arrangement, than in the study of astronomy. Some subjects involved in
this study appear very difficult and perplexing to the learner, before
he has fully learned the doctrine of the sphere, and gained a certain
familiarity with astronomical doctrines, which would seem very easy to
him after he had made such attainments. Such an order ought to be
observed, as shall bring out the facts and doctrines of the science just
in the place where the mind of the learner is prepared to receive them.
Some writers on astronomy introduce their readers at once to the most
perplexing part of the whole subject,--the planetary motions. I have
thought a different course advisable, and have therefore commenced these
Letters with an account of those bodies which are most familiarly known
to us, the earth, the sun, and the moon. In connexion with the earth, we
are able to acquire a good knowledge of the artificial divisions and
points of reference that are established on the earth and in the
heavens, constituting the doctrine of the sphere. You thus became
familiar with many terms and definitions which are used in astronomy.
These ought to be always very clearly borne in mind; and if you now meet
with any term, the definition of which you have either partially or
wholly forgotten, let me strongly recommend to you, to turn back and
review it, until it becomes as familiar to you as household words.
Indeed, you will find it much to your advantage to go back frequently,
and reiterate the earlier parts of the subject, before you advance to
subjects of a more intricate nature. If this process should appear to
you a little tedious, still you will find yourself fully compensated by
the clear light in which all the succeeding subjects will appear. This
clear and distinct perception of the ground we have been over shows us
just where we are on our journey, and helps us to find the remainder of
the way with far greater ease than we could otherwise do. I do not,
however, propose by any devices to relieve you from the trouble of
thinking. Those who are not willing to incur this trouble can never
learn much of astronomy.
In introducing you to the planets, (which next claim our attention,) I
will, in the first place, endeavor to convey to you some clear views of
these bodies individually, and afterwards help you to form as correct a
notion as possible of their motions and mutual relations.
The name _planet_ is derived from a Greek word, (= planêtês=,
_planetes_,) which signifies a _wanderer_, and is applied to this class
of bodies, because they shift their positions in the heavens, whereas
the fixed stars constantly maintain the same places with respect to each
other. The planets known from a high antiquity are, Mercury, Venus,
Earth, Mars, Jupiter, and Saturn. To these, in 1781, was added Uranus,
(or _Herschel_, as it is sometimes called, from the name of its
discoverer;) and, as late as the commencement of the present century,
four more were added, namely, Ceres, Pallas, Juno, and Vesta. These
bodies are designated by the following characters:
1. Mercury, [Planet: Mercury]
2. Venus, [Planet: Venus]
3. Earth, [Planet: Earth]
4. Mars, [Planet: Mars]
5. Vesta, [Planet: Vesta]
6. Juno, [Planet: Juno]
7. Ceres, [Planet: Ceres]
8. Pallas, [Planet: Pallas]
9. Jupiter, [Planet: Jupiter]
10. Saturn, [Planet: Saturn]
11. Uranus, [Planet: Uranus]
The foregoing are called the _primary_ planets. Several of these have
one or more attendants, or satellites, which revolve around them as they
revolve around the sun. The Earth has one satellite, namely, the Moon;
Jupiter has four; Saturn, seven; and Uranus, six. These bodies are also
planets, but, in distinction from the others, they are called
_secondary_ planets. Hence, the whole number of planets are twenty-nine,
namely, eleven primary, and eighteen secondary, planets.
You need never look for a planet either very far in the north or very
far in the south, since they are always near the ecliptic. Mercury,
which deviates furthest from that great circle, never is seen more than
seven degrees from it; and you will hardly ever see one of the planets
so far from it as this, but they all pursue nearly the same great route
through the skies, in their revolutions around the sun. The new planets,
however, make wider excursions from the plane of the ecliptic,
amounting, in the case of Pallas, to thirty-four and a half degrees.
