not only on Venus, but on the
other planets, is always a subject of general interest. Professor
Barnard of the Yerkes Observatory had the following convictions on the
subject:
"There are possibilities of life on some of our brother worlds, though
that is not in any way a necessity, possibly on Mars and probably on
Venus, with Mercury very doubtful but more probably lifeless. The moon
we will not consider, for we believe that it is dead long ago--even if
it ever had any form of life upon it. It may even be that some world of
our solar system has ceased to bear life and it has been suggested that
possibly Mars is such a world. This is in no wise unreasonable. It is
entirely probable that some of them are not in a life-bearing condition,
just as a tree in your orchard may not have obtained maturity yet.
Their heyday is yet to come, perhaps this will be when the earth and all
its present life are cold and dead--dead of old age! for a world must
die just as a man must die."
Jupiter, Saturn, Uranus and Neptune are considered, he says, to be
worlds of the future, their low density and other considerations leaving
little doubt of their unripeness for life. Perhaps millions of
years will pass before these giant worlds are in a condition to welcome
life of any description.
Many astronomers believe that the conditions for life such as we know it
on earth, are more likely to be found on Venus than on any other planet;
others think that Mars, being evolutionally older, may have even passed
the stage in which we now find ourselves, or that it may have developed,
to suit the conditions on Mars, along quite a different line. Life
adapts itself to many extremes, it may live in intense heat and freezing
cold, in the air, in the water and under the ground. Speculation on this
subject has disclosed the most interesting data, but as yet we do not
know, and may never know, if there is life on any of the planets.
THE GIANT PLANET OF JUPITER, THE KING OF THE GODS
_Diameter--88,300 miles_
Jupiter is the largest planet in our solar system. With high magnifying
power it is seen to be a great "orange-shaped" globe, larger than the
full moon to the unaided eye, with a bulging equator and flattened
poles. This polar depression is ¹⁄₁₆th of its diameter causing
the polar diameter of 84,400 miles to be several thousand miles less
than its equatorial diameter, which is 90,200 miles.
This colossal world is favorably situated for observation three months
out of every year, traveling eastward along the zodiac, and spending
around twelve months in each zodiacal constellation. It requires twelve
of our years for Jupiter to complete a revolution around the sun.
Even an opera-glass will give a view of Jupiter as a minute disk with
two bands across its center and four luminous points on either side.
These luminous points are satellites, three of which are larger than our
moon and one almost as large. Jupiter has nine satellites in all
but only four are visible in any but the most powerful telescope and
three cannot be seen except in photographs. The four large satellites
may be seen to change their positions from hour to hour, occasionally
disappearing in the shadow behind Jupiter, and then reappearing with
startling abruptness on the other side.
[Figure 149: THE PLANET JUPITER.
Photographed by Yerkes Observatory through the 40-inch refractor.]
With a stronger glass they may be seen moving as bright spots across the
face of the planet, followed or preceded by the dark disks of their
shadows.
With a large telescope, various colored belts are seen to streak this
globe like dark stripes from east to west, the most prominent ones being
the two deep red bands some 7000 miles broad which lie on either side of
a faint golden equatorial belt, some 6000 miles across. Besides these,
there are lesser belts to the north and south which are red,
green, brown, black and brilliant white. The spots on the equatorial
belt move rapidly toward the west with a speed of over 270 miles an
hour, with respect to the other markings on the planet. By watching
these impressive colorings and markings the gigantic globe may actually
be seen to turn around on its axis! It does this in only 9 hours and 50
minutes--less than half the time that it takes the earth to rotate.
With a still larger telescope, however, it was discovered that 9 hours
and 50 minutes was only the average time that it takes Jupiter to rotate
and that the day varies according to the location on the planet. Thus,
like the sun, its surface is not a fixed one, for at the equator the
rate of rotation is 9 hours and 47 minutes, a little distance at either
side 9 hours and 56 minutes, and in higher latitudes over 10 hours. But
Jupiter is not as hot a globe as the sun, which has been described as
somewhat resembling "thick molasses," for if it shone by its own light
it would not cast a shadow, nor would the shadows of its satellites be
dark spots upon its surface.
Such facts have led to the general belief that Jupiter is still in an
early stage of development with its globe probably in a semi-fluid
condition so hot that the gases which will some day form its waters and
be cradled in the hollows of its crust, now rise as hot vapor into its
thick, warm atmosphere and fall as incessant rains. The surface visible
to us on earth must therefore be in the nature of dense clouds and
vapors while its real surface is probably in a "hot, muddy thickish
liquid" condition, quite unattractive to think about.
Yet there are some considerations that tend to disprove the cloud
theory. Barnard mentions that the semi-permanent nature often shown by
the markings is against it. The great Red Spot has been visible at
intervals for a hundred years or more, and other markings may be traced
with but little change of form for months, although these objects
are free to drift around and their rotation periods are not constant.
Professor Barnard, using the powerful Yerkes telescope, said that when
seen under the finest conditions the appearance of the surface is more
of a pasty nature.
The great oval-shaped spot mentioned above and famed as the "Great Red
Spot" appeared in the southern hemisphere in 1878, or at least it
appeared more distinctly on that date, for early drawings show that
traces of it had been observed long before that. It was at first of a
very strong red color and stretched above the equator for a distance of
30,000 miles. It is now of a delicate pinkish tinge, extremely faint but
still visible.
