[Illustration: _Plate I._
The COPERNICAN or SOLAR SYSTEM.
_The comparative Distances of the Planets from the Sun_
_T. Conder Sculp^t._]
THE
STUDY OF ASTRONOMY,
ADAPTED TO THE
CAPACITIES OF YOUTH:
_IN TWELVE FAMILIAR DIALOGUES_,
BETWEEN
A TUTOR AND HIS PUPIL:
Explaining the General PHÆNOMENA of the HEAVENLY
BODIES, the THEORY of the TIDES, &c.
_ILLUSTRATED WITH COPPER-PLATES._
BY JOHN STEDMAN.
_LONDON_:
PRINTED FOR C. DILLY, IN THE POULTRY.
M.DCC.XCVI.
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ERRATA.
Page 20. line 8. _for_ he _read_ the.
—— 22. — 6. ⎫
⎪
—— 23. — 2. ⎪ — disk — disc.
⎬
—— 42. — last ⎪
⎪
—— 79. — 5. ⎭
—— 74. — 6. — it axis — its axis.
—— 78. — 19. _dele_ Mercury.
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PREFACE.
It has long been a matter of surprize to those who are interested in the
education of youth, that, among the numerous publications intended for
their improvement, so few attempts have been made to facilitate the
study of Astronomy.
Many excellent treatises have been written on this important and useful
science; but if it be considered that they abound with technical terms,
unintelligible to juvenile minds, it cannot be expected that they should
derive any great advantage from the perusal of them.
To remove these difficulties, the Author has endeavoured, whenever he
had occasion to use them, to give such illustrations as to leave no
doubt on the young student’s mind respecting their true meaning.
The subject appeared to him to be best calculated for dialogues, which
are certainly more agreeable as well as more perspicuous to young
persons, than the discouraging formality of a treatise. And it is
presumed the language will be found natural and easy.
In the order he has chosen, he has been careful not to introduce any
thing new, till the former part, on which it depends, has been clearly
explained.
On the whole, it has been his aim to render it as concise and plain as
the nature of the subject will admit; and he flatters himself, that at a
time when the sciences are so universally studied, the introduction now
offered to the public will not be unacceptable.
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CONTENTS.
DIALOGUE I. p. 1.
Introduction. Definition. The sun and planets. A
globe defined. Sun’s distance and magnitude.
Planets, what; their names, periods, and
distances from the sun; their magnitudes,
compared with the earth; called inferior and
superior, why. Comets; derivation of the name.
Solar system; why so called.
DIALOGUE II. p. 10.
Different systems explained. Planets appear like
stars; they shine by reflection; how known
from stars; they never twinkle, why. Stars
shine with their own native light; their
inconceivable distance; are suns, the centers
of other systems. Plurality of worlds.
DIALOGUE III. p. 20.
The earth has the appearance of a star to Venus.
Remote objects appear at equal distances from
us. Our earth is a moon to the moon. The orb
of the moon visible soon after the change; her
disc and bulk compared with the earth; her
mean distance. Sun’s disc compared with hers.
Our sun a star, if seen from a planet of
another system. Stars as far from each other
as the nearest is to us. Stars distinguished
by their apparent magnitude. The Milky Way
innumerable stars. Number of stars visible at
one time to the naked eye.
DIALOGUE IV. p. 29.
Stars divided into constellations; necessary for
ascertaining the situation of the planets, and
of the stars with each other. Planets motion
regular if seen from the sun; irregular as
seen from the earth, the motion being
sometimes direct, sometimes retrograde; at
others they appear stationary. Superior and
inferior conjunction, and opposition, what.
Venus has the different phases of the moon.
Planets, how distinguished from each other.
DIALOGUE V. p. 39.
Ecliptic, what. Inclination of the orbits of the
planets. Nodes of the planets, what. A plane,
what. Planets move in unbounded space. Mercury
and Venus seen on the sun’s disc. Number of
signs in the zodiac. Zodiac, what. A degree,
what. Names of the signs. Number of degrees in
each sign. Sun’s place in the ecliptic. Table
of signs, their characters, &c. To find the
sun’s place in the ecliptic for any day in the
year.
DIALOGUE VI. p. 50.
The orbits of the planets are not true circles,
but somewhat elliptical. Perihelion, aphelion,
and mean distance, what. Attraction, what.
Laws of attraction. Attraction of gravitation,
its effects. Simple motion rectilineal.
Attractive or centripetal, and projectile or
centrifugal forces, what.
DIALOGUE VII. p. 61.
Bodies moving in circles have a tendency to fly
off. Planets kept in their orbits by the joint
action of the centripetal and centrifugal
forces; they describe equal areas in equal
times. Orbits of the comets very elliptical.
The earth in its perihelion in December.
Equation of time. Center of gravity, what; sun
and planets move round it. Sun the center of
the system.
DIALOGUE VIII. p. 73.
The earth revolves on its axis. Cause of day and
night. The motion of the earth so uniform as
not to be perceived. The apparent motion of
the sun caused by the earth’s motion on its
axis. An objection to the earth’s motion
answered. The sun and some of the planets
revolve on their axes. Atmosphere, what; cause
of twilight. Horizon, what; the sun and moon
appear largest near the horizon, why; they
appear above the horizon when below it; caused
by refraction; proved by experiment.
DIALOGUE IX. p. 87.
Inclination of the earth’s axis. An angle, what.
The poles, what. Equinoctial, what. Earth’s
parallelism described. The axis of the earth
points to the same parts of the heavens.
