appeared to be a mass of rolling and ever-changing flame. Figs. 223 and
224 give the appearance of this portion of the prominence at intervals
of fifteen minutes. Other similar eruptions have been observed.
[Illustration: Fig. 223.]
[Illustration: Fig. 224.]
V. THE CORONA.
202. _General Appearance of the Corona._--At the time of a total eclipse
of the sun, if the sky is clear, the moon appears as a huge black ball,
the illumination at the edge of the disk being just sufficient to bring
out its rotundity. "From behind it," to borrow Professor Young's vivid
description, "stream out on all sides radiant filaments, beams, and
sheets of pearly light, which reach to a distance sometimes of several
degrees from the solar surface, forming an irregular stellate halo, with
the black globe of the moon in its apparent centre. The portion nearest
the sun is of dazzling brightness, but still less brilliant than the
prominences which blaze through it like carbuncles. Generally this inner
corona has a pretty uniform height, forming a ring three or four minutes
of arc in width, separated by a somewhat definite outline from the outer
corona, which reaches to a much greater distance, and is far more
irregular in form. Usually there are several _rifts_, as they have been
called, like narrow beams of darkness, extending from the very edge of
the sun to the outer night, and much resembling the cloud-shadows which
radiate from the sun before a thunder-shower; but the edges of these
rifts are frequently curved, showing them to be something else than real
shadows. Sometimes there are narrow bright streamers, as long as the
rifts, or longer. These are often inclined, occasionally are even nearly
tangential to the solar surface, and frequently are curved. On the
whole, the corona is usually less extensive and brilliant over the solar
poles, and there is a recognizable tendency to accumulations above the
middle latitudes, or spot-zones; so that, speaking roughly, the corona
shows a disposition to assume the form of a quadrilateral or four-rayed
star, though in almost every individual case this form is greatly
modified by abnormal streamers at some point or other."
[Illustration: Fig. 225.]
203. _The Corona as seen at Recent Eclipses._--The corona can be seen
only at the time of a total eclipse of the sun, and then for only a few
minutes. Its form varies considerably from one eclipse to another, and
apparently also during the same eclipse. At least, different observers
at different stations depict the same corona under very different forms.
Fig. 225 represents the corona of 1857 as observed by Liais. In this
view the _petal-like_ forms, which have been noticed in the corona at
other times, are especially prominent.
[Illustration: Fig. 226.]
Fig. 226 shows the corona of 1860 as it was observed by Temple.
[Illustration: Fig. 227.]
Fig. 227 shows the corona of 1867. This is interesting as being a corona
at the time of sun-spot minimum.
[Illustration: Fig. 228.]
Fig. 228 represents the corona of 1868. This is a larger and more
irregular corona than usual.
[Illustration: Fig. 229.]
The corona of 1869 is shown in Fig. 229.
[Illustration: Fig. 230.]
Fig. 230 is a view of the corona of 1871 as seen by Capt. Tupman.
[Illustration: Fig. 231.]
Fig. 231 shows the same corona as seen by Foenander.
[Illustration: Fig. 232.]
Fig. 232 shows the same corona as photographed by Davis.
[Illustration: Fig. 233.]
Fig. 233 shows the corona of 1878 made up from several views as combined
by Professor Young.
204. _The Spectrum of the Corona._--The chief line in the spectrum of
the corona is the one usually designated as 1474, and now known as the
_coronal_ line. It is seen as a dark line on the disk of the sun; and a
spectroscope of great dispersive power shows this dark line to be
closely double, the lower component being one of the iron lines, and the
upper the coronal line. This dark line is shown at _x_, Fig. 234.
[Illustration: Fig. 234.]
Besides this bright line, the hydrogen lines appear faintly in the
spectrum of the corona. The 1474 line has been sometimes traced with the
spectroscope to an elevation of nearly twenty minutes above the moon's
limb, and the hydrogen lines nearly as far; and the lines were just as
strong _in the middle of a dark rift_ as anywhere else.
The substance which produces the 1474 line is unknown as yet. It seems
to be something with a vapor-density far below that of hydrogen, which
is the lightest substance of which we have any knowledge. It can hardly
be an "allotropic" form of any terrestrial element, as some scientists
have suggested; for in the most violent disturbances in prominences and
near sun-spots, when the lines of hydrogen, magnesium, and other metals,
are contorted and shattered by the rush of the contending elements, this
line alone remains fine, sharp, and straight, a little brightened, but
not otherwise affected. For the present it remains, like a few other
lines in the spectrum, an unexplained mystery.