Mercury and Venus are called _inferior_ planets, because they have their
orbits nearer to the sun than that of the earth; while all the others,
being more distant from the sun than the earth, are called _superior_
planets. The planets present great diversities among themselves, in
respect to distance from the sun, magnitude, time of revolution, and
density. They differ, also, in regard to satellites, of which, as we
have seen, three have respectively four, six, and seven, while more than
half have none at all. It will aid the memory, and render our view of
the planetary system more clear and comprehensive, if we classify, as
far as possible, the various particulars comprehended under the
foregoing heads. As you have had an opportunity, in preceding Letters,
of learning something respecting the means which astronomers have of
ascertaining the distances and magnitudes of these bodies, you will not
doubt that they are really as great as they are represented; but when
you attempt to conceive of spaces so vast, you will find the mind wholly
inadequate to the task. It is indeed but a comparatively small space
that we can fully comprehend at one grasp. Still, by continual and
repeated efforts, we may, from time to time, somewhat enlarge the
boundaries of our mental vision. Let us begin with some known and
familiar space, as the distance between two places we are accustomed to
traverse. Suppose this to be one hundred miles. Taking this as our
measure, let us apply it to some greater distance, as that across the
Atlantic Ocean,--say three thousand miles. From this step we may advance
to some faint conception of the diameter of the earth; and taking that
as a new measure, we may apply it to such greater spaces as the distance
of the planets from the sun. I hope you will make trial of this method
on the following comparative statements respecting the planets.
_Distances from the Sun, in miles._
1. Mercury, 37,000,000
2. Venus, 68,000,000
3. Earth, 95,000,000
4. Mars, 142,000,000
5. Vesta, 225,000,000
6. Juno, }
7. Ceres, } 261,000,000
8. Pallas, }
9. Jupiter, 485,000,000
10. Saturn, 890,000,000
11. Uranus, or Herschel, 1800,000,000
The _dimensions_ of the planetary system are seen from this table to be
vast, comprehending a circular space thirty-six hundred millions of
miles in diameter. A rail-way car, travelling constantly at the rate of
twenty miles an hour, would require more than twenty thousand years to
cross the orbit of Uranus.
_Magnitudes._
Diam. in miles.
1. Mercury, 3140
2. Venus, 7700
3. Earth, 7912
4. Mars, 4200
5. Ceres, 160
6. Jupiter, 89,000
7. Saturn, 79,000
8. Uranus, 35,000
We remark here a great diversity in regard to magnitude,--a diversity
which does not appear to be subject to any definite law. While Venus, an
inferior planet, is nine tenths as large as the earth, Mars, a superior
planet, is only one seventh, while Jupiter is twelve hundred and
eighty-one times as large. Although several of the planets, when nearest
to us, appear brilliant and large, when compared with most of the fixed
stars, yet the angle which they subtend is very small,--that of Venus,
the greatest of all, never exceeding about one minute, which is less
than one thirtieth the apparent diameter of the sun or moon. Jupiter,
also, by his superior brightness, sometimes makes a striking figure
among the stars; yet his greatest apparent diameter is less than one
fortieth that of the sun.
_Periodic Times_.
Mercury revolves around the sun in nearly 3 months.
Venus, " " " " 7-1/2 "
Earth, " " " " 1 year.
Mars, " " " " 2 years.
Ceres, " " " " 4-2/3 "
Jupiter, " " " " 12 "
Saturn, " " " " 29 "
Uranus, " " " " 84 "
From this view, it appears that the planets nearest the sun move most
rapidly. Thus, Mercury performs nearly three hundred and fifty
revolutions while Uranus performs one. The apparent progress of the most
distant planets around the sun is exceedingly slow. Uranus advances only
a little more than four degrees in a whole year; so that we find this
planet occupying the same sign, and of course remaining nearly in the
same part of the heavens, for several years in succession.
After this comparative view of the planets in general, let us now look
at them individually; and first, of the inferior planets, Mercury and
Venus.