Equator, ecliptic, polar circles, and
meridians, explained. Difference of time
between places lying under different
meridians. Longitude, what. How to reduce
longitude to time, and time to longitude.
Latitude, what.
DIALOGUE X. p. 101.
The seasons. Vernal and autumnal equinoxes. Days
and nights always equal, if the axis of the
earth were perpendicular to the plane of its
orbit. Seasons occasioned by the inclination
of the earth’s axis. Seasons continued. Days
and nights equal at all times under the
equator. The sun above the horizon of the
poles six months; and six months below them
alternately, so that they have but one day and
one night in the year; the longest day under
the polar circles is twenty-four hours. The
sun rises on different points of the compass
at different seasons of the year. Twilight in
the polar regions of long duration. We are
nearest the sun in winter, yet it is our
coldest season, why. The earth divided into
zones; proved to be globular, but is not a
true sphere.
DIALOGUE XI. p. 120.
The moon. Her diameter, synodical and periodical
revolutions. Her phases. Has always the same
side to the earth, and makes a revolution on
her axis every lunation. Has mountains and
valleys, but no seas nor atmosphere; yet may
be inhabited. Her real and apparent motion
described. Eclipses. Of the sun; total and
partial eclipses. Digit, what. Eclipse of the
moon. Penumbra, what. Central and total
eclipse. Why we have not an eclipse at every
full and change of the moon. She does not
always rise with the sun at change; nor when
he sets at full. She is visible when totally
eclipsed.
DIALOGUE XII. p. 136.
Tides. Occasioned by the attraction of the sun
and moon, and their centrifugal forces;
exemplified by an experiment. Spring and neap
tides. Tides not highest directly under and
opposite the moon, but after she has passed
the meridian. They are later and later every
day. Rule for finding the proportional
magnitudes of the planets compared with the
earth; or the proportion that one globe bears
to another. A cube number, what. Table of
roots, squares, and cubes; an example. Rule
for finding the mean distances of the planets
from the sun. Dr. Turner’s rule for extracting
the cube root; an example to explain the rule.
Example to find the mean distance of Mercury
from the sun. Table of diameters, &c.
Conclusion.
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DIALOGUE I.
TUTOR.
Well, Sir! I suppose this early visit is in consequence of my promise,
and your anxiety to become an astronomer.
PUPIL. It is, Sir.—And as astronomy is a science of which I have a very
imperfect idea, I must beg of you to explain it to me.
TUTOR. That I shall do with pleasure. But you surely cannot wholly
forget what I have formerly told you. However, as I mean to treat the
subject as if you had no previous knowledge of it, you will have an
opportunity from what you can recollect, to make such remarks, and ask
such questions, as may appear most material to you.
PUPIL. I thank you, Sir, it is just what I wish.
TUTOR. By astronomy then is meant a knowledge of the heavenly bodies,
the sun, moon, planets, comets, and stars, respecting their nature,
magnitudes, distances, motions, &c.
PUPIL. I fear I shall find it a difficult study.
TUTOR. Have patience.——
“The wise and prudent conquer difficulties,
“By daring to attempt them. Sloth and folly
“Shiver and shrink at sight of toil and danger,
“And make the impossibility they fear.”
PUPIL. This gives me encouragement, and, if you will have patience with
me, I will endeavour to profit by your instructions.——Pray, Sir, what is
the sun?
TUTOR. The sun, the source of light and heat, has been considered a
globe of fire, round which seven other spherical bodies revolve at
different distances from him, and in different periods of time, from
west by south to east. These are the planets[1].
[Footnote 1: From _Planeta_, roving or wandering.]
PUPIL. Any round ball is a globe, is it not?
TUTOR. A sphere or globe is defined a round solid body, every part of
whose surface is equally distant from a point within called its center;
and a line drawn from one side through the center to the opposite side,
is called its diameter.
PUPIL. You say the sun has been considered a globe of fire. Is he not
now thought to be so?
TUTOR. [2]Doctor Herschell, from some late observations, is of a
different opinion.—But what think you of his magnitude?
[Footnote 2: See his letter read at the Royal Society, December
18th, 1794.]
PUPIL. I really cannot conjecture.—This I know, that when I saw him
through the fog the other day, he appeared about the size of a common
plate.
TUTOR. You must not always judge by appearances. You will find that
there is a material difference between his real and apparent magnitude,
which I think you will be convinced of when I tell you, that he is no
less than 95 millions of miles from our earth.
PUPIL. Ninety-five millions of miles! You astonish me.
TUTOR. You will, I dare say, be no less surprized at being told, that he
is more than a million of times as large as our earth.
PUPIL. It is almost incredible! And what are the planets?
TUTOR. The planets are opaque, that is dark bodies, which receive their
light from the sun; and, as I told you, revolve about him. The first, or
that nearest the sun, is called Mercury, the next Venus, then the Earth,
Mars, Jupiter, Saturn, and Georgian, or the Georgium Sidus.[3] These are
called primary planets.
[Footnote 3: Their characters are,
Sun, Merc. Venus, Earth, Mars, Jup. Saturn, Georgian,
☉ ☿ ♀ ♁ ♂ ♃ ♄ ♅ .]
PUPIL. Are there then any others?
TUTOR. Yes. There are fourteen others, which move round their respective
primaries as their centers, and with them round the sun, and are called
secondaries, satellites or moons.
PUPIL. Have all the primaries secondaries?
TUTOR. Only four of them have moons. The earth, I need not tell you, has
one; Jupiter has four; Saturn seven, besides a stupendous ring which
surrounds his body; and Georgian two.
PUPIL. In what time, and at what distances, from the sun, do the planets
perform their periodical revolutions?
