A compendium of astronomy; containing the elements of the science, familiarly explained and illustrated, with the latest discoveries. Adapted to the use of schools and academies, and of the general reader

Front matter

X s

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* •1

Preface

This small volume is intended to afford to the General Reader, and to the more advanced pupils of our Schools and Academies, a comprehensive outline of Astronomy with its latest discoveries. For its perusal, no further acquaintance with mathematics is necessary, than a knowledge of common arithmetic ; although some slight knowledge, at least, of geometry and trigonometry will prove very useful.

By omitting mathematical formulae, and employing much familiar illustration, we have endeavored to bring the leading facts and doctrines of this noble and interesting science, within the comprehension of every attentive and intelligent reader. In no science, more than in this, are greater advantages to be derived from a lucid arrangement — an order winch brings out every fact and doctrine of the science, just in the place where the mind is ready to receive it. A certain maturity of mind, and power of reflection, are, however, indispensable for understanding this science. Astronomy is no study for children. Let them be employed on subjects more suited to the state of their capacities, until those faculties are more fully developed, which will enable them to learn to conceive correctly of the celestial motions. A work on Astronomy that is very easy, must be very superficial, and will be found to enter little into the arcana of the science. The riches of this mine lie deep ; and no one can acquire them, who is either incompetent or unwilling to penetrate beneath the surface.

Although this treatise is based on the larger work of the author, (" Introduction to Astronomy,") prepared for the students of Yale College, yet it is not merely an abridgment of that. It contains much original jnatter adapted to the peculiar exigencies of the class of readers for whom it is intended. The few passages taken verbatim from astronomical writers,

IV PREFACE.

are not, as in the larger work, always accredited to their respective authors, as this was deemed unimportant in a work of this description.

It is strongly recommended to all who study this science, even in its most elementary form, early to commence learning the names of the constellations, and of the largest of the individual stars, in the order in which they are described in the last part of the work. A celestial globe will be found a most useful auxiliary in this as in every other part of Astronomy. If it cannot supersede, it may greatly aid reflection. The reader also should, if in his power, take frequent opportunities of viewing the heavenly bodies through the telescope. This will add much to his intelligence, and increase his interest in the study.

ADVERTISEMENT.

Since the stereotype edition of this work was first published, several new and interesting discoveries have been added to Astronomy, an account of which will be found in the Supplement. They make no change in the great facts and doctrines of the science, but these remain unaltered and immutable ; while the new discoveries extend still further our knowledge of the Universe. We have, therefore, no occasion to alter the text, except perhaps very slightly in one or two statements, but by giving whatever is new and important in the form of a supplement, (to which we may add as every successive discovery is made,) we shall endeavor to secure to this treatise the freshness and accuracy of the most recent compilations, as well as furnish to the schools what has been thoroughly tested and approved by the most able teachers of the Union.

Part II. OF THE SOLAR SYSTEM

Chapter I. — Of the Sun — Solar Spots — Zodiacal Light, 70 Chapter II. Of the Apparent Annual Motion of the

Sun — Seasons — Figure of the Earth's Orbit, - 79 Chapter III. — Of Universal Gravitation — Kepler's

Laws, — Motion in an Elliptical Orbit — Precession

of the Equinoxes, - - - - - 91

Chapter IV. — Of the Moon — Phases, Revolutions, - 110 Chapter V. — Of Eclipses, - - - - - 137 Chapter VI.— Of Longitude— Tides, - - - 150

Chapter VIII. — Of the Superior Planets — Mars, Jupiter

Saturn and Uranus — Ceres, Pallas, Juno and Vesta, 183 vl CONTENTS.

VJ Pag«

Chapter IX.— Of the Motions of the Planetary System —Quantity of Matter in the Sun and Planets- Stability of the Solar System, - - - - 205

Chapter IV.— Of the System of the World, - - 265 COMPENDIUM OF ASTRONOMY

PRELIMINARY OBSERVATIONS.