Besides bright lines, the corona shows also a faint continuous spectrum,
in which have been observed a few of the more prominent _dark_ lines of
the solar spectrum.
This shows, that, while the corona may be in the main composed of
glowing gas (as indicated by the bright lines of its spectrum), it also
contains considerable matter in such a state as to reflect the sunlight,
probably in the form of dust or fog.
V. ECLIPSES.
[Illustration: Fig. 235.]
205. _The Shadows of the Earth and Moon._--The shadows cast by the earth
and moon are shown in Fig. 235. Each shadow is seen to be made up of a
dark portion called the _umbra_, and of a lighter portion called the
_penumbra_. The light of the sun is completely excluded from the umbra,
but only partially from the penumbra. The umbra is in the form of a
cone, with its apex away from the sun; though in the case of the earth's
shadow it tapers very slowly. The penumbra surrounds the umbra, and
increases in size as we recede from the sun. The axis of the earth's
shadow lies in the plane of the ecliptic, which in the figure is the
surface of the page. As the moon's orbit is inclined five degrees to the
plane of the ecliptic, the axis of the moon's shadow will sometimes lie
above, and sometimes below, the ecliptic. It will lie on the ecliptic
only when the moon is at one of her nodes.
206. _When there will be an Eclipse of the Moon._--The moon is eclipsed
_whenever it passes into the umbra of the earth's shadow_. It will be
seen from the figure that the moon can pass into the shadow of the earth
only when she is in opposition, or _at full_. Owing to the inclination
of the moon's orbit to the ecliptic, the moon will pass either above or
below the earth's shadow when she is at full, unless she happens to be
near her node at this time: hence there is not an eclipse of the moon
every month.
When the moon simply passes into the penumbra of the earth's shadow, the
light of the moon is somewhat dimmed, but not sufficiently to attract
attention, or to be denominated an eclipse.
[Illustration: Fig. 236.]
207. _The Lunar Ecliptic Limits._--In Fig. 236 the line _AB_ represents
the plane of the ecliptic, and the line _CD_ the moon's orbit. The large
black circles on the line _AB_ represent sections of the umbra of the
earth's shadow, and the smaller circles on _CD_ represent the moon at
full. It will be seen, that, if the moon is full at _E_, she will just
graze the umbra of the earth's shadow. In this case she will suffer no
eclipse. Were the moon full at any point nearer her node, as at _F_, she
would pass into the umbra of the earth's shadow, and would be
_partially_ eclipsed. Were the moon full at _G_, she would pass through
the centre of the earth's shadow, and be _totally_ eclipsed.
It will be seen from the figure that full moon must occur when the moon
is within a certain distance from her node, in order that there may be a
lunar eclipse; and this space is called the _lunar ecliptic limits_.
The farther the earth is from the sun, the less rapidly does its shadow
taper, and therefore the greater its diameter at the distance of the
moon; and, the nearer the moon to the earth, the greater the diameter of
the earth's shadow at the distance of the moon. Of course, the greater
the diameter of the earth's shadow, the greater the ecliptic limits:
hence the lunar ecliptic limits vary somewhat from time to time,
according to the distance from the earth to the sun and from the earth
to the moon. The limits within which an eclipse is inevitable under all
circumstances are called the _minor ecliptic limits_; and those within
which an eclipse is possible under some circumstances, the _major
ecliptic limits_.
[Illustration: Fig. 237.]
208. _Lunar Eclipses._--Fig. 237 shows the path of the moon through the
earth's shadow in the case of a _partial eclipse_. The magnitude of such
an eclipse depends upon the nearness of the moon to her nodes. The
magnitude of an eclipse is usually denoted in _digits_, a digit being
one-twelfth of the diameter of the moon.
[Illustration: Fig. 238.]
Fig. 238 shows the path of the moon through the earth's shadow in the
case of a _total eclipse_. It will be seen from the figure that it is
not necessary for the moon to pass through the centre of the earth's
shadow in order to have a total eclipse. When the moon passes through
the centre of the earth's shadow, the eclipse is both _total_ and
_central_.
At the time of a total eclipse, the moon is not entirely invisible, but
shines with a faint copper-colored light. This light is refracted into
the shadow by the earth's atmosphere, and its amount varies with the
quantity of clouds and vapor in that portion of the atmosphere which the
sunlight must graze in order to reach the moon.