TUTOR. _Mercury_ revolves about the sun in 88 days, at the distance of
36 millions of miles.
_Venus_, at the distance of 68 millions of miles, completes her
revolution in 224 days.
_Earth_, on which we live, at the distance of 95 millions of miles,
performs its period in one year.[4]
[Footnote 4: The motion of the earth in its orbit is at the rate of
68 thousand miles an hour.]
_Mars_, at the distance of 145 millions of miles, in little less than
two of our years.
_Jupiter_, at the distance of 494 millions of miles, in near 12 years.
_Saturn_, at the distance of 906 millions of miles, in about 30 years.
_Georgian_, discovered a few years since by Dr. Herschell, performs its
period at the distance of 1812 millions of miles, in about 83 years.[5]
[Footnote 5: As the distances of the planets, when marked in miles,
are a burthen to the memory, astronomers often express their mean
distances in a shorter way, by supposing the distance of the earth
from the sun to be divided into ten parts. Mercury may then be
estimated at four of such parts from the sun, Venus at seven, the
Earth at ten, Mars at fifteen, Jupiter at fifty-two such parts,
Saturn at ninety-five, and Georgian 190 parts. See Plate I. Fig. 1.
These are calculated by multiplying the respective distances of the
planets by 10, and dividing by 95, the mean distance of the earth
from the sun; and may be set off by any scale of equal parts.]
PUPIL. What proportion does the earth bear in magnitude to the other
planets?
TUTOR. The earth is fourteen times as large as Mercury, very little
larger than Venus, and three times as large as Mars. But Jupiter is more
than fourteen hundred times as large as the earth; Saturn above a
thousand times as large, exclusive of his ring; and Georgian eighty-two
times as large.
PUPIL. Have you any thing else, Sir, to remark concerning the planets?
TUTOR. There are several other things I intend to make you acquainted
with, namely, their nature, appearances, motions, &c. At present I shall
only say, that Mercury and Venus are called [6]inferior planets, their
orbits or paths described in going round the sun, being within that of
the earth; and the other four, whose orbits are without the earth’s
orbit, [7]superior planets.
[Footnote 6: Perhaps with more propriety _interior_ or _inward_.]
[Footnote 7: _Exterior_ or _outward_.]
PUPIL. There is one thing more I wish to know, if——
TUTOR. I suppose you were going to say if not too much trouble; that is
quite unnecessary, as you well know that where I see a desire to learn,
teaching is to me a pleasure.—What is it?
PUPIL. That you will be so kind as to inform me what the comets are, and
if they have any motion?
TUTOR. The knowledge we have of comets is very imperfect, as they afford
few observations on which to ground conjecture. They are generally
supposed to be planetary bodies, forming a part of our system: for, like
the planets, they revolve about the sun, but in different directions,
and in extremely long elliptic curves, being sometimes near the sun, at
others staying far beyond the orbit of the outermost planet; whereas the
orbits of the planets are nearly circular. The period of one, which
appeared in 1680, is computed to be 575 years.
PUPIL. Whence do they derive their name?
TUTOR. From _Cometa_, a _hairy star_, because they appear with long
tails, somewhat resembling hair: some, however, have been seen without
this appendage, as well defined and round as planets.
PUPIL. You say _our_ system: what am I to understand by it?
TUTOR. The word system, in an astronomical sense, means a number of
bodies moving round one common center or point: and, because the planets
and comets revolve about the sun, it is called the _Solar System_ (Plate
I. fig. 2.); and we say _our_ system, as the earth is one of the
planets. Other systems have been invented for solving the appearances
and motions of the heavenly bodies, a description of which I shall leave
till I next see you.
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DIALOGUE II.
PUPIL.
I am afraid, Sir, I am come before you are prepared for me: but the very
great pleasure I received yesterday, induced me to be with you as early
as possible.
TUTOR. I am glad to see you, and happy to find you are so well pleased
with your difficult study. It will, I assure you, give you more exalted
ideas of the Deity than any that I know of. The Psalmist was undoubtedly
of this opinion when he said, The Heavens declare the glory of God, and
the Firmament sheweth his handy work.
PUPIL. I will no longer call it a difficult, but a pleasing study, and
feel myself ashamed at having used the expression. I shall now beg you
to explain to me the different systems.
TUTOR. The system I have been describing to you was known and taught by
Pythagoras, a Greek philosopher, who flourished about 500 years before
Christ, as he found it impossible, in any other way, to give a
consistent account of the heavenly motions.
This system, however, was so extremely opposite to all the prejudices of
sense and opinion, that it never made any great progress, nor was ever
widely spread in the ancient world.
Ptolemy, an Egyptian philosopher, who flourished 130 years after Christ,
supposed that the earth was fixed in the center, and that the sun and
the rest of the heavenly bodies moved round it in twenty-four hours, or
one natural day, as this seemed to correspond with the sensible
appearances of the cœlestial motions. This system was maintained from
the time of Ptolemy to the revival of learning in the sixteenth century.
At length, Copernicus, a native of Poland, a bold and original genius,
adopted the Pythagorean system, and published it to the world in the
year 1530. This doctrine had been so long in obscurity, that the
restorer of it was considered as the inventor.
Europe, however, was still immersed in ignorance; and the general ideas
of the world were not able to keep pace with those of a refined
philosophy. This occasioned Copernicus to have few abettors, but many
opponents. Tycho Brahe, in particular, a noble Dane, sensible of the
defects of the Ptolemaic system, but unwilling to acknowledge the motion
of the earth, endeavoured, about 1586, to establish a new system of his
own; but, as this proved to be still more absurd than that of Ptolemy,
it was soon exploded, and gave way to the [8]Copernican or true Solar
System.
