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30 FIANNA
covered as recently as 1577 by Professor Asavll Hall ""5
Washington with the great 26-inch telescope of the Navzll
Obsciiatory. The inner one is at a distance of only about
t‘i,1)00 miles from the centre of the planet (or l<‘>T>1- 11111“
-1.000 miles from its surface), and its “inoiitli" is but
71,’; hours long, so that it ris s in the zI.'r?.s-I and sets In
the cast for any possible iiiliabit-ants of that plaiict. The
outer satellite rises in the east and sets in the west,
but its orbital iiiotioii in the opposite directizm is so
nearly equal to the etlcct of the diurnal motion that it is
nearly 132 hours between rising and setting. Very C‘s11‘1.Y
in the applicatioii of the telescope to Astronomy it was
discovered that the surface of Mars is diversified with a
iiuniber of inarkiiigs, which are commonly known by the
names of continents and seas. There are also to be seen
near the poles of each lieinisphcre white spots much bright-
er than the rest of the planet; these probably consist
of snow and ice, since they increase at the time of winter
and diniinish Ll1.lI'lIl,‘.',' the suiniiicr of each region. There
appear also to be a great number of very long, narrow,
dark streaks; these are known as "caiials,” crossing both
(-ontincnts and seas. Occasionally some of these streaks
are seen double-two parallel streaks are seen instead
of one. The nature of these "caiials” is still very uncertain,
sonic observers failiiig to see them, and in consequence
almost denying their existence, others regardiiig them
as perhaps purely optical effects, others again seeing
in them artiticial coiistriictioiis, the work of Martian
eiigiiieers, who by their means convey water from the
polar regions towards the drier equatorial districts.
Along these canals luxuriant vegetation is thought to
;:row, and this forms a band on each side. of sutiicicnt
i-Iidth to be visible in the telescope, thougli the canals
tlieiiisclves are probably too narrow to be seen at all at
our distance. As is well known, much controversy has
ariseii as to whether the planet is inhabited or no, but
the evidence we as yet possess is certainly iiisuflicicnt to
enable us to decide one way or the other. Air and
water, land and sea, certainly exist, but the conditions of
temperature prevailiiig are probably so far dillerciit from
any with which we are familiar that it is ditiicult to
inia':.'iiic how beings organised like ourselves could endure
such a rigorous climate. At the distance of Mars, the
Sun imparts less than half the light and heat given to ‘
corresponding areas on the Earth, and this is not much
bcttere(l by the very thin atiiiosplierc; but there is no
reason why rational beings of an altogether diilerciit type
should not exist on this planet. On our own Earth seveii-
tenths of the surface is covered by sea. but on Mars there
seeins to be a much greater proportion of land, perhaps
one-lial.f.
Jlixoii I’L.i.Vr:rs.s'l‘lie hundreds of small planets cir-
enlating round the Sun. wiliicli are found at distances
from the latter between those of liars and of Jupiter,
are imiie of them more than 300 miles in diameter, if
so much, and the total quantity of material in all put‘
togetlicr does not perhaps amount to one-quarter of that
of our own Earth. One of them, Vesta, is just visible to
the unaided eye of a keeii-siglitcd person, when it is
nearest; to the Earth. Unlike the larger planets, many
of these bodies move round the Sun in paths by no
incans circular but distinctly oval, and they coiiscquently
vary very much in their distances from the Sun and
Earth at different times, when nearest the latter. One
of these, to which the name of Eros (the Greek Cupid)
has been given, at times approaches the Earth nearer
than any other celestial body except the Moon, and ad-
vantage has been taken of this circumstance to ascertain
with considerable accuracy the distance in miles of our
Earth and the other planets from the Sun, or what is
technically called the Sun’s parallax (the apparent change
(mi
of positiml in an object, due to tllO'Cl1a1.lg0 of place of
an Observer, being greater as the obicct is nearer, is of
course then more easy to measure). So soon as the
distance of any one planet from the ‘Sun is accurately
kiiowii. we have the means of ziscertullllrlg the distance
of awry M1,“-, for there is a relation between the time
of revolution of 51115’ I’1“l19t “ml Its“ dbmnce from, ll?“
Sun, known as Kepler’s third law ( squares of .pL‘1‘lDdlC
times of any planets are as the cubes-of their iiieaii
dismnm from the Sun”); since tlieqieriods are known
with great accuracy fI'0111.l()l1;!,‘.SQI‘l.CS of observatioiis
we are thus able to ascertain their distances. Of course,
in practice the method is by no means so simple may
appear from this brief stateniciit, but the principle on
which it depends is not (lifiicult to nnd.crstaud.
Jm.1T1.;R,e “'e now come to the giant planet of the
S0131. Sy5t(.,n’ Jupiter, whose diameter is nearly elcveii
times that of the Earth; though only one-tliousaiidtli
of the Sun, it nevertheless exceeds in bulk all the other
planets put together. Taking nearly 12 years to perform
one revolution round the central orb, at a distance of
about 480,000,000 miles, he turns once round on his own
axis in less than 10 hours, so that his year contains
thousands of his days. That his shape is by no iricaiis
circular, but distinctly oval, is evident from more in-
speetioii through the telescope, and this polar iiattmiiiig
is considered to be a result of his rapid axial motion.
He is attended by no fewer than eight moons, four of
which were discovered by Galileo in 1610, and tlicse are
easily visible through a good opera glass; it is said that
they have occasionally been seen by the unaided eye when
sufiicic-iitly far from the planet. The other four are much
smaller, and were only discovered within the last few
years. The eclipses and oecnltatioiis (when they pass
behind the planet) of the larger satellites are frequent
plienoineiia, observable with quite small telescopes. The
surface of the planet is seen to be covered with a variety
of beautiful (and coloured) details, markings known as
the belts of Jupiter, bands parallel to the equator of the
planet (running east and west), which soon cliaiige in
shape and relative position. One very remarkable feature,
however, is of a more pcrinaiicnt character. The great
red spot has been persistently seen for over 30 years; as
its name indicates, it is usually of a reddish colour,
though at times very faint. lts shape has changed but
little, it being of a roughly oval form. Though from his
remoteness from the Sun. Jupiter receives but a small
amount of light and heat (huiiiaiily speaking) from that
source, yet from the rapidity of the changes upon its
surface the belts may be, as it were, clouds in atmosplicre)
and the low density, only about 1 % times that of water,
it seems not improbable that this planet is at a very high
temperature-in short, a kind of “scini-sun.” liardly. if
at all. solidified as yet, but in a liquid and perhaps partly
gaseous conditioii. Though occasionally surpassed by V0‘
nus, Jupiter is, on the whole, the brightest planet in our
sky, being four times as bright as Sirius, the liriglitest
fixed star.
SATURX.-Next in order of distance from the Sun.
the furthest of the planets known to the ancients, comes
Saturn, regarded by them, perhaps from his slower move-
ment’. and the somewhat leaden tint of its light, as a
Ulilligllilllt Object. To the modern astronomer, on the
contrary, it is an object of the highest interest and beauty.
1391118 Sllrroullded by awoiiderful and unique system of
Ylflgs and acconipaiiied by, at least, nine moons. Its
distance from the Sun is 9 V2 times that of our own
Earth’ and 1'3 requires ‘29 years to perform one circuit
,r0u“(l its llrlmafyi whilst it rotates very swiftly on its
axis ill 8 Period not much longer than does Jupiter (10 1,4
1101113). which planet it resembles in ‘more respects than