1. Astronomy is that science which treats of the heavenly bodies.

More particularly, its object is to teach what is known respecting the Sun, Moon, Planets, Comets, and Fixed Stars ; and also to explain the methods by which this knowledge is acquired.

Astronomy is sometimes divided into Descriptive, Physical, and Practical. Descriptive Astronomy respects facts ; Physical Astronomy, causes ; Practical Astronomy, the means of investigating the facts, whether by instruments, or by calculation. It is the province of Descriptive Astronomy to observe, classify, and record, all the phenomena of the heavenly bodies, whether pertaining to those bodies individually, or resulting from their motions and mutual relations. It is the part of Physical Astronomy to explain the causes of these phenomena by investigating and applying the general laws on which they depend ; especially by tracing out all the consequences of the law of universal gravitation. Practical Astronomy lends its aid to both the other departments.

2. Astronomy is the most ancient of all the sciences. At a period of very high antiquity, it was cultivated in Egypt, in Chaldea, and in India. Such knowledge of the heavenly bodies as could be acquired by close and long continued observation, without the aid of instru-

1, Define Astronomy. What does it teach ? Name the three parU into which it is divided. What does Descriptive Astronomy respect ? What does Physical Astronomy ? What does Practical Astronomy 1 What is the peculiar province of each '

2 PRELIMINARY OBSERVATIONS.

ments, was diligently amassed ; and tables of the celestial motions were constructed, which could be used in predicting eclipses, and other astronomical phenomena.

About 500 years before the Christain era, Pythagoras, of Greece, taught astronomy at the celebrated school at Crotona, (a Greek town on the southeastern coast of Italy,) and exhibited more correct views of the nature of the celestial motions, than were entertained by any other astronomer of the ancient world. His views, however, were not generally adopted, but lay neglected for nearly 2000 years, when they were revived and established by Copernicus and Galileo. The most celebrated astronomical school of antiquity, was at Alexandria in Egypt, which was established and sustained by the Ptolemies, (Egyptian princes,) 300 years before the Christian era. The employment of instruments for measuring angles, and bringing in trigonometrical calculations to aid the naked powers of observation, gave to the Alexandrian astronomers great advantages over all their predecessors.

The most able astronomer of the Alexandrian school was Hipparchus, who was distinguished above all the ancients for the accuracy of his astronomical measurements and determinations. The knowledge of astronomy possessed by the Alexandrian school, and recorded in the Almagest, or great work of Ptolemy, constituted the chief of what was known of our science during the middle ages, until the fifteenth and sixteenth centuries, when the labors of Copernicus of Prussia, Tycho Brake

2. Trace the history of Astronomy. Among what ancient nations was it cultivated ? What kind of knowledge of the heavenly bodies was amassed ? Who was Pythagoras? When and where did he live 1 Where was his school ? How correct were his views 1 Were they generally adopted 1 Give an account of the Alexandrian school. When was it established and by whom 1 What gave it great advantages over all its predecessors ? Give some account of Hipparchus — of Ptolemy — of Copernicus — of Tycho Brahe — of Kepler — of Galileo — of Newton — of La Place. Specify the respective labors of each.

PRELIMINARY OBSERVATIONS.

of Denmark, Kepler of Germany, and Galileo of Italy, laid the solid foundations of modern astronomy. Copernicus expounded the true system of the world, or the arrangement and motions of the heavenly bodies ; Tycho Brahe carried the use of instruments, and the art of astronomical observation, to a far higher degree of accuracy than had ever been done before ; Kepler discovered the great laws which regulate the movements of the planets ; and Galileo, having first enjoyed the aid of the telescope, made innumerable discoveries in the solar system. Near the beginning of the eighteenth century, Sir Isaac Newton discovered, in the law of universal gravitation, the great principle mat explains the causes of all celestial phenomena ; and recently, La Place has more fully completed what Newton begun, having followed out all the consequences of the law of universal gravitation, in his great work, the Mecanique Celeste.