[Footnote 8: See Plate I. fig. 2.]
PUPIL. I confess, I should have thought with Ptolemy, that the earth was
in the center, and that the sun moved round it.
TUTOR. You must at present content yourself with knowing that it is not
so; and it shall be my business to prove it.
PUPIL. May I beg the favour of the information you intended respecting
the planets?
TUTOR. I will grant it with pleasure. The planets are spherical bodies,
which appear like stars, but are not luminous; that is, they have no
light in themselves; though they give us light; for they shine by
reflecting the light of the sun.
PUPIL. You say, Sir, that they appear like stars; if so, how am I to
know them from stars?
TUTOR. Very easily: for the stars, or as they are more properly called
fixed stars, always keep the same situation with respect to each other;
whereas the planets, as they move round the sun, must be continually
changing their places among the fixed stars, and with one another.
PUPIL. Is there any other method of distinguishing them besides what you
have mentioned?
TUTOR. Yes. The planets never twinkle like the fixed stars, and are seen
earliest in the evening and latest in the morning.
PUPIL. How is the twinkling of the stars in a clear night accounted for?
TUTOR. It arises from the continual agitation of the air or atmosphere
through which we view them; the particles of air being always in motion,
will cause a twinkling in any distant luminous body, which shines with a
strong light.
PUPIL. Then, I suppose, the planets not being luminous, is the reason
why they do not twinkle.
TUTOR. Most certainly. The feeble light with which they shine is not
sufficient to cause such an appearance.
PUPIL. Have the stars then light in themselves?
TUTOR. They undoubtedly shine with their own native light, or we should
not see even the nearest of them: the distance being so immensely great,
that if a cannon-ball were to travel from it to the sun, with the same
velocity with which it left the cannon, it would be more than 1 million,
868 thousand years, before it reached it.[9]
[Footnote 9: The distance of Syrius is 18,717,442,690,526 miles. A
cannon-ball going at the rate of 1143 miles an hour, would only
reach the sun in about 1,868,307 years, 88 days.
Adams’s Lectures, vol. 4. page 44.]
PUPIL. This is wonderful indeed! what then are they supposed to be?
TUTOR. Suns.
PUPIL. Suns! the fixed stars suns!
TUTOR. Yes, suns.
“One sun by day, by night ten thousand shine.”
And what will increase your astonishment, each of them is the center of
a system of planets, which move round him.[10]
[Footnote 10: Dr. Herschell says, that in some clusters of stars he
has observed, they appear too close together to admit any planets to
revolve about them.]
“Observe how system into system runs.”
“What other planets circle other suns.”
PUPIL. I am almost lost.—I used to think they were designed to give us
light.
TUTOR. This is a vulgar error.—They were doubtless created for a much
nobler purpose, since thousands of them are invisible to us without the
help of a telescope; and we receive more light from the moon than from
all the stars together.
PUPIL. How do you know they are suns? Is their being luminous a proof of
their being so?
TUTOR. No. But we know that the sun shines with his own light on all the
planets belonging to our system; and from what I have told you, have the
greatest reason to believe that the stars shine with their own light: we
therefore from analogy conclude, that they are so many suns conveying
light and heat to other worlds[11].
[Footnote 11: Dr. Herschell thinks it probable that the sun and
fixed stars may be inhabited.]
PUPIL. Are there then other worlds besides this we live in?
TUTOR. Consider.—Has not the earth we inhabit a moon to enlighten it?
PUPIL. Yes, Sir.
TUTOR. And have I not told you that Jupiter, Saturn, and Georgian, have
also moons?
PUPIL. This I well remember.
TUTOR. For what purpose then do you suppose those orbs were designed?
PUPIL. Indeed, I cannot tell.
TUTOR. You surely cannot imagine that they were intended for our use,
since we knew nothing of them till after the invention of telescopes.
PUPIL. That is what I think no one can suppose.
TUTOR. And do not all the planets enjoy the benefit of the sun in common
with us?
PUPIL. Undoubtedly.
TUTOR. Well, then; of what use would the light and heat be which is
conveyed to them from the sun; or the light which they receive from
their moons if there are no inhabitants?
PUPIL. I know of none.
TUTOR. Can you then have any doubt about their being inhabited?
PUPIL. No, Sir.—But you say that the stars are suns, each of which is
the center of a system of planets or worlds.
TUTOR. If you are satisfied that the planets belonging to our system are
inhabited, and that the fixed stars are suns, the centers of other
systems, what reasonable objection can you have to all the planets in
the universe being so?
PUPIL. It is what I cannot comprehend.
TUTOR. It may be so.—But is not the same Almighty Power, who does
nothing in vain, as capable of making ten thousand worlds if he pleased,
as well as one?
PUPIL. I will not presume to dispute his power; but are we not told that
all mankind descended from Adam?
TUTOR. Yes; Moses wrote concerning this earth, he has not made us
acquainted with the inhabitants of the other planets: for aught we know
they might descend from other Adams.—To-morrow evening, I hope to see
you again.
------------------------------------------------------------------------
DIALOGUE III.
PUPIL.
I recollect, Sir, you mentioned last night, that the planets appear like
stars. Our earth is a planet; how can it have the appearance of a star?
TUTOR. If you were on the planet Venus, the earth would have as much the
appearance of a star as Venus has to us.
PUPIL. But Venus appears amongst the fixed stars.