The duration of an eclipse varies between very wide limits, being, of
course, greatest when the eclipse is central. A total eclipse of the
moon may last nearly two hours, or, including the _partial_ portions of
the eclipse, three or four hours.
Every eclipse of the moon, whether total or partial, is visible at the
same time to the whole hemisphere of the earth which is turned towards
the moon; and the eclipse will have exactly the same magnitude at every
point of observation.
209. _When there will be an Eclipse of the Sun._--There will be an
eclipse of the sun _whenever any portion of the moon's shadow is thrown
on the earth_. It will be seen from Fig. 235 that this can occur only
when the moon is in conjunction, or at _new_. It does not occur every
month, because, owing to the inclination of the moon's orbit to the
ecliptic, the moon's shadow is usually thrown either above or below the
earth at the time of new moon. There can be an eclipse of the sun only
when new moon occurs at or near one of the nodes of her orbit.
210. _Solar Ecliptic Limits._--The distances from the moon's node within
which a new moon would throw some portion of its shadow on the earth so
as to produce an eclipse of the sun are called the _solar ecliptic
limits_. As in the case of the moon, there are _major_ and _minor_
ecliptic limits; the former being the limits within which an eclipse of
the sun is _possible_ under some circumstances, and the latter those
under which an eclipse is _inevitable_ under all circumstances.
The limits within which a solar eclipse may occur are greater than those
within which a lunar eclipse may occur. This will be evident from an
examination of Fig. 235. Were the moon in that figure just outside of
the lines _AB_ and _CD_, it will be seen that the penumbra of her shadow
would just graze the earth: hence the moon must be somewhere within the
space bounded by these lines in order to cause an eclipse of the sun.
Now, these lines mark the prolongation to the sun of the cone of the
umbra of the earth's shadow: hence, in order to produce an eclipse of
the sun, new moon must occur somewhere within this prolongation of the
umbra of the earth's shadow. Now, it is evident that the diameter of
this cone is greater on the side of the earth toward the sun than on the
opposite side: hence the solar ecliptic limits are greater than the
lunar ecliptic limits.
211. _Solar Eclipses._--An observer in the umbra of the moon's shadow
would see a _total_ eclipse of the sun, while one in the penumbra would
see only a _partial_ eclipse. The magnitude of this partial eclipse
would depend upon the distance of the observer from the umbra of the
moon's shadow.
[Illustration: Fig. 239.]
[Illustration: Fig. 240.]
The umbra of the moon's shadow is just about long enough to reach the
earth. Sometimes the point of this shadow falls short of the earth's
surface, as shown in Fig. 239, and sometimes it falls upon the earth, as
shown in Fig. 240, according to the varying distance of the sun and moon
from the earth. The diameter of the umbra at the surface of the earth is
seldom more than a hundred miles: hence the belt of a total eclipse is,
on the average, not more than a hundred miles wide; and a total eclipse
seldom lasts more than five or six minutes, and sometimes only a few
seconds. Owing, however, to the rotation of the earth, the umbra of the
moon's shadow may pass over a long reach of the earth's surface. Fig.
241 shows the track of the umbra of the moon's shadow over the earth in
the total eclipse of 1860.
[Illustration: Fig. 241.]
[Illustration: Fig. 242.]
Fig. 242 shows the track of the total eclipse of 1871 across India and
the adjacent seas.
[Illustration: Fig. 243.]
[Illustration: Fig. 244.]
In a partial eclipse of the sun, more or less of one side of the sun's
disk is usually concealed, as shown in Fig. 243. Occasionally, however,
the centre of the sun's disk is covered, leaving a bright ring around
the margin, as shown in Fig. 244. Such an eclipse is called an _annular_
eclipse. An eclipse can be annular only when the cone of the moon's
shadow is too short to reach the earth, and then only to observers who
are in the central portion of the penumbra.
212. _Comparative Frequency of Solar and Lunar Eclipses._--There are
more eclipses of the sun in the year than of the moon; and yet, at any
one place, eclipses of the moon are more frequent than those of the sun.