3. Among the ancients, astronomy was studied chiefly as subsidiary to astrology. Astrology was the art of di* vining future events by the stars. It was of two kinds, natural and judicial. Natural Astrology, aimed at predicting remarkable occurrences in the natural world, as eathquakes, volcanoes, tempests, and pestilential diseases. Judicial Astrology, aimed at foretelling the fates of individuals, or of empires.

4. Astronomers of every age, have been distinguished for their persevering industry, and their great love of accuracy. They have uniformly aspired to an exactness in their inquiries, far beyond what is aimed at in most geographical investigations, satisfied with nothing short of numerical accuracy wherever this is attainable ; and years of toilsome observation, or laborious calculation, have been spent with the hope of attaining a few se-

3. Define Astrology. What was Natural and what Judicial Astrology 1

4. What is said of the industrv and accuracy of astronomers \ Can this science be taught by artificial aids alone ?

4 PRELIMINARY OBSERVATIONS.

conds nearer to the truth. Moreover, a severe but de lightful labor is imposed on all, who would arrive at a clear and satisfactory knowledge of the subject of astronomy. Diagrams, artificial globes, orreries, and familiar comparisons and illustrations, proposed by the author or the instructor, may aiford essential aid to the learner, but nothing can convey to him a perfect comprehension of the celestial motions, without much diligent study and reflection.

5. In this treatise, we shall for the present assume the Copernican system as the true system of the world, postponing the discussion of the evidence on which it rests to a late period, when the learner has been made extensively acquainted with astronomical facts. This system maintains (1,) That the apparent diurnal revolution of the heavenly bodies, from east to west, is owing to the real revolution of the earth on its own axis from west to east, in the same time ; and (2,) That the sun is the center around which the earth and planets all revolve from west to east, contrary to the opinion that the earth is the center of motion of the sun and planets.

5. What system is assumed as the true system of the world * Specify the two leading points in the Copernican system.

PART I.— OF THE EARTH.

Chapter I

OF THE FIGURE AND DIMENSIONS OF THE EARTH, AND THE DOCTRINE OF THE SPHERE.

6. The figure of the earth is nearly globular. This fact is known, first, by the circular form of its shadow cast upon the moon in a lunar eclipse ; secondly, from analogy, each of the other planets being seen to be spherical ; thirdly, by our seeing the tops of distant objects while the other parts are invisible, as the topmast of a ship, while either leaving or approaching the shore, or the lantern of a light-house, which when first descried at a distance at sea, appears to glimmer upon the very surface of the water ; fourthly, by the testimony of navigators who have sailed around it ; and, finally, by actual observations and measurements, made for the express purpose of ascertaining the figure of the earth, by means of which astronomers are enabled to compute the distances from the center of the earth of various places on its surface, which distances are found to be nearly equal. •

The effect of the rotundity of the earth upon the appearance of a ship, when either leaving or approaching the spectator, is illustrated by Fig. 1.

As light proceeds in straight lines, it is evident that, if the earth is round, the top of the ship ought to come into view before the lower parts, when the ship is approaching the spectator at A, and to remain longest in view when the ship is leaving him. But, were the earth

6. What is the figure of the earth 1 Enumerate the various proofs of its rotundity.

THE EARTH.

Fig. 1.

a continued plane, then the spectator would see all parts of the ship at the same time, as is represented in the annexed figure.

Fig. 2.

7. The foregoing considerations show that the form of the earth is spherical ; but more exact determinations prove, that the earth, though nearly globular, is not exactly so ; its diameter from the north to the south pole is about 26 miles less than through the equator, giving to the earth the form of an oblate spheroid, or a flattened sphere resembling an orange. We shall reserve the ex-

FIGURE AND DIMENSIONS. 7

planations of the methods by which this fact is established, until the learner is better prepared than at present to understand them.