MERCURY and VENUS, having their orbits so far within that of the earth,
appear to us as attendants upon the sun. Mercury never appears further
from the sun than twenty-nine degrees, and seldom so far; and Venus,
never more than about forty-seven degrees. Both planets, therefore,
appear either in the west soon after sunset, or in the east a little
before sunrise. In high latitudes, where the twilight is long, Mercury
can seldom be seen with the naked eye, and then only when its angular
distance from the sun is greatest. Copernicus, the great Prussian
astronomer, (who first distinctly established the order of the solar
system, as at present received,) lamented, on his death-bed, that he had
never been able to obtain a sight of Mercury; and Delambre, a
distinguished astronomer of France, saw it but twice. In our latitude,
however, we may see this planet for several evenings and mornings, if we
will watch the time (as usually given in the almanac) when it is at its
greatest elongations from the sun. It will not, however, remain long for
our gaze, but will soon run back to the sun. The reason of this will be
readily understood from the following diagram, Fig. 50. Let S represent
the sun, E, the earth, and M, N, Mercury at its greatest elongations
from the sun, and O Z P, a portion of the sky. Then, since we refer all
distant bodies to the same concave sphere of the heavens, it is evident
that we should see the sun at Z, and Mercury at O, when at its greatest
eastern elongation, and at P, when at its greatest western elongation;
and while passing from M to N through Q, it would appear to describe the
arc O P; and while passing from N to M through R, it would appear to run
back across the sun on the same arc. It is further evident that it would
be visible only when at or near one of its greatest elongations; being
at all other times so near the sun as to be lost in his light.
[Illustration Fig. 50.]
A planet is said to be in _conjunction_ with the sun when it is seen in
the same part of the heavens with the sun. Mercury and Venus have each
two conjunctions, the inferior and the superior conjunction. The
_inferior conjunction_ is its position when in conjunction on the same
side of the sun with the earth, as at Q, in the figure; the _superior
conjunction_ is its position when on the side of the sun most distant
from the earth, as at R.
The time which a planet occupies in making one entire circuit of the
heavens, from any star, until it comes round to the same star again, is
called its _sidereal revolution_. The period occupied by a planet
between two successive conjunctions with the earth is called its
_synodical revolution_. Both the planet and the earth being in motion,
the time of the synodical revolution of Mercury or Venus exceeds that of
the sidereal; for when the planet comes round to the place where it
before overtook the earth, it does not find the earth at that point, but
far in advance of it. Thus, let Mercury come into inferior conjunction
with the earth at C, Fig. 51. In about eighty-eight days, the planet
will come round to the same point again; but, mean-while, the earth has
moved forward through the arc E E´, and will continue to move while the
planet is moving more rapidly to overtake her; the case being analogous
to that of the hour and minute hand of a clock.
[Illustration Fig. 51.]
The synodical period of Mercury is one hundred and sixteen days, and
that of Venus five hundred and eighty-four days. The former is increased
twenty-eight days, and the latter, three hundred and sixty days, by the
motion of the earth; so that Venus, after being in conjunction with the
earth, goes more than twice round the sun before she comes into
conjunction again. For, since the earth is likewise in motion, and moves
more than half as fast as Venus, by the time the latter has gone round
and returned to the place where the two bodies were together, the earth
is more than half way round, and continues moving, so that it will be a
long time before Venus comes up with it.
The motion of an inferior planet is _direct_ in passing through its
superior conjunction, and _retrograde_ in passing through its inferior
conjunction. You will recollect that the motion of a heavenly body is
said to be direct when it is in the order of the signs from west to
east, and retrograde when it is contrary to the order of the signs, or
from east to west. Now Venus, while going from B through D to A, (Fig.
51,) moves from west to east, and would appear to traverse the celestial
vault B´ S´ A´, from right to left; but in passing from A through C to
B, her course would be retrograde, returning on the same arc from left
to right. If the earth were at rest, therefore, (and the sun, of course,
at rest,) the inferior planets would appear to oscillate backwards and
forwards across the sun. But it must be recollected that the earth is
moving in the same direction with the planet, as respects the signs, but
with a slower motion. This modifies the motions of the planet,
accelerating it in the superior, and retarding it in the inferior,
conjunction. Thus, in Fig. 51, Venus, while moving through B D A, would
seem to move in the heavens from B´ to A´, were the earth at rest; but,
mean-while, the earth changes its position from E to E´, on which
account the planet is not seen at A´, but at A´´, being accelerated by
the arc A´ A´´, in consequence of the earth's motion. On the other hand,
when the planet is passing through its inferior conjunction A C B, it
appears to move backwards in the heavens from A´ to B´, if the earth is
at rest, but from A´ to B´´, if the earth has in the mean time moved
from E to E´, being retarded by the arc B´ B´´. Although the motions of
the earth have the effect to accelerate the planet in the superior
conjunction, and to retard it in the inferior, yet, on account of the
greater distance, the apparent motion of the planet is much slower in
the superior than in the inferior conjunction, Venus being the whole
breadth of her orbit, or one hundred and thirty-six millions of miles
further from us when at her greatest, than when at her least, distance,
as is evident from Fig. 51. When passing from the superior to the
inferior conjunction, or from the inferior to the superior, through the
greatest elongations, the inferior planets are _stationary_. Thus, (Fig.
