Illustrated common school astronomy

Front matter

Class

D Q Q

Gojyright^N?-

Index

INTRODUCTORY CHAPTER.

Astronomy defined, .page 9

Solar System, 10

Mode of conducting Astronomical Investigations, 11

EXPLANATORY CHAPTER.

Angle, 14

Right Angle, 15

Triangles, 16

Similar Triangles, 16

A Plane Surface, 17

A Plane Figure, 17

Ellipse, 17

To construct an Ellipse, 18

Eccentricity, 19

A Sphere, 19

Poles of a Circle of a Sphere, 20

PAET FIEST.

THE EARTH IN ITS RELATION TO OTHER HEAVENLY BODIES.

Chapter I. Its Form, Size, And Rotation

The Form of the Earth, „ 22

Size, 24

Rotation, 25

Chapter II. The Horizon

Sensible Horizon, 26

Rational Horizon, 27

Plane of the Horizon not fixed in Space, » 27 iv INDEX

Zenith and Nadir, page 28

Changing Aspect of the Heavens, arising from the Kotation of the

Earth, 28

Why the Stars appear to describe Circles, 30

Pole of the Heavens, and Pole Star, 31

Changing Aspect of the Heavens, arising from Change in Latitude,. . 31

Circle of Perpetual Apparition, 33

Circle of Perpetual Occultation, 34

Aspect of the Heavens at the Equator, 37

At North Latitude Forty Degrees, 37

At the North Pole, * 38

Latitude of any Place equal to the Elevation of the Pole of the Heavens, 88 .

CHAPTEE III.

OF THE MODE OF DETERMINING THE PLACE OF A HEAVENLY BODY.

Celestial Sphere, Poles, Axes, and Meridians, 40

Equators, 42

Vertical Circles, , 42

The Position of a Star — how determined, 42

Azimuth and Altitude of a Star, 43

Declination and Eight Ascension, 44

Ecliptic, 45

The Signs, . 46

Zodiac, 46

CHAPTEE IV.

OF THE MEASUREMENT OF TIME.

Of the Time occupied by the Earth in performing one Rotation — how

determined, 47

Standard Unit of Time, 49

Of the Sidereal and Sola?' Day, 50

Apparent Time, 52

Mean Solar Time, . 52

Astronomical Time, 52

The Tropical Year, 53

Egyptians, 54

Mexicans, 54

CHAPTER V.

OF THE EARTH'S ORBIT, AND THE SEASONS.

The Earth's Orbit, 56

The Seasons, 57

Spring, 59

INDEX

Summer, page 59

Autumn, 62

Winter, 62

Polar Winters, 64=

Part Second. Solar System

CHAPTER I.

THE SUN.

Diameter, 66

Size or Bulk, 67

Quantity of Matter, 68

Weight of Bodies at the Surface of the Snn, 68

Solar Spots, 70

Size and Number, 71

Motion of the Spots, 72

Rotation of the Sun on its Axis, 75

Physical Nature of the Sun, 75

Temperature at the Sun's Surface, 77

CHAPTER II.

THE MOON.

Distance — Diameter, 78

Moon's Phases, 79

From New Moon to the First Quarter, 79

From the First Quarter to Full Moon, 80

From Fall Moon to the Third Quarter, 81

From the Third Quarter to New Moon, 81

What the Phases prove, 84

Sidereal Month, 84

Synodical or Lunar Month, 85

Physical Aspects of the Moon, 85

Lunar Mountains, 87

Lunar Craters, 88 .

Lunar Volcanoes,. 91

Bulk— Mass— Density, 92

The Moon's Orbit, 92

The Line of the Nodes, 92

The Moon always turns the same Face towards the Earth, 93

Length of the Lunar Day, 95

The Appearance of the Earth, as seen from the Moon, 95 vi INDEX.