The mean or average diameter of the earth, is 7912.4 miles, a measure which the learner should fix in his memory as a standard of comparison in astronomy, and of which he shoulc endeavor to form the most adequate conception in his power. The circumference of the earth is about 25,000 miles. Although the surface of the earth is uneven, sometimes rising in high mountains, and sometimes descending in deep valleys, yet these elevations and depressions are so small in comparison with the immense volume of the globe, as hardly to occasion any sensible deviation from a surface uniformly curvilinear. The irregularities of the earth's surface, in this view, are no greater than the rough points on the rind of an orange, which do not perceptibly interrupt its continuity ; for the highest mountain on the globe is only about five miles above the general level ; and the deepest mine hitherto opened is only about half a mile.*

'51

Now = , or about one sixteen hundredth part of the whole diameter, an inequality which, in an artificial globe of eighteen inches diameter, amounts to only the eighty eighth part of an inch.

8. The greatest difficulty in the way of acquiring correct views in astronomy, arises from the erroneous notions that pre-occupy the mind. To divest himself

7. What is the exact figure of the earth 1 How much greater is its diameter through the equator than through the poles 1 What is the mean average diameter of the earth ? What is its circumference ? Do the inequalities on the earth's surface affect its rotundity ? To what may these be compared ? How high is the highest mountain above the general level ? How deep is the deepest mine? To how much would this amount on an artificial globe eighteen inches in diameter ?

* Sir John Herschel.

of these, the learner should conceive of the earth as a huge globe occupying a small portion of space, and encircled on all sides with the starry sphere. He should free his mind from its habitual proneness to consider one part of space as naturally up and another down, and view himself as subject to a force which binds him to the earth as truly as though he were fastened to it by some invisible cords or wires, as the needle attaches itself to all sides of a spherical loadstone. He should

Fig. 3.

dwell on this point until it appears to him as truly up in the direction of BB, CC, DD, (Fig. 3,) when he is at B, C, and D, respectively, as in the direction AA, when he is at A.

DOCTRINE OF THE SPHERE.

9. The definitions of the different lines, points, and circles, which are used in astronomy, and the propositions founded upon them, compose the Doctrine of the Sphere,

8. Whence arises the greatest difficulty in acquiring correct views in astronomy 1 How should the learner conceive of the earth ? Illustrate by figure 3.

9. Doctrine of the sphere — define it.

DOCTRINE OF TtfE SPHERE.

10. A section of a sphere by a plane cutting it in any manner, is a circle. Great circles are those which pass through the center of the sphere, and divide it into two equal hemispheres : Small circles, are such as do not pass through the center, but divide the sphere into two unequal parts. Every circle, whether great or small, is divided into 360 equal parts called degrees. A degree, therefore, is not any fixed or definite quantity, but only a certain aliquot part of any circle.*

The axis of a circle, is a straight line passing through its center at right angles to its plane.

* As this work may be read by some who are unacquainted with even the rudiments of geometry, we annex a few particulars respecting angular measurements. . .

A line drawn from the center to the circumference of a circle is

called a radius, as CD, fig. 4. „.,.-,■, na atj

Any part of the circumference of a circle is called an arc, as AC,

orBD.

Fig. 4.

An angle is measured by the arc included between two radii.

Thus, in the annexed figure, the angle contained between the two radii CA and CB, that is, the angle ACB, is measured by the arc AB. But this arc is the same part of the smaller circle that EF is of the greater. The arc AB therefore contains the same number of degrees as the arc EF, and either may be taken for the measure of the angle ACB. As the whole circle contains 360°, it is evident that the quarter of a circle, or quadrant ABD, contains 90°, and the semicircle ABDG contains 180°.

The complement of an arc or an- _

gle is what it wants of 90°. Thus BD is the complement of AB, and ABis the complement of BD. If AB denotes a certain number of degrees of latitude, BD will be the complement of the latitude or the colatitude , as it is commonly written.

The supplement of an arc or angle, is what it wants ot 1H0°. Thus BA is the supplement of GDB, and GDB, is the supplement of BA. If BA were 20° of longitude, GDB its supplement would

An angle is said to be subtended by the side which is opposite to it. Thus in the triangle ACK, the angle at C is subtended by the side AK, the angle at A by CK, and the angle at K by CA. In like manner a side is said to be subtended by an angle, as AK by the angle at C.