51,) when the planet is at A, the earth being at E, as the planet's
motion is directly towards the spectator, he would constantly project it
at the same point in the heavens, namely, A´; consequently, it would
appear to stand still. Or, when at its greatest elongation on the other
side, at B, as its motion would be directly from the spectator, it would
be seen constantly at B´. If the earth were at rest, the stationary
points would be at the greatest elongations, as at A and B; but the
earth itself is moving nearly at right angles to the planet's motion,
which makes the planet appear to move in the opposite direction. Its
direct motion will therefore continue longer on the one side, and its
retrograde motion longer on the other side, than would be the case, were
it not for the motion of the earth. Mercury, whose greatest angular
distance from the sun is nearly twenty-nine degrees, is stationary at an
elongation of from fifteen to twenty degrees; and Venus, at about
twenty-nine degrees, although her greatest elongation is about
forty-seven degrees.
Mercury and Venus exhibit to the telescope _phases_ similar to those of
the moon. When on the side of their inferior conjunction, as from B to C
through D, Fig. 52, less than half their enlightened disk is turned
towards us, and they appear horned, like the moon in her first and last
quarters; and when on the side of the superior conjunction, as from C to
B through A, more than half the enlightened disk is turned towards us,
and they appear gibbous. At the moment of superior conjunction, the
whole enlightened orb of the planet is turned towards the earth, and the
appearance would be that of the full moon; but the planet is too near
the sun to be commonly visible.
[Illustration Fig. 52.]
We should at first thought expect, that each of these planets would be
largest and brightest near their inferior conjunction, being then so
much nearer to us than at other times; but we must recollect that, when
in this situation, only a small part of the enlightened disk is turned
toward us. Still, the period of greatest brilliancy cannot be when most
of the illuminated side is turned towards us, for then, being at the
superior conjunction, its light will be diminished, both by its great
distance, and by its being so near the sun as to be partially lost in
the twilight. Hence, when Venus is a little within her place of greatest
elongation, about forty degrees from the sun, although less than half
her disk is enlightened, yet, being comparatively near to us, and
shining at a considerable altitude after the evening or before the
morning twilight, she then appears in greatest splendor, and presents an
object admired for its beauty in all ages. Thus Milton,
"Fairest of stars, last in the train of night,
If better thou belong not to the dawn,
Sure pledge of day that crown'st the smiling morn
With thy bright circlet."
Mercury and Venus both _revolve on their axes_ in nearly the same time
with the earth. The diurnal period of Mercury is a little greater, and
that of Venus a little less, than twenty-four hours. These revolutions
have been determined by means of some spot or mark seen by the
telescope, as the revolution of the sun on his axis is ascertained by
means of his spots. Mercury owes most of its peculiarities to its
proximity to the sun. Its light and heat, derived from the sun, are
estimated to be neatly seven times as great as on the earth, and the
apparent magnitude of the sun to a spectator on Mercury would be seven
times greater than to us. Hence the sun would present to an inhabitant
of that planet, with eyes like ours, an object of insufferable
brightness; and all objects on the surface would be arrayed in a light
more glorious than we can well imagine. (See Fig. 53.) The average heat
on the greater portion of this planet would exceed that of boiling
water, and therefore be incompatible with the existence both of an
animal and a vegetable kingdom constituted like ours.
The motion of Mercury, in his revolution round the sun, is swifter than
that of any other planet, being more than one hundred thousand miles
every hour; whereas that of the earth is less than seventy thousand.
Eighteen hundred miles every minute,--crossing the Atlantic ocean in
less than two minutes,--this is a velocity of which we can form but a
very inadequate conception, although, as we shall see hereafter, it is
far less than comets sometimes exhibit.
Venus is regarded as the most beautiful of the planets, and is well
known as the _morning and evening star_. The most ancient nations,
indeed, did not recognise the morning and evening star as one and the
same body, but supposed they were different planets, and accordingly
gave them different names, calling the morning star Lucifer, and the
evening star Hesperus. At her period of greatest splendor, Venus casts a
shadow, and is sometimes visible in broad daylight. Her light is then
estimated as equal to that of twenty stars of the first magnitude. In
the equatorial regions of the earth, where the twilight is short, and
Venus, at her greatest elongation, appears very high above the
horizon, her splendors are said to be far more conspicuous than in
our latitude.