TUTOR. Yes. And so would the earth appear from Venus.
PUPIL. How can it be?
TUTOR. Because, in whatever part of the universe we are, we appear to be
in the center of a concave, that is hollow, sphere, where remote objects
appear at equal distances from us: so that, whether we are on the planet
Venus or on the earth, in this particular the effect will be the same.
PUPIL. Then the light _we_ receive from the sun is by reflection
conveyed to the other planets.
TUTOR. No doubt of it. And our earth appears as a moon to the
inhabitants of the moon, and undergoes the various changes of that
planet.
PUPIL. Have you any proof of this, Sir?
TUTOR. Nothing can be clearer; for, on a fine evening, soon after the
change of the moon, when the earth appears nearly as a full moon to the
moon, and we see a faint streak of light, the whole body of the moon is
visible to us.
PUPIL. I remember to have seen it.
TUTOR. You do?—The earth then will appear there thirteen times as large
as the moon does to us; of course it must reflect a strong light on the
body of the moon, and it is by that light we see that part of the moon
which is turned from the sun.
PUPIL. Is the earth, then, only thirteen times as big as the moon?
TUTOR. In solidity it is about fifty times as large; but its disc or
face is only thirteen times.
PUPIL. What is the moon’s distance from the earth?
TUTOR. 240 thousand miles, which is about 400 times less than that of
the sun.
PUPIL. And yet she appears as far distant as the sun.
TUTOR. You are now, I hope, convinced of what I said relative to distant
objects.
PUPIL. I am, Sir: and I suppose the reason of the moon’s appearing as
large as the sun, is because she is so much nearer to us.
TUTOR. It is so.—For, at a total eclipse of the sun, which happens when
the moon is in a right line between the sun and the earth, the sun is
obscured from our sight, although his disc is 160 thousand times as
large as that of the moon. In like manner would the moon, when at full,
be hid by placing your cricket-ball in a line between your eye and her,
yet, you know, the ball is not so large as the moon; but being nearer
the eye, it is apparently so.
PUPIL. This is very clear. But——
TUTOR. I conjecture you were going to ask me to explain the nature of
eclipses.
PUPIL. That was certainly my intention, Sir.
TUTOR. There are other things you must be made acquainted with before
you will be able to comprehend it, and which I will endeavour to make
you understand before we enter on the subject.
PUPIL. Whenever you please, Sir.
TUTOR. You have taken a view of the earth from the planet Venus.—Suppose
I transport you to one of the planets belonging to another system; what
description do you think you should give of it?
PUPIL. I must consider. What I now call a star would be a sun. The
planets of that system I should see as I now do those belonging to ours:
our sun would be a star; and the earth, with all the other planets,
would be invisible.
TUTOR. Very well, Sir. Can you then find it difficult to conceive that
all the stars are as far from each other in unbounded space as our sun
is from the nearest star?
PUPIL. It is hard to conceive: but when I consider that wherever I am,
every remote object appears at an equal distance from me, the difficulty
vanishes.
TUTOR. That you might form some idea of the immense distance of the
fixed stars, you must recollect, I mentioned the time a cannon-ball
would be in reaching the nearest of them.
PUPIL. I do, Sir. More than 1,868,000 years.
TUTOR. You have an excellent memory. I suppose then you know the
distance of the earth from the sun?
PUPIL. Yes, Sir. I wrote it down; and, it made so strong an impression
on my memory, that I believe I shall never forget it.—95 millions of
miles.
TUTOR. Now, suppose the earth to be in that part of its orbit which is
nearest to the star, it would be 95 millions of miles nearer to it than
the sun is.
PUPIL. Certainly.
TUTOR. And, in the opposite side of its orbit, as much farther from the
star.
PUPIL. Without doubt.
TUTOR. Then you find that the earth is 190 millions of miles nearer to
the star at one time of the year than it is at another; and yet the
magnitude of the star does not appear the least altered, nor is its
distance affected by it.
PUPIL. A proof of its amazing distance.—I was going to ask a silly
question.
TUTOR. What is it? perhaps not so simple as you may imagine.
PUPIL. Whether the most conspicuous stars are not supposed to be the
nearest to us?
TUTOR. Undoubtedly.—And are called stars of the first magnitude; the
next in splendor, stars of the second magnitude; and so on to the sixth
magnitude; and those beyond, which are not visible to the naked eye, are
called telescopic stars.
PUPIL. The distance of the telescopic stars must be great indeed, beyond
all conception.
TUTOR. You judge rightly; and their numbers are beyond all computation.
Doctor Herschell says, he has not a doubt but that the broad circle in
the heavens, called the Milky Way, is a most extensive stratum of stars,
he having discovered in it many thousands. Besides, some stars appear to
him double, others treble, &c. not that they are really so, but are
stars at different distances from us, which appear nearly in a right
line.
“As in the milky-way a shining white
“O’erflows the heav’ns with one continued light,
“That not a single star can shew his rays,
“Whilst jointly all promote the common blaze.”
PUPIL. I have heard of numbering the stars; but that, I find, is
impossible.
TUTOR. If you mean that immense host of stars I have been describing, it
is impossible; but, though in a clear winter’s night, without moonshine,
they seem to be innumerable, which is owing to their strong sparkling,
and our looking at them in a confused manner; yet when the whole
firmament is divided as it has been done by the ancients, the number
that can be seen at a time, by the naked eye, is not above a thousand.
PUPIL. Pray, Sir, how did the ancients divide the firmament?
TUTOR. I would willingly answer your question; but, as I find I shall
not have time to give you that information I wish, I shall postpone it
till I see you to-morrow evening.