The pole of a great circle, is the point on the sphere where its axis cuts through the sphere. Every great circle has two poles, each of which is every where 90° from the great circle.

All great circles of the sphere cut each other in two points diametrically opposite, and consequently, their points of section are 180° apart.

A great circle which passes through the pole of another great circle, cuts the latter at right angles.

The great circle which passes through the pole of another great circle and is at right angles to it, is called a secondary to that circle.

The angle made by two great circles on the surface of the sphere, is measured by the arc of another great circle, of which the angular point is the pole, being the arc of that great circle intercepted between those two circles.

1 1. In order to fix the position of any plane, either on the surface of the earth or in the heavens, both the earth and the heavens are conceived to be divided into separate portions by circles, which are imagined to cut through them in various ways. The earth thus intersected is called the terrestrial, and the heavens the celestial sphere. The learner will remark, that these circles have no existence in nature, but are mere landmarks, artificially contrived for convenience of refer-

10. What figure is produced by the section of a sphere? Define great circles. Define small circles. Into how many degrees is every circle divided ? Is a degree any fixed or definite quantity ? What is the axis of a circle ? What is the pole of a circle ? How do all great circles cut each other? How is a great circle cut by another great circle passing through its pole 1 What is the secondary of a circle ? How is the angle made by two great circles on the surface of the sphere measured?

11. How are the earth and the heavens conceived to be divided ? What constitutes the terrestrial sphere ? What the celestial ? Have these circles any existence in nature ? In what do the heavenly bodies appear to be fixed ?

ence On account of the immense distance of the heavenly bodies, they appear to us, wherever we are placed, to be fixed in the same concave surface, or celestial vault The great circles of the globe, extended every way to meet the concave surface of the heavens, become circles of the celestial sphere.

12 The Horizon is the great circle which divides the earth into upper and lower hemispheres and separates the visible heavens from the invisible. 1 his is the rational horizon. The sensible horizon, is a circle touching the earth at the place of the spectator, and is bounded by the line in which the earth and skies seem to meet The sensible horizon is parallel to the rational, but is distant from it by the semi-diameter of the earth or nearly 4,000 miles. Still, so vast is the distance of the starry sphere, that both these planes appear to cut that sphere in the same line ; so that we see the same hemisphere of stars that we should see if the upper half of the earth were removed, and we stood on the rational horizon.

13. The poles of the horizon are the zenith and nadir. ' The Zenith is the point directly over our head, and the Nadir that directly under our feet. The plumb line is in the axis of the horizon, and consequently directed towards its poles.

Every place on the surface of the earth has its own horizon ; and the traveller has a new horizon at every step, always extending 90 degrees from him in all directions.

12 Define the horizon. Distinguish between the rational and the sensible horizon. What is the distance between the sensible and rational horizons ? How do both appear to cut

the starry heavens 1

13 What are the poles of the horizon 1 Define the zenith. Define the nadir. How is the plumb line situated with respect to the horizon? How manv horizons are there on the earth ?

14. Vertical circles are those which pass through the ooles of the horizon, perpendicular to it.

The Meridian is that vertical circle which passes" through the north and south points.

The Prime Vertical, is that vertical circle wluch passes through the east and west points.

The Altitude of a body, is its elevation above the horizon, measured on a vertical circle.

The Azimuth of a body, is its distance measured on the horizon from the meridian to a vertical circle passing through the body.

The Amplitude of a body, is its distance on the horizon, from the prime vertical, to a vertical circle passing through the body.

Azimuth is reckoned 90° from either the north or south point ; and amplitude 90°; from either the east or west point. Azimuth and amplitude are mutually complements of each other. When a point is on the horizon, it is only necessary to count the number of degrees of the horizon between that point and the meridian, in order to find its azimuth ; but if the point is above the horizon, then its azimuth is estimated by passing a vertical circle through it, and reckoning the azimuth from the point where this circle cuts the horizon.

The Zenith Distance of a body is measured on a vertical circle, passing through that body. It is the complement of the altitude.