The cause of this prominent marking is not known. It has been suggested
that it might have been a gigantic eruption. Gigantic indeed, if it
covered 30,000 miles! It also has a curious repellent nature, for all
the belts and spots that come near it, instead of passing onto it or
over it, seem to be forcibly pushed to one side and held at a certain
distance until they have completely passed this whatever-it-is.
It has also been suggested that a new continent may be forming here, but
the fact that this feature like other features on Jupiter's apparent
surface does not seem to have a fixed position has caused it to be
likened rather to a vast drifting island. This "vast drifting island"
sometimes moves fast, sometimes slowly and sometimes remains perfectly
stationary! The last suggestion was brought forward before the more or
less irregular drift was noticed which seems to prove after all that it
could not be a true part of the real surface of Jupiter.
If Jupiter were composed of materials as dense and solid as the earth,
he would not be as large a planet as he now is. The materials of which
this great world is composed are only about one fourth as dense as the
earth or 1.25 as dense as water.
[Figure 150: JUPITER AND FOUR OF HIS MOONS.
Photograph by the Yerkes Observatory.]
The axis of the planet is nearly perpendicular to the plane of its
motion, the inclination being only about three degrees from the
perpendicular. Thus there is no great variety of seasons on Jupiter
although his year is twelve times as long as our year. If his axis were
inclined as much to his ecliptic as the axis of the earth, his polar
regions would remain in darkness for six years at a time.
JUPITER'S SATELLITES
With a telescope magnifying thirty-three times which he had constructed
himself, Galileo discovered the moons of Jupiter. Three of them were
first seen on the 7th of January, 1610, and the fourth on the 13th of
the same month. These moons, which are sometimes called the "Galilean
quartet," were the first heavenly bodies which had not been known from
time immemorial.
Delighted beyond measure, Galileo sent his drawings and an account of
his observations to his patron Cosmo de' Medici, Great Duke of Tuscany,
in honor of whom he called the moons the "Medician Stars." Equally
anxious to show the world, Galileo set his telescope on the tower of St.
Mark where the advocates of the Copernican system hailed them with
joyful satisfaction for here was a miniature of the sun and its planets
hung up in the heavens to demonstrate the truth of the new doctrines.
But the stubborn followers of Ptolemy, viewing them argued that such
pretended discoveries were absurd, and that since we had seven openings
in the head--two ears, two eyes, two nostrils and the mouth, there could
be in the heavens but seven planets. He had therefore either bewitched
them or the telescope, for which they threw him into prison.
After the moons had finally been accepted as a fact, they were given the
names of Io, Europa and Callisto, three lovely maidens of whom the great
Olympian God was fond, and Ganymede, his cupbearer, although astronomers
usually designate them as I, II, III (Ganymede) and IV, in order
of their distance from Jupiter. Their orbits lie in the plane of
Jupiter's equator and as that lies always in the plane of our vision,
the satellites seem to swing back and forth in their journey around him,
sometimes in one position and sometimes in another--sometimes all four
on one side, sometimes two on one side and two on the other, or perhaps
one of them will be hidden by the planet. They present a different
aspect and relation to each other every successive evening and even in a
few hours may be seen to change. Frequently these moons may be seen as
bright spots against the surface of the planet or their shadows appear
as conspicuous round black spots traveling along one of the colorful
belts.
Ganymede, satellite III, is the largest of all known moons in the solar
system, having a diameter of 3560 miles, actually exceeding the planet
Mercury in size. Satellite I exceeds our moon in diameter by a couple of
hundred miles, satellites III and IV by over a thousand miles.
Io, the satellite closest to Jupiter (named after the maiden who was
transformed into a cow and chased to the end of the world), is a very
swiftly moving satellite, traveling completely around its orbit in 1
day, 18 hours and 28 minutes. This means that it must move at the
tremendous speed of 320 miles a minute! Our moon takes 27½ days or
nearly a month to travel around the earth in an orbit of almost the same
size, but the attractive force of our earth is so much less than the
attractive force of Jupiter that our moon does not have to travel so
fast in order to keep from being pulled down on its surface.
Europa's period of revolution is 3 days, 13 hours and 14 minutes;
Ganymede's, 7 days, 3 hours and 43 minutes and Callisto's 16 days, 16
hours and 32 minutes.
In 1892 Professor Barnard discovered through the Lick telescope a moon
closer to Jupiter than Io. This little moon is only
¹⁄₆₀₀th part as bright as the other satellites and is only 30
miles across.
In 1905 two other satellites were discovered by photography, still
another in 1908 and another in 1914, making a retinue of nine satellites
in all. These last two, which are very remote from the planet, were
found to be revolving in the opposite direction from the other seven,
which is from west to east, the direction that generally prevails
throughout the heavens. Other exceptions to this rule are the outermost
moon of the planet Saturn, the moon of Neptune and all four moons of
Uranus. The ninth moon of Jupiter is about 15,400,000 miles from the
planet and takes nearly 3 years to complete a revolution. Only 11 hours
and 57 minutes for Barnard's satellite and nearly 3 years for the most
distant one, is quite a contrast in periods. Although Jupiter's 5-hour
night is rich in moons, he is not nearly as well supplied with moonlight
as is the earth.
It has already been mentioned how Roemer, a Danish astronomer of the
17th century, discovered the finite velocity of light through the
disappearance and reappearance of Jupiter's satellites within his
shadow. This was later found true through direct experiment.
At least a peep at this mammoth and colorful world, with its bands and
spots and swiftly moving satellites, should be the privilege of every
man, woman and child.