ASTRONOMY
27
ASTRONOMY
The solar system, as at present kBown, consists of
four interior planets, Mercurj', Venus, the Earth,
and Mars; four exterior, and relatively colossal
planets, Jupiter, Saturn, Uranus, and Neptune, the
diffuse crowd of pygniy globes called asteroids, or
minor planets, and an outlying array of comets with
their attendant meteor-systems. All the planets
rotate on their axes, though in very different periods.
That of Mercury was determined by Signor Scliia-
parelli of Milan in 1889 to be 88 days, the identical
time of his revolution round the sun, and Venus
was, in the following year, shown by him to be, in all
likelihood, similarly conditioned, the common period
of rotation and circulation being, in her case, 225
days. This implies that both planets keep the same
hemisphere always turned towards the sun, as the
moon does towards the earth; nor can we doubt
that the friction of tidal waves was, on the three
bodies, the agency by which the observed sjmchro-
nism was brouglit about. All the planets travel
round the sun from west to east, or coimter clock-
wise, and most of the satellites move in the same
direction round their primaries. But there are
e-xceptions. Phcebe. Saturn's remotest moon, cir-
culates oppositely to the other members of the
system; the four moons of Uranus are retrograde,
their plane of movement being inclined at more
than a right angle to the ecliptic; and the satellite
of Neptune travels quite definitely backward. These
anomalies are of profound import to theories of
planetary origin. The "canals" of Mars were
recognized by Schiaparelli in .\ugust. 1877, and he
caught sight of some of them duplicated two years
later. Their photographic registration at the Lowell
observatory in 1905 proves them to be no optical
illusion, but their nature remains enigmatical.
Co.METS AND Meteoks. — The predicted return of Halley's comet in 1759 afforded the first proof that bodies of the kind are permanently attached to the smi. They accompany its march through space, traversing, in either ilirection indifferently, higlily eccentric orbits inclined at all possible angles to the ecliptic. They are accordingly subject to violent, even subversive disturbances from the great planets. Jupiter, in particular, sways the movements of a group of over thirty " captured " comets, which have had their periods curtailed, and their primitive velocities reduced by his influence. Schiaparelli announced in 1866 that the August .shooting-stars, or Perseids, pursue the same orbit with a bright comet visible in 1862; and equally striking accord- ances of movement between three other comets and the Leonid, Lyraid, and Andromede meteor-swarms were soon afterwards established by Leverrier and Weiss. The obvious inference is that meteors are the disintegration-products of their cometary fellow- travellers. -\ theorj' of comets' tails, based upon the varying efficacy of electrical repulsion upon chemically different kinds of matter, was announced by Theodor Bredikhine of Moscow in 1882, and gave a satisfactory account of the appearances it was invented to explain. Latterly, however, the author- ity of Arrhenius of Stockholm has lent vogue to a "light-pressure" hypothesis, according to which, cometary appendages are formed of particles driven from the sun by the mechanical stress of liis radia- tions. But the singular and rapid changes pho- tographically disclosed as taking place in the tails of comets, remain unassociated with any known cause.
SiDERE.VL Astronomy. — Sir William Herschel's discovery, in 1802, of binary stars, imperfectly antici- pated by leather Christian Maj'er in 1778, was one of far-reaching scope. It virtually proved the realm of gravity to include sidereal regions; and the relations it intimated liave since proved to be much more widely prevalent than could have been miagined
beforehand. Mutually circling stars exist in such
profusion as probably to amoimt to one in three or
four of those unaccompanied. The}' are of limit-
less variety, some of the systems formed by tliem
being exceedingly close and rapid, while others
describe, in millennial periods , vastly extended orbits.
Many, too, comprise three or more members; and
the multiple stars thus constituted merge, by pro-
gressive increments of complexity, into actual clus-
ters, globular and irregular. The latter class is
exemplified by the Pleiades and the Hyades, by the
Beehive cluster in Cancer, just visible to the naked
eye, and by the double cluster in Perseus, which
makes a splendid show with an opera-glass. Globu-
lar clusters are compressed "balls" of minute stars,
of which more than one hundred have been cata-
logued. The scale on which these marvellous sys-
tems are constructed remains conjectural, since
their distances from the earth are entirely unknown.
Variable stars are met with in the utmost diversity.
Some are temporary apparitions, which spring up
from invisibility often to an astonisliing pitch of
splendour, then sink back more slowly to quasi-
extinction. Nova Persei, which blazed 22 February,
1901, and was photographically studied by Father
Sidgreaves at Stonyhurst, is the most noteworthy
recent instance of the phenomenon. ■ Stars, the
vicissitudes of which are comprised in cycles of
seven to twenty months, or more, are called "long-
period variables". About 400 had been recorded
down to 1906. They not uncommonly attain, at
maximum, to 1,000 times their minimum brightness.
Mira, the "wonderful" star in the Whale. di.scovered
by David Fabricius in 1596, is the exemplar of the
class. The fluctuations of "short-period variables"
take place in a few days or hours, and with far more
punctuality. A certain proportion of them are
"eclipsing stars" (about 35 have .so far been recog-
nized as such), which owe their regularly recurring
failures of light to the interposition of large satellites.
Algol in Perseus, the variations of which were per-
ceived by Montanari in 1669, is the best-known
specimen. Hundreds of rapid variables have been
recently detected among the components of globular
clusters; but their course of change is of a totally
different nature from that of eclipsing stars. Ed-
mund Halley (1656-1742), the second Astronomer
Royal, announced in 1718 that the stars, far from
being fixed, move onward, each on its own account,
across the sky. He arrived at this conclusion by
comparing modern with antique observations; and
stellar "proper motions" now constitute a wide
and expansive field of research. A preliminarv'
attempt to regularize them was made by Herschel's
determination, in 178-3, of the sun's line of travel.
His success depended upon the fact that the apparent
displacements of the stars include a common element,
transferred by perspective from the solar advance.
Tlieir individual, or "peculiar" movements, however,
show no certain trace of method. A good many
stars, too, have been ascertained to travel at rates
probably uncontrollable by the gravitational power
of the entire sidereal system. Arcturus, with its
portentous velocity of 250 miles a second, is one of
these "runaway" stars. The sun's pace of about
12 miles a second, seems, by comparison, extremely
sedate; and it is probably only half the average
stellar speed. The apex of the sun's way, or the
point towards which its movement at present tends,
is located by the best recent investigations near the
bright star Vega.
Distances of the Sun .\Js'd St.\rs. — The dis- tances of the heavenly bodies can only be determined (speaking generally) by measuring their parallaxes, in other words, their apparent changes of position when seen from different points of view. That of the Sim is simply the angle subtended at his distance
