THE INFERIOR PLANETS
Starting from the centre of the solar system, the first body we meet
with is the planet Mercury. It circulates at an average distance from
the sun of about thirty-six millions of miles. The next body to it is
the planet Venus, at about sixty-seven millions of miles, namely, about
double the distance of Mercury from the sun. Since our earth comes next
again, astronomers call those planets which circulate within its orbit,
_i.e._ Mercury and Venus, the Inferior Planets, while those which
circulate outside it they call the Superior Planets.[11]
In studying the inferior planets, the circumstances in which we make our
observations are so very similar with regard to each, that it is best to
take them together. Let us begin by considering the various positions of
an inferior planet, as seen from the earth, during the course of its
journeys round the sun. When furthest from us it is at the other side of
the sun, and cannot then be seen owing to the blaze of light. As it
continues its journey it passes to the left of the sun, and is then
sufficiently away from the glare to be plainly seen. It next draws in
again towards the sun, and is once more lost to view in the blaze at
the time of its passing nearest to us. Then it gradually comes out to
view on the right hand, separates from the sun up to a certain distance
as before, and again recedes beyond the sun, and is for the time being
once more lost to view.
To these various positions technical names are given. When the inferior
planet is on the far side of the sun from us, it is said to be in
_Superior Conjunction_. When it has drawn as far as it can to the left
hand, and is then as east as possible of the sun, it is said to be at
its _Greatest Eastern Elongation_. Again, when it is passing nearest to
us, it is said to be in _Inferior Conjunction_; and, finally, when it
has drawn as far as it can to the right hand, it is spoken of as being
at its _Greatest Western Elongation_ (see Fig. 11, p. 148).
The continual variation in the distance of an interior planet from us,
during its revolution around the sun, will of course be productive of
great alterations in its apparent size. At superior conjunction it
ought, being then farthest away, to show the smallest disc; while at
inferior conjunction, being the nearest, it should look much larger.
When at greatest elongation, whether eastern or western, it should
naturally present an appearance midway in size between the two.
[Illustration: Various positions, and illumination by the Sun, of an
Inferior Planet in the course of its orbit.
Corresponding views of the same situations of an Inferior Planet as seen
from the Earth, showing consequent phases and alterations in apparent
size.
FIG. 11.--Orbit and Phases of an Inferior Planet.]
From the above considerations one would be inclined to assume that the
best time for studying the surface of an interior planet with the
telescope is when it is at inferior conjunction, or, nearest to us. But
that this is not the case will at once appear if we consider that the
sunlight is then falling upon the side away from us, leaving the side
which is towards us unillumined. In superior conjunction, on the other
hand, the light falls full upon the side of the planet facing us; but
the disc is then so small-looking, and our view besides is so dazzled by
the proximity of the sun, that observations are of little avail. In the
elongations, however, the sunlight comes from the side, and so we see
one half of the planet lit up; the right half at eastern elongation, and
the left half at western elongation. Piecing together the results given
us at these more favourable views, we are enabled, bit by bit, to gather
some small knowledge concerning the surface of an inferior planet.
From these considerations it will be seen at once that the inferior
planets show various phases comparable to the waxing and waning of our
moon in its monthly round. Superior conjunction is, in fact, similar to
full moon, and inferior conjunction to new moon; while the eastern and
western elongations may be compared respectively to the moon's first and
last quarters. It will be recollected how, when these phases were first
seen by the early telescopic observers, the Copernican theory was felt
to be immensely strengthened; for it had been pointed out that if this
system were the correct one, the planets Venus and Mercury, were it
possible to see them more distinctly, would of necessity present phases
like these when viewed from the earth. It should here be noted that the
telescope was not invented until nearly seventy years after the death of
Copernicus.
The apparent swing of an inferior planet from side to side of the sun,
at one time on the east side, then passing into and lost in the sun's
rays to appear once more on the west side, is the explanation of what is
meant when we speak of an _evening_ or a _morning star_. An inferior
planet is called an evening star when it is at its eastern elongation,
that is to say, on the left-hand of the sun; for, being then on the
eastern side, it will set after the sun sets, as both sink in their turn
below the western horizon at the close of day. Similarly, when such a
planet is at its western elongation, that is to say, to the right-hand
of the sun, it will go in advance of him, and so will rise above the
eastern horizon before the sun rises, receiving therefore the
designation of morning star. In very early times, however, before any
definite ideas had been come to with regard to the celestial motions, it
was generally believed that the morning and evening stars were quite
distinct bodies. Thus Venus, when a morning star, was known to the
ancients under the name of Phosphorus, or Lucifer; whereas they called
it Hesperus when it was an evening star.