1 5. The Axis of the Earth is the diameter, on which the earth is conceived to turn in its diurnal revolution. The same line continued until it meets the starry concave, constitutes the axis of the celestial sphere.

14. Define vertical circles — the meridian — the prime vertical — altitude — azimuth — amplitude. How many degrees of azimuth are reckoned ? from what points ? How are azimuth and amplitude related to each other ? Define zenith distance — How is it related to the altitude ?

15. Define the axis of the earth — the axis of the celestial sphere — the poles of the earth — the poles of the heavens.

The Poles of the Earth are the extremities of the earth's axis : the Poles of the Heavens, the extremities of the celestial axis.

16. The Equator is a great circle cutting the axis of the earth at right angles. Hence the axis of the earth is the axis of the equator, and its poles are the poles of the equator. The intersection of the plane of the equator with the surface of the earth, constitutes the terrestrial, and with the concave sphere of the heavens, the celestial equator. The latter, by way of distinction, is sometimes denominated the equinoctial.

17. The secondaries to the equator, that is, the great circles passing through the poles of the equator, are called Meridians, because that secondary which passes through the zenith of any place is the meridian of that place, and is at right angles both to the equator and the horizon, passing as it does through the poles of both. These secondaries are also called Hour Circles, because the arcs of the equator intercepted between them are used as measures of time.

18. The Latitude of a place on the earth, is its distance from the equator north or south. The Polar Distance, or angular distance from the nearest pole, is the complement of the latitude.

19. The Longitude of a place is its distance from some standard meridian, either east or west, measured on the equator. The meridian usually taken as the standard, is that of the Observatory of Greenwich, in London. If a place is directly on the equator, we have only to inquire how many degrees of the equator there

16. Define the equator. What constitutes the terrestrial equator? what the celestial equator ? What is this also called?

17. What are the secondaries of the equator called 7

18. Define the Latitude of a place— the polar distance.

are between that place and the point where the meridian of Greenwich cuts the equator. If the place is north or south of the equator, then its longitude is the arc of the equator intercepted between the meridian which passes through the place, and the meridian of Greenwich.

20. The Ecliptic is a great circle in which the earth performs its annual revolution around the sun. It passes through the center of the earth and the center of the sun. It is found by observation that the earth does not lie with its axis at right angles to the plane of the ecliptic, but that it is turned about 23J degrees out of a perpendicular direction, making an angle with the plane itself of 66i°. The equator, therefore, must be turned the same distance out of a coincidence with the ecliptic, the two circles making an angle with each other of 23J°. It is particularly important for the learner to form correct ideas of the ecliptic, and of its relations to the equator, since to these two circles a great number of astronomical measurements and phenomena are referred.

21. The Equinoctial Points, or Equinoxes* are the intersections of the ecliptic and equator. The time when the sun crosses the equator in going northward is called the vernal, and in returning southward, the autumnal equinox. The vernal equinox occurs about the 21st of March, and the autumnal the 22d of September.

19. Define the Longitude of a place. What is the standard meridian ? When a place is on the equator, how is its longitude measured ? how when it is north or south of the equator ?

20. Define the ecliptic. How does it pass with respect to the earth and the sun 1 How is it situated with respect to the

equator ?

21. Define the equinoctial points. When is the vernal equinox, and when the autumnal ?

*

* The term Equinoxes strictly denotes the times when the sun arrives at the equinoctial points, but it is frequently used to denote those Giants themselves.

22. The Solstitial Points are the two points of the ecliptic most distant from the equator. The times when the sun' comes to them are called solstices. The summer solstice occurs about the 22d of June, and the winter solstice about the 22d of December.

The ecliptic is divided into twelve equal parts of 30° each, called signs, which, beginning at the vernal equinox, succeed each other in the following order :

Northern.

Southern.

Aries op

7. Libra

-Ti.

Taurus

8. Scorpio m

Gemini n

9. Sagittarius t

Cancer

£d

10. Capricornus vs

Leo

SI

11. Aquarius