[Illustration Fig. 53. APPARENT MAGNITUDES OF THE SUN, AS SEEN FROM THE
DIFFERENT PLANETS.]
[Illustration Figures 54, 55, 56. VENUS AND MARS.]
Every eight years, Venus forms her conjunction with the sun in the same
part of the heavens. Whatever appearances, therefore, arise from her
position with respect to the earth and the sun, they are repeated every
eight years, in nearly the same form.
Thus, every eight years, Venus is remarkably conspicuous, so as to be
visible in the day-time, being then most favorably situated, on several
accounts; namely, being nearest the earth, and at the point in her orbit
where she gives her greatest brilliancy, that is, a little within the
place of greatest elongation. This is the period for obtaining fine
telescopic views of Venus, when she is seen with spots on her disk. Thus
two figures of the annexed diagram (Fig. 54) represent Venus as seen
near her inferior conjunction, and at the period of maximum brilliancy.
The former situation is favorable for viewing her inequalities of
surface, as indicated by the roughness of the line which separates the
enlightened from the unenlightened part, (the _terminator_.) According
to Schroeter, a German astronomer, Venus has mountains twenty-two miles
high. Her mountains, however, are much more difficult to be seen than
those of the moon.
The sun would appear, as seen from Venus, twice as large as on the
earth, and its light and heat would be augmented in the same proportion.
(See Fig. 53.) In many respects, however, the phenomena of this planet
are similar to those of our own; and the general likeness between Venus
and the earth, in regard to dimensions, revolutions, and seasons, is
greater than exists between any other two bodies of the system.
I will only add to the present Letter a few words on the _transits_ of
the inferior planets.
The transit of Mercury or Venus is its passage across the sun's disk, as
the moon passes over it in a solar eclipse. The planet is seen projected
on the sun's disk in a small, black, round spot, moving slowly over the
face of the sun. As the transit takes place only when the planet is in
inferior conjunction, at which time her motion is retrograde, it is
always from left to right; and, on account of its motion being retarded
by the motion of the earth, (as was explained by Fig. 51, page 232,) it
remains sometimes a long time on the solar disk. Mercury, when it makes
its transit across the sun's centre, may remain on the sun from five to
seven hours.
You may ask, why we do not observe this appearance every time one of the
inferior planets comes into inferior conjunction, for then, of course,
it passes between us and the sun. It must, indeed, at this time, cross
the meridian at the same time with the sun; but, because its orbit is
inclined to that of the sun, it may cross it (and generally does) a
little above or a little below the sun. It is only when the conjunction
takes place at or very near the point where the two orbits cross one
another, that is, near the _node_, that a transit can occur. Thus, if
the orbit of Mercury, N M R, Fig. 50, (page 231,) were in the same plane
with the earth's orbit, (and of course with the sun's apparent orbit,)
then, when the planet was at Q, in its inferior conjunction, the earth
being at E, it would always be projected on the sun's disk at Z, on the
concave sphere of the heavens, and a transit would happen at every
inferior conjunction. But now let us take hold of the point R, and lift
the circle which represents the orbit of Mercury upwards seven degrees,
letting it turn upon the diameter _d b_; then, we may easily see that a
spectator at E would project the planet higher in the heavens than the
sun; and such would always be the case, except when the conjunction
takes place at the node. Then the point of intersection of the two
orbits being in one and the same plane, both bodies would be referred to
the same point on the celestial sphere. As the sun, in his apparent
revolution around the earth every year, passes through every point in
the ecliptic, of course he must every year be at each of the points
where the orbit of Mercury or Venus crosses the ecliptic, that is, at
each of the nodes of one of these planets;[12] and as these nodes are on
opposite sides of the ecliptic, consequently, the sun will pass through
them at opposite seasons of the year, as in January and July, February
and August. Now, should Mercury or Venus happen to come between us and
the sun, just as the sun is passing one of the planet's nodes, a transit
would happen. Hence the transits of Mercury take place in May and
November, and those of Venus, in June and December.