Since an inferior planet circulates between us and the sun, one would be
inclined to expect that such a body, each time it passed on the side
nearest to the earth, should be seen as a black spot against the bright
solar disc. Now this would most certainly be the case were the orbit of
an inferior planet in the same plane with the orbit of the earth. But we
have already seen how the orbits in the solar system, whether those of
planets or of satellites, are by no means in the one plane; and that it
is for this very reason that the moon is able to pass time after time in
the direction of the sun, at the epoch known as new moon, and yet not to
eclipse him save after the lapse of several such passages. Transits,
then, as the passages of an inferior planet across the sun's disc are
called, take place, for the same reason, only after certain regular
lapses of time; and, as regards the circumstances of their occurrence,
are on a par with eclipses of the sun. The latter, however, happen much
more frequently, because the moon passes in the neighbourhood of the
sun, roughly speaking, once a month, whereas Venus comes to each
inferior conjunction at intervals so long apart as a year and a half,
and Mercury only about every four months. From this it will be further
gathered that transits of Mercury take place much oftener than transits
of Venus.
Until recent years _Transits of Venus_ were phenomena of great
importance to astronomers, for they furnished the best means then
available of calculating the distance of the sun from the earth. This
was arrived at through comparing the amount of apparent displacement in
the planet's path across the solar disc, when the transit was observed
from widely separated stations on the earth's surface. The last transit
of Venus took place in 1882, and there will not be another until the
year 2004.
_Transits of Mercury_, on the other hand, are not of much scientific
importance. They are of no interest as a popular spectacle; for the
dimensions of the planet are so small, that it can be seen only with the
aid of a telescope when it is in the act of crossing the sun's disc. The
last transit of Mercury took place on November 14, 1907, and there will
be another on November 6, 1914.
The first person known to have observed a transit of an inferior planet
was the celebrated French philosopher, Gassendi. This was the transit of
Mercury which took place on the 7th of December 1631.
The first time a transit of Venus was ever seen, so far as is known, was
on the 24th of November 1639. The observer was a certain Jeremiah
Horrox, curate of Hoole, near Preston, in Lancashire. The transit in
question commenced shortly before sunset, and his observations in
consequence were limited to only about half-an-hour. Horrox happened to
have a great friend, one William Crabtree, of Manchester, whom he had
advised by letter to be on the look out for the phenomenon. The weather
in Crabtree's neighbourhood was cloudy, with the result that he only got
a view of the transit for about ten minutes before the sun set.
That this transit was observed at all is due entirely to the remarkable
ability of Horrox. According to the calculations of the great Kepler, no
transit could take place that year (1639), as the planet would just pass
clear of the lower edge of the sun. Horrox, however, not being satisfied
with this, worked the question out for himself, and came to the
conclusion that the planet would _actually_ traverse the lower portion
of the sun's disc. The event, as we have seen, proved him to be quite in
the right. Horrox is said to have been a veritable prodigy of
astronomical skill; and had he lived longer would, no doubt, have become
very famous. Unfortunately he died about two years after his celebrated
transit, in his _twenty-second_ year only, according to the accounts.
His friend Crabtree, who was then also a young man, is said to have been
killed at the battle of Naseby in 1645.
There is an interesting phenomenon in connection with transits which is
known as the "Black Drop." When an inferior planet has just made its way
on to the face of the sun, it is usually seen to remain for a short time
as if attached to the sun's edge by what looks like a dark ligament (see
Fig. 12, p. 153). This gives to the planet for the time being an
elongated appearance, something like that of a pear; but when the
ligament, which all the while keeps getting thinner and thinner, has at
last broken, the black body of the planet is seen to stand out round
against the solar disc.
[Illustration: FIG. 12.--The "Black Drop."]
This appearance may be roughly compared to the manner in which a drop of
liquid (or, preferably, of some glutinous substance) tends for a while
to adhere to an object from which it is falling.
When the planet is in turn making its way off the face of the sun, the
ligament is again seen to form and to attach it to the sun's edge before
its due time.
The phenomenon of the black drop, or ligament, is entirely an illusion,
and, broadly speaking, of an optical origin. Something very similar will
be noticed if one brings one's thumb and forefinger _slowly_ together
against a very bright background.
This peculiar phenomenon has proved one of the greatest drawbacks to the
proper observation of transits, for it is quite impossible to note the
exact instant of the planet's entrance upon and departure from the solar
disc in conditions such as these.
The black drop seems to bear a family resemblance, so to speak, to the
phenomenon of Baily's beads. In the latter instance the lunar peaks, as
they approach the sun's edge, appear to lengthen out in a similar manner
and bridge the intervening space before their time, thus giving
prominence to an effect which otherwise should scarcely be noticeable.