------------------------------------------------------------------------
DIALOGUE IV.
TUTOR.
The ancients, in reducing astronomy to a science, combined the fixed
stars into constellations, allowing several stars to make one
constellation: and, for the better distinguishing and observing them,
they reduced the constellations to the forms of animals, or to the
images of some known things, by which means they were enabled to signify
to others any particular star they meant to notice. Job mentions two of
the constellations, namely, Orion and Pleiades, which shews the study of
astronomy to be very ancient.
PUPIL. Pray, Sir, how may I know them?
TUTOR. By studying the use of the cælestial globe, on which they are
drawn.
PUPIL. Will you be kind enough to instruct me, Sir?
TUTOR. At some future time I probably may: at present you are not
prepared for it.
PUPIL. I am satisfied.—Have you any thing more to remark of the
constellations, Sir?
TUTOR. Yes. The situation of the planets, as they are continually
changing their places, could not be pointed out without first dividing
the stars into constellations: hence, necessity was the mother of
invention.
PUPIL. And I think a very ingenious one.—If I may be allowed a
comparison, I will suppose the different kingdoms of the world on my
dissected map, to represent so many constellations; then, if I hear of
London, I know it is in England; if of Paris, in France; of Lisbon, in
Portugal; and so on. These I would compare with stars of the first
magnitude, being the chief cities of their respective kingdoms; inferior
cities, stars of the second magnitude; principal towns of the third, &c.
TUTOR. A very apt comparison indeed. Now if you hear of a traveller
setting off from London to Dover, thence to Calais, Paris, Bern, and so
on to Rome, you know that he must go through part of England, Flanders,
France, Switzerland, and Italy, passing many towns and villages on his
way.
PUPIL. That is very evident.
TUTOR. Very well, then; in like manner would the planets, if seen from
the sun, be traced from star to star, from constellation to
constellation, through their whole periods.
PUPIL. It is not possible to view them from the sun, surely, is it?
TUTOR. No, certainly.
PUPIL. Why then do you say if seen from the sun?
TUTOR. Because it is there only their motions can appear uniform; as
seen from the earth they apparently move very irregularly.—Suppose you
were in the center of a circular course; and, whilst a horse was going
round, you kept your eye on him: cannot you conceive that you should see
him run round the course in a regular manner, moving the whole time the
same way?
PUPIL. It is not at all difficult to conceive.
TUTOR. Again. Imagine yourself placed at a considerable distance on the
outside of the course, where you could see the horse the whole time he
was going round, would he appear to move as uniformly as before?
PUPIL. Certainly not: on the opposite side of the course his motion
would be the same as when I stood in the center of it; when he was
approaching me, I should scarcely see him move; in that part of the
course next to me he would move in a direction contrary to what he did
at first; and again when going from me, his motion would be scarcely
visible.
TUTOR. This I think will give you a tolerable idea of the irregular
motion of the inferior planets, as seen from the earth. When farthest
from us their motion is said to be direct; when nearest to us
retrograde, because they appear to be moving back again; and, when
approaching, or going from us, we say they are stationary; because, if
then observed in a line with any particular star, they will continue so
for a considerable time: now these appearances could not happen if they
moved round the earth.
PUPIL. Nothing can be plainer: for if the earth were in the center we
should always see them move the same way.
TUTOR. When the planet is nearest to us, that is in a line between us
and the sun, we say it is in its inferior conjunction; when farthest
from us, and the sun is between us and the planet, in its superior
conjunction. But the superior planets have alternately a conjunction and
an opposition.
PUPIL. A conjunction, I suppose, when the sun is between the earth and
the planet, and an opposition when the earth is between the sun and the
planet; that is, when the planet is nearest to us, and appears to be
opposite to the sun?
TUTOR. You are right.—Therefore, when in conjunction it rises and sets,
nearly with the sun; but in opposition, it rises nearly when the sun
sets, and sets when he rises.
PUPIL. Why do you say nearly, Sir?
TUTOR. Because it cannot be exactly, but when the sun, earth, and planet
are in a _right_ line, which seldom happens.
PUPIL. How do you account for this, Sir?
TUTOR. At present I fear you will not be able to comprehend what I wish
to explain, as I must use a term you are unacquainted with. The reason
is, that the planets are very seldom in or near their nodes at their
conjunctions or oppositions.
PUPIL. I do not indeed understand what you mean by the word _nodes_.
TUTOR. It will be explained to you in due time, and I shall conclude
this evening with a few more remarks relative to the appearance of the
planets.
PUPIL. Any thing you please, Sir.
TUTOR. You know that the planets, being opaque bodies, receive their
light from the sun; and that only that part which is turned to the sun
can be enlightened by him, whilst the opposite side must remain in
darkness.
PUPIL. This is self-evident: if I hold my ball to the candle it will
have the same effect.
TUTOR. Tell me then how you think they will appear as seen from the
earth.
PUPIL. If, when you shewed me Venus, she had not appeared perfectly
round, I should say that, both before and after her superior conjunction
I should see her nearly with a full face; when stationary, only half
enlightened, like the moon at first quarter; because, an equal portion
of the dark and bright parts will be turned towards us; the bright part
will be decreasing till her inferior conjunction, when the dark side
will be turned towards us, and consequently invisible; the light will
then increase; and, when she is again stationary, she will appear like
the moon at last quarter.
TUTOR. When seen through a telescope she has the different appearances
you have mentioned; and when I next see you I will shew you that both
Venus and Mercury may sometimes be seen when in their inferior
conjunctions; the superior planets always appear with nearly a full
face.
