Part 1

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.

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.

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.

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.

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.

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?