Chapter III. Eclipses Of The Sun And Moon

Lunar Eclipses, page 97

Of the Earth's Shadow, 98

Red Light of the Disk, 100

Earliest Observations of Lunar Eclipses, 101

Eclipses of the Sun, 101

Form of the Eclipse, 102

Shadow of the Moon, 103

Total Eclipse of the Sun, , 104

Solar and Lunar Eclipses — Points of Difference, 108

Frequency of Eclipses, 108

Chapter V. The Planets

Universal Attraction, 110

Kepler's Laws, Ill

The Planets and their Solar Distances, 112

Apparent Size, 118

Kepler's Law of Distances, 113

Division of the Planets, • 114

Inferior Planets, 114

Mercury, 116

Rotation on its Axis, 117

Phases, 118

Transit of Mercury, 118

Mass and Density, 119

Venus, 119

Rotation — Phases, 120

Splendor of Venus, 122

Transit of Venus, 122

Mass and Density, 122

The Earth, 123

Superior Planets, 1 23

Mars, 123

Splendor — Physical Aspect, 124

The Asteroids, 126

Table of Asteroids, 128

Jupiter, 127

Physical Aspect of Jupiter — Belts, 129

Satellites of Jupiter—Their Discovery, 130

Their Magnitudes — Diameters — Distances — Periods of Revolution, 131

INDEX. vii

Kepler's Laws — Eotation, page 131

Transits and Eclipses of the Satellites, 132

Saturn, 132

Physical Aspect, 1 33

Eing of Saturn — its Discovery, 133

Form and Constitution, 133

Eotation and Position, 134

Divisions of the Eing, 134

Dimensions of the Eings,. 136

Satellites of Saturn, 137

Hyperion, 137

Uranus or Herschel, 137

Aspect — Diameter — Mass,. 138

Rotation, 138

Distance — Periodic Time, 138

Satellites of Uranus, 138

Neptune — History of its Discovery, 139

Name — Diameter— Mass — Density, 140

Orbit — Distance — Periodic Time, 140

Has Neptune a Eing ? , 141

The Satellite of Neptune, 141

Chapter VI Comets

Constitution, 141

Number of Comets, 142

Splendor and Size, 142

Orbits, 144

The Comets of Haliey, Encke, Biela, and Faye, 145

Nature of Comets, 145

CHAPTER VIL

TIDES.

Tides denned, 146

Cause of the Tides, 148

Solar Influence, 148

Spring and Neap Tides, 149

Actual Heights of the Tide, 151

No Tides except on the Ocean, and on Seas connected with it, 152

Tiii INDEX.

Chapter I. On The Fixed Stars In General, And The Constellations

The Fixed Stars, page 153

Magnitudes, 153

Number of Stars, 154

The Constellations — Their Use, , 155

Principal Constellations, , 156

Distance of the Fixed Stars, 157

Nature and intrinsic Splendor of the Fixed Stars, 158

Chapter II. Different Kinds Of Stars — Stellar Motions — Binary Systems

Periodical Stars, 158

Mira, 158

Algol, 159

Temporary Stars, 159

Double Stars, 161

Castor — Alpha Centauri, 161

Colored Double Stars, 161

Triple and Quadruple or Multiple Stars, 162

Binary Stars, 162

Orbits — Periodic Times, 163

Chapter III

STARRY CLUSTERS — NEBULAE — NEBULOUS STARS — ZODIACAL LIGHT — MAGELLAN CLOUDS — STRUCTURE OF THE HEAVENS.

Starry Clusters, 164

Milky Way or Galaxy, 164

Nebulae— Their Constitution, 166, 168

Number and Distance of Stellar Clusters and Nebulie, 168

Their Physical Structure, 169

Nebulous Stars, 160

Zodiacal Light, 169

Aspects — Size— Nature, 170

Structure of the Heavens, ^ 171

Ptolemaic, Tychonic, and Copernican Systems, 171

ASTRONOMY.

INTRODUCTORY CHAPTER.

1. Astronomy is that branch of Natural Science which treats of the magnitudes, distances, constitutions, and motions of the Heavenly Bodies, and the laws which regulate them.

2. The Heavenly Bodies consist of moons, planets, 1 comets, 2 and suns ; and possibly a fifth class exist called nebulae?- To moons, planets, and suns, the general name of stars is often given.

3. The mode of union existing among the heavenly bodies is the following : One or more moons revolve aoout a planet ; several planets with their attendant moons revolve about a sun, around which, likewise, sweeps a numerous train of comets. A sun with its assemblage of planets and comets constitutes a system.

1. Planet, from the Greek word planetes, signifying a wanderer: a star that changes its place in the heavens.

2. Comet, from the Latin word coma, a head of hair, this body presenting a hairy appearance.

3. The name of nebulce is given by astronomers to certain objects in the distant heavens which appear like small clouds, or specks of mist. True nebulae are supposed to be vast collections of unformed matter, thinly diffused through space. Nebulae is a Latin word, signifying mists, or clouds.

What is Astronomy ? What do the heavenly bodies consist of % What is the mode of union between heavenly bodies ?

1*

ASTRONOMY.

4. The investigations of astronoiners tend to prove that these systems are not fixed in space, but revolve like planets about some common central point, or body. And we have reason for believing that this mode of arrangement extends throughout all space, groups of systems rising one above the other in magnitude ; the lesser circling around the greater, until at length their vast aggregate embraces and completes the Universe.