There are more lunar than solar eclipses, because, as we have seen, the
limits within which a solar eclipse may occur are greater than those
within which a lunar eclipse may occur. There are more eclipses of the
moon visible at any one place than of the sun; because, as we have seen,
an eclipse of the moon, whenever it does occur, is visible to a whole
hemisphere at a time, while an eclipse of the sun is visible to only a
portion of a hemisphere, and a total eclipse to only a very small
portion of a hemisphere. A total eclipse of the sun is, therefore, a
very rare occurrence at any one place.
The greatest number of eclipses that can occur in a year is seven, and
the least number, two. In the former case, five may be of the sun and
two of the moon, or four of the sun and three of the moon. In the latter
case, both must be of the sun.
VI. THE THREE GROUPS OF PLANETS.
I. GENERAL CHARACTERISTICS OF THE GROUPS.
213. _The Inner Group._--The _inner group_ of planets is composed of
_Mercury_, _Venus_, the _Earth_, and _Mars_; that is, of all the planets
which lie between the asteroids and the sun. The planets of this group
are comparatively small and dense. So far as known, they rotate on their
axes in about twenty-four hours, and they are either entirely without
moons, or are attended by comparatively few.
The comparative sizes and eccentricities of the orbits of this group are
shown in Fig. 245. The dots round the orbits show the position of the
planets at intervals of ten days.
[Illustration: Fig. 245.]
214. _The Outer Group._--The _outer group_ of planets is composed of
_Jupiter_, _Saturn_, _Uranus_, and _Neptune_. These planets are all very
large and of slight density. So far as known, they rotate on their axes
in about ten hours, and are accompanied with complicated systems of
moons. Fig. 246, which represents the comparative sizes of the planets,
shows at a glance the immense difference between those of the inner and
outer group. Fig. 247 shows the comparative sizes and eccentricities of
the orbits of the outer planets. The dots round the orbits show the
position of the planets at intervals of a thousand days.
[Illustration: Fig. 246.]
[Illustration: Fig. 247.]
215. _The Asteroids._--Between the inner and outer groups of planets
there is a great number of very small planets known as the _minor
planets_, or _asteroids_. Over two hundred planets belonging to this
group have already been discovered. Their orbits are shown by the dotted
lines in Fig. 247. The sizes of the four largest of these planets,
compared with the earth, are shown in Fig. 248.
[Illustration: Fig. 248.]
The asteroids of this group are distinguished from the other planets,
not only by their small size, but by the great eccentricities and
inclinations of their orbits. If we except Mercury, none of the larger
planets has an eccentricity amounting to one-tenth the diameter of its
orbit (43), nor is any orbit inclined more than two or three degrees to
the ecliptic; but the inclinations of many of the minor planets exceed
ten degrees, and the eccentricities frequently amount to an eighth of
the orbital diameter. The orbit of Pallas is inclined thirty-four
degrees to the ecliptic, while there are some planets of this group
whose orbits nearly coincide with the plane of the ecliptic.
[Illustration: Fig. 249.]
Fig. 249 shows one of the most and one of the least eccentric of the
orbits of this group as compared with that of the earth.
[Illustration: Fig. 250.]
The intricate complexity of the orbits of the asteroids is shown in Fig.
250.
II. THE INNER GROUP OF PLANETS.
Mercury.
216. _The Orbit of Mercury._--The orbit of Mercury is more eccentric
than that of any of the larger planets, and it has also a greater
inclination to the ecliptic. Its eccentricity (43) is a little over a
fifth, and its inclination to the ecliptic somewhat over seven degrees.
The mean distance of Mercury from the sun is about thirty-five million
miles; but, owing to the great eccentricity of its orbit, its distance
from the sun varies from about forty-three million miles at aphelion to
about twenty-eight million at perihelion.
[Illustration: Fig. 251.]
217. _Distance of Mercury from the Earth._--It is evident, from Fig.
251, that an inferior planet, like Mercury, is the whole diameter of its
orbit nearer the earth at inferior conjunction than at superior
conjunction: hence Mercury's distance from the earth varies
considerably. Owing to the great eccentricity of its orbit, its distance
from the earth at inferior conjunction also varies considerably. Mercury
is nearest to the earth when its inferior conjunction occurs at its own
aphelion and at the earth's perihelion.
[Illustration: Fig. 252.]
218. _Apparent Size of Mercury._--Since Mercury's distance from the
earth is variable, the apparent size of the planet is also variable.
Fig. 252 shows its apparent size at its extreme and mean distances from
the earth. Its apparent diameter varies from five seconds to twelve
seconds.
[Illustration: Fig. 253.]