PUPIL. How are the planets distinguished from each other?
TUTOR. _Mercury_, from his vicinity to the sun, is seldom seen, being
lost in the splendor of the solar brightness. When seen, he emits a very
bright white light.
_Venus_, known by the names of the morning and evening star, is the
brightest, and to appearance, the largest of all the planets; her light
is of a white colour, and so considerable, that in a dusky place she
projects a sensible shade. She is visible only for three or four hours
in the morning or evening, according as she is before or after the sun.
_Mars_ is the least bright of all the planets. He appears of a dusky
reddish hue, and much larger at some periods than at others, according
as he is nearer to, or farther from us.
_Jupiter_ is distinguished by his peculiar magnitude and light. To the
naked eye he appears almost as large as Venus, but not altogether so
bright.
_Saturn_ shines but with a pale feeble light, less bright than Jupiter,
though less ruddy than Mars.
_The Georgium Sidus_ cannot be readily perceived without the assistance
of a telescope.
------------------------------------------------------------------------
DIALOGUE V.
TUTOR.
Before I proceed to explain what I promised you, it is necessary you
should be informed that the earth as seen from the sun, in its
periodical revolutions, will describe a circle among the stars which
astronomers call the _ecliptic_, and sometimes _the sun’s annual path_,
because the sun, as seen from the earth, always appears in that line.
PUPIL. Do not all the planets move in the ecliptic?
TUTOR. No.—On account of the obliquity of their orbits, they are, in
every revolution, one half of their periods above the ecliptic, and the
other half below it.
PUPIL. I think I comprehend your meaning; but shall be obliged to you,
Sir, if you can make it clearer to me.
TUTOR. I have here a little design, (Plate II. Fig. 1.) which will
answer our purpose: where S represents the sun; ABCD, the orbit of the
earth; and EFGH, the orbit of one of the inferior planets, suppose
Venus.
[Illustration: _Plate II._
_T. Conder Sculp^t._]
PUPIL. Now I understand it perfectly: the half EHG rises above, and the
other half EFG sinks below it, from the points EG, which I perceive are
in a line with the orbit of the earth. But pray, Sir, have you any name
for that dotted line?
TUTOR. Yes, it is called the _line_ of the nodes; and the points EG the
_nodes_ of the planet: the latter is called the ascending node, because,
when the planet is in G, it is ascending or rising above the orbit of
the earth; or, which is the same thing, above the ecliptic: and when in
E, it is descending or sinking below it, whence _it_ is called the
descending node. But you must remember that the orbits of all the
planets do not cross or intersect the ecliptic in the same points; but
that their nodes or intersections are at different parts of it.
PUPIL. How can the orbit of the earth and the ecliptic be the same?
TUTOR. They are very different; but being in the same plane, if the
orbit of any planet inclines to one it must incline equally to the
other.
PUPIL. You will, I fear, Sir, think me very stupid: but I must beg of
you to inform me what you mean by a plane?
TUTOR. Any flat surface is a plane. You may therefore suppose the edge
of a round tea-table to represent the ecliptic, and a circle within it,
drawn from the center of the table, the orbit of the earth: will they
not be both in the same plane?
PUPIL. Certainly.
TUTOR. You must not imagine, when I am speaking to you of the plane of
the ecliptic, or plane of the earth’s orbit, that it is a visible flat
surface, or, in speaking of the orbits of the planets, I mean solid
rings.—No. The planets perform their revolutions with the utmost
regularity, in unbounded space; and, like a bird thro’ the air, leave no
track behind them.
PUPIL. How then are they retained in their orbits?
TUTOR. The question, I confess, is natural, and is what I expected; but
I must of necessity postpone it to another opportunity; and shall now
fulfil the promise I made of shewing you in what manner the inferior
planets may be seen when in their inferior conjunctions. Cast your eye
again on the little design I gave you, and consider, if Venus were in
her ascending node at G, when the earth is at _b_; or, in her descending
node, at E, when the earth is at _a_, what the effect would be.
PUPIL. She would be in a line with the sun.
TUTOR. And, on the sun’s disc, she would appear a dark round spot,
passing over it. These appearances, which are called transits, happen
very seldom: because she is very seldom in or near her nodes at her
inferior conjunctions. There was one in June 1761, one in June 1769; and
the next will be in the year 1874. And as Mercury is seen in the same
manner, it is a proof that their orbits must be within that of the
earth.
PUPIL. I thank you, Sir, and shall be obliged to you to inform me how
many constellations the earth pastes over in every revolution?
TUTOR. Twelve, which correspond with the months of the year, and are
called the twelve signs of the zodiac.
PUPIL. What is the zodiac?
TUTOR. That part of the heavens which contains the twelve signs, and
which you may conceive to be a zone or belt extending eight degrees on
each side the ecliptic, in which the planets constantly revolve: so that
no planet is ever seen more than eight degrees either north or south,
that is above or below the ecliptic.
PUPIL. What am I to understand by a degree?
TUTOR. All circles, whether great or small, are supposed to be divided
into 360 equal parts, called degrees, and each degree into 60 equal
parts, called minutes: therefore, if I speak of a circle in the heavens,
the circumference of the earth, or any other circle, by a degree is
meant the 360th part of that circle; and a minute the 60th part of a
degree.
PUPIL. What are the names of the twelve signs?
TUTOR. The first is called Aries, which you know signifies a Ram;
Taurus, the Bull; Gemini, the Twins; Cancer, the Crab; Leo, the Lion;
Virgo, the Virgin; Libra, the Balance; Scorpio, the Scorpion;
Sagittarius, the Archer; Capricorn, the Goat; Aquarius, the
Water-bearer; and Pisces, the Fishes.