The last transit of Mercury, which, as has been already stated, took
place on November 14, 1907, was not successfully observed by astronomers
in England, on account of the cloudiness of the weather. In France,
however, Professor Moye, of Montpellier, saw it under good conditions,
and mentions that the black drop remained very conspicuous for fully a
minute. The transit was also observed in the United States, the reports
from which speak of the black drop as very "troublesome."
Before leaving the subject of transits it should be mentioned that it
was in the capacity of commander of an expedition to Otaheite, in the
Pacific, to observe the transit of Venus of June 3, 1769, that Captain
Cook embarked upon the first of his celebrated voyages.
In studying the surfaces of Venus and Mercury with the telescope,
observers are, needless to say, very much hindered by the proximity of
the sun. Venus, when at the greatest elongations, certainly draws some
distance out of the glare; but her surface is, even then, so dazzlingly
bright, that the markings upon it are difficult to see. Mercury, on the
other hand, is much duller in contrast, but the disc it shows in the
telescope is exceedingly small; and, in addition, when that planet is
left above the horizon for a short time after sunset, as necessarily
happens after certain intervals, the mists near the earth's surface
render observation of it very difficult.
Until about twenty-five years ago, it was generally believed that both
these planets rotated on their axes in about twenty-four hours, a
notion, no doubt, originally founded upon an unconscious desire to bring
them into some conformity with our earth. But Schiaparelli, observing in
Italy, and Percival Lowell, in the clear skies of Arizona and Mexico,
have lately come to the conclusion that both planets rotate upon their
axes in the same time as they revolve in their orbits,[12] the result
being that they turn one face ever towards the sun in the same manner
that the moon turns one face ever towards the earth--a curious state of
things, which will be dealt with more fully when we come to treat of our
satellite.
The marked difference in the brightness between the two planets has
already been alluded to. The surface of Venus is, indeed, about five
times as bright as that of Mercury. The actual brightness of Mercury is
about equivalent to that of our moon, and astronomers are, therefore,
inclined to think that it may resemble her in having a very rugged
surface and practically no atmosphere. This probable lack of atmosphere
is further corroborated by two circumstances. One of these is that when
Mercury is just about to transit the face of the sun, no ring of
diffused light is seen to encircle its disc as would be the case if it
possessed an atmosphere. Such a lack of atmosphere is, indeed, only to
be expected from what is known as the _Kinetic Theory of Gases_.
According to this theory, which is based upon the behaviour of various
kinds of gas, it is found that these elements tend to escape into space
from the surface of bodies whose force of gravitation is weak. Hydrogen
gas, for example, tends to fly away from our earth, as any one may see
for himself when a balloon rises into the air. The gravitation of the
earth seems, however, powerful enough to hold down other gases, as, for
instance, those of which the air is chiefly composed, namely, oxygen and
nitrogen. In due accordance with the Kinetic theory, we find the moon
and Mercury, which are much about the same size, destitute of
atmospheres. Mars, too, whose diameter is only about double that of the
moon, has very little atmosphere. We find, on the other hand, that
Venus, which is about the same size as our earth, clearly possesses an
atmosphere, as just before the planet is in transit across the sun, the
outline of its dark body is seen to be surrounded by a bright ring of
light.
The results of telescopic observation show that more markings are
visible on Mercury than on Venus. The intense brilliancy of Venus is,
indeed, about the same as that of our white clouds when the sun is
shining directly upon them. It has, therefore, been supposed that the
planet is thickly enveloped in cloud, and that we do not ever see any
part of its surface, except perchance the summit of some lofty mountain
projecting through the fleecy mass.
With regard to the great brilliancy of Venus, it may be mentioned that
she has frequently been seen in England, with the naked eye in full
sunshine, when at the time of her greatest brightness. The writer has
seen her thus at noonday. Needless to say, the sky at the moment was
intensely blue and clear.
The orbit of Mercury is very oval, and much more so than that of any
other planet. The consequence is that, when Mercury is nearest to the
sun, the heat which it receives is twice as great as when it is farthest
away. The orbit of Venus, on the other hand, is in marked contrast with
that of Mercury, and is, besides, more nearly of a circular shape than
that of any of the other planets. Venus, therefore, always keeps about
the same distance from the sun, and so the heat which she receives
during the course of her year can only be subject to very slight
variations.
[11] In employing the terms Inferior and Superior the writer bows to
astronomical custom, though he cannot help feeling that, in the
circumstances, Interior and Exterior would be much more appropriate.
[12] This question is, however, uncertain, for some very recent
spectroscopic observations of Venus seem to show a rotation period of
about twenty-four hours.