219. _Volume and Density of Mercury._--The real diameter of Mercury is
about three thousand miles. Its size, compared with that of the earth,
is shown in Fig. 253. The earth is about sixteen times as large as
Mercury; but Mercury is about one-fifth more dense than the earth.
220. _Greatest Elongation of Mercury._--Mercury, being an _inferior_
planet (or one within the orbit of the earth), appears to oscillate to
and fro across the sun. Its greatest apparent distance from the sun, or
its _greatest elongation_, varies considerably. The farther Mercury is
from the sun, and the nearer the earth is to Mercury, the greater is its
angular distance from the sun at the time of its greatest elongation.
Under the most favorable circumstances, the greatest elongation amounts
to about twenty-eight degrees, and under the least favorable to only
sixteen or seventeen degrees.
221. _Sidereal and Synodical Periods of Mercury._--Mercury accomplishes
a complete revolution around the sun in about eighty-eight days; but it
takes it a hundred and sixteen days to pass from its greatest elongation
east to the same elongation again. The orbital motion of this planet is
at the rate of nearly thirty miles a second.
In Fig. 251, _P'''_ represents elongation east of the sun, and _P'_
elongation west. It will be seen that it is much farther from _P'_
around to _P'''_ than from _P'''_ on to _P'_. Mercury is only about
forty-eight days in passing from greatest elongation east to greatest
elongation west, while it is about sixty-eight days in passing back
again.
222. _Visibility of Mercury._--Mercury is too close to the sun for
favorable observation. It is never seen long after sunset, or long
before sunrise, and never far from the horizon. When visible at all, it
must be sought for low down in the west shortly after sunset, or low in
the east shortly before sunrise, according as the planet is at its east
or west elongation. It is often visible to the naked eye in our
latitude; but the illumination of the twilight sky, and the excess of
vapor in our atmosphere near the horizon, combine to make the telescopic
study of the planet difficult and unsatisfactory.
[Illustration: Fig. 254.]
223. _The Atmosphere and Surface of Mercury._--Mercury seems to be
surrounded by a dense atmosphere. One proof of the existence of such an
atmosphere is furnished at the time of the planet's _transit_ across the
disk of the sun, which occasionally happens. The planet is then seen
surrounded by a border, as shown in Fig. 254. A bright spot has also
been observed on the dark disk of the planet during a transit, as shown
in Fig. 255. The border around the planet seems to be due to the action
of the planet's atmosphere; but it is difficult to account for the
bright spot.
[Illustration: Fig. 255.]
[Illustration: Fig. 256.]
Schröter, a celebrated German astronomer, at about the beginning of the
present century, thought that he detected spots and shadings on the disk
of the planet, which indicated both the presence of an atmosphere and of
elevations. The shading along the terminator, which seemed to indicate
the presence of a twilight, and therefore of an atmosphere, are shown in
Fig. 256. It also shows the blunted aspect of one of the cusps, which
Schröter noticed at times, and which he attributed to the shadow of a
mountain, estimated to be ten or twelve miles high. Fig. 257 shows this
mountain near the upper cusp, as Schröter believed he saw it in the year
1800. By watching certain marks upon the disk of Mercury, Schröter came
to the conclusion that the planet rotates on its axis in about
twenty-four hours. Modern observers, with more powerful telescopes, have
failed to verify Schröter's observations as to the indications of an
atmosphere and of elevations. Nothing is known with certainty about the
rotation of the planet.
[Illustration: Fig. 257.]
The border around Mercury, and the bright spot on its disk at the time
of the transit of the planet across the sun, have been seen since
Schröter's time, and the existence of these phenomena is now well
established; but astronomers are far from being agreed as to their
cause.
224. _Intra-Mercurial Planets._--It has for some time been thought
probable that there is a group of small planets between Mercury and the
sun; and at various times the discovery of such bodies has been
announced. In 1859 a French observer believed that he had detected an
intra-Mercurial planet, to which the name of _Vulcan_ was given, and for
which careful search has since been made, but without success. During
the total eclipse of 1878 Professor Watson observed two objects near the
sun, which he thought to be planets; but this is still matter of
controversy.
Venus.
225. _The Orbit of Venus._--The orbit of Venus has but slight
eccentricity, differing less from a circle than that of any other large
planet. It is inclined to the ecliptic somewhat more than three degrees.
The mean distance of the planet from the sun is about sixty-seven
million miles.