PUPIL. Do you wish me to commit these to memory, Sir?
TUTOR. It is very requisite; but as I know you are fond of verse, you
shall hear what Doctor Watts says—
The Ram, the Bull, the heav’nly Twins,
And next the Crab the Lion shines,
The Virgin, and the Scales:
The Scorpion, Archer, and Sea-goat,
The Man that holds the Water-pot,
And Fish with glitt’ring tails.
PUPIL. I like it much, as it will assist my memory.
TUTOR. As the twelve signs correspond with the months of the year, the
earth must pass over nearly one degree every day, one sign every month,
and in twelve months complete a whole circle, or 360 degrees; therefore
every sign must contain 30 degrees, because 30 multiplied by 12 is equal
to 360.
PUPIL. It must be so.
TUTOR. You must remember, that when the earth is in any sign, as seen
from the sun, the sun will be in the opposite sign, as seen from the
earth: for instance, if the earth be in Aries, the sun will be in Libra;
if in Taurus, the sun will be in Scorpio, &c. therefore, as by the
earth’s annual motion, the sun _appears_ to move, we always speak of the
sun’s, not the earth’s place, in the ecliptic.—You do not seem to
understand me?
PUPIL. Not perfectly, Sir.
TUTOR. Take this orange, and put it in the middle of the round table
before us, and place an apple on the opposite side next the window: the
orange may represent the sun, the apple the earth, and the window the
sign Aries. Now go round the table to the apple; look at the orange, and
tell me to what part of the room the eye will be directed.
PUPIL. To the part opposite to the window, Sir.
TUTOR. If then you suppose the door, which is opposite to the window, to
be the sign Libra, the sun will be in Libra when the earth is in
Aries—will it not?
PUPIL. It is very plain.
TUTOR. I shall now give you a table of the signs, their characters, the
corresponding months, and the days of the month the sun enters each
sign, by means of which, if you reckon a degree for a day, you may find
the sun’s place, nearly, for any day in the year.
PUPIL. This will give me much pleasure, and I shall be happy to have it.
THE TABLE.
NORTHERN SIGNS.
Aries, Taurus, Gemini, Cancer, Leo, Virgo.
♈ ♉ ♊ ♋ ♌ ♍
March, April, May, June, July, Aug.
20, 20, 21, 21, 23, 23.
SOUTHERN SIGNS.
Libra, Scorpio, Sagittarius, Capricorn, Aqua. Pisces.
♎ ♏ ♐ ♑ ♒ ♓
Sept. October, November, Decem. Jan. Feb.
23, 23, 21, 21, 20, 18.
PUPIL. Why do you write northern and southern signs, Sir?
TUTOR. Because they are situated north and south of a circle in the
heavens, called the equinoctial, which circle crosses the ecliptic in
the points Aries and Libra, and extends 23-1/2 degrees on each side of
it; and which I shall have occasion to mention to you another time.
PUPIL. When you think proper, Sir, I shall be glad to have it explained
to me.
TUTOR. Look at your table, and tell me what sign and what degree the sun
is in the 30th of March, and 20th of October.
PUPIL. The sun enters Aries the 20th of March, of course he must be 10
degrees in that sign the 30th; and, as he does not enter Scorpio till
the 23d of October, he must want three degrees of completing the sign
Libra; he must therefore, on the 20th of October, be in 27 degrees of
Libra.
TUTOR. Very well.—Do you learn the table, as you will have a farther use
for it.
------------------------------------------------------------------------
DIALOGUE VI.
PUPIL.
Since I was last with you, Sir, I have been thinking of what you then
told me, that the planets perform their revolutions in open space: I
have not the least idea how this can be; if convenient, I shall be happy
to have it explained.
TUTOR. It will be necessary first to inform you, that the orbits or
paths described by the revolution of the planets round the sun, are not
true circles (as Plate II. fig. 2.) but somewhat elliptical, that is,
longer one way than the other, as fig. 3.
PUPIL. This is exceedingly plain.
TUTOR. In a circle, the periphery or circumference is equally distant
from a point within called its center, as A; but an ellipsis has two
points called the focuses or foci, as B C. In one of these, called its
lower focus, is the sun: so that you see in every revolution of the
planet it must be nearer to the sun in one part of its orbit, than it is
in another.
PUPIL. I see it clearly.
TUTOR. Now let S (Plate II. fig. 4.) represent the sun, A B C D a planet
in different parts of its orbit; when it is nearest to the sun, as at A
it is said to be in its _perihelion_; when at B its _aphelion_; but when
at C or D its middle or mean distance, because the distance S C or S D
is the middle between A S the least and B S the greatest distance; and
half the distance between the two focuses is called the _eccentricity_
of its orbit, as S E or E F.
PUPIL. This I will endeavour to understand; but I find it will take me
some time to be perfected in it.
TUTOR. You may study it at your leisure, as it will not prevent our
proceeding to the thing proposed, namely, the laws which govern the
motion of the planets, or ATTRACTION OF GRAVITATION.
PUPIL. By attraction I think you mean that property in bodies whereby
they have a tendency to approach each other. I remember you told me that
the magnet I had the other day attracted the needle.
TUTOR. Yes. And you may recollect that when I took a feather suspended
by a thread, and put it near the conductor of the electrical machine, it
was strongly attracted by it, and adhered to it as long as the machine
was kept in motion.
PUPIL. I remember it well. But what am I to understand by attraction of
gravitation?