5. Solab System. The sun with his train of planets, moons, and comets, forms the Solar System.

6. The number of planets already known is fifty* Thirty-nine of these have been discovered within the last twelve years, and others will doubtless be detected. The names of the planets are given in the following table, in the order of their distances from the sun, beginning with the nearest.

TABLE OF THE PLANETS.

MERCURY, (nearest), EARTH, VENUS, MARS.

The Asteroids, between Maes and Jupiter.

FLORA,

PHOCEA,

POMONA,

CALLIOPE,

HARMONIA,

MASS ALIA,

PROSERPINE,

PYSCHE,

ISIS,

HEBE,

FIDES,

THEMIS,

MELPOMENE,

LUTETIA,

EUNOMIA,

HYGEIA,

CLIO,

FORTUNA.

THALIA,

EUPHROSYNE,

EUTERPE,

PARTHENOPE,

JUNO,

URANIA,

THETIS,

CIRCE,

VESTA,

AMPHITRITE,

CERES,

LEUCOTHEA,

POLYMNIA,

EGERIA,

LjETITIA,

ATALANTA,

METIS,

ASTREA,

PALLAS,

LEDA,

IRIS,

IRENE,

BELLONA,

DAPHNE.

JUPITER, SATURN

, HERSCHEL, or

URANUS, NEPTUNE (most distant).

What is a system ? What is the solar system 1 How many planets are now known? How many have been discovered within the last twelve years ? Give the names of the planets.

ASTRONOMY. H

7. All the planets between Mars and Jupiter are termed Asteroids. 1 In the annexed cut, Fig. 1, a view of the solar system is presented. The Roman numeral, I, represents the orbit 2 of Mercury ; II, that of Venus ; III, that of the Earth ; IY, that of Mars ; V, the orbit of the nearest asteroid ; VI, that of the most remote asteroid ; VII, is the orbit of Jupiter ; VIII, of Saturn ; IX, of Herschel ; and X, of Neptune. In the cut the distances of the several planets from the sun bear the same relation to each other as their actual distances.

8. The Mode of Conducting Astronomical Investigations. "When an artisan wishes to ascertain the dimensions of a stick of timber, he does so by means of a rule, the length of which he knows, and thus he obtains the solidity of the log in feet and inches.

When, likewise, we wish to determine the speed of a locomotive, we measure by the aid of a w-atch the time taken to pass over a known number of miles. Thus unknown magnitudes and motions are ascertained by comparing them with such as are known.

9. In astronomical investigations we pursue a like course, and begin w T ith determining the size, motions, and form of the Eajrth, with other important particulars that are within our reach. We thus obtain fixed standards of measurement, whereby we are ena-

1. Asteroids. From two Greek words : aster, a star, and eidos, like. Like a star, because all these planets are very small.

2. Orbit means the path of a planet about the sun. So called from the Latin word, orbis, a circle, a circuit.

What are the Asteroids, and where situated? Explain the figure. In what manner are astronomical investigations conducted?

bled to push our inquiries beyond the earth, and to compute the distances, times, motions, and velocities of many of the bright orbs 3 that glitter about us, and the extent of the vast spaces through which they move. In the study of Astronomy our attention is, therefore, directed First to the Earth in its relation to the rest of the heavenly bodies. Secondly ', to the Solar System. Thirdly, to the Starry Heavens, of which this system is a part. .

3. The stars are frequently called orbs, from their round figures. (Latin,) a circle.

FIG. 1.

SOLAR SYSTEM.

EXPLANATOKY CHAPTER.

10. In learning Astronomy it is necessary for the pupil at the outset to know the meaning of certain mathematical and philosophical terms and expressions which are constantly occurring - in the discussion of astronomical subjects. These must "be mastered in order to obtain a clear understanding of the science, and yet they are by no means difficult to comprehend. The most important of these are explained in the present chapter. The meaning of other terms and phrases will be given as they occur in the book.

11. Angle. An angle is the opening or inclination between two lines that meet each other ; thus, in Fig.

FIG. 2.

AN ANGLE.

2, the line AB meets the line BC, and the opening between them is called the angle B, or the angle ABC ; the letter at the point of meeting always being placed in the middle.

The size of an angle is computed as follows. The

In learning astronomy, what is it necessary for the pupil to do at the outset 1 What ii an angle ? How is its size computed? Describe from the figure.

circumference of any circle being divided into 360 equal parts, each part is called a degree / a degree being divided into 60 equal parts, each part is called a minute * a minute being divided into 60 equal parts, each part is called a second. If, now, we take the point B, as the centre of a circle, and describe the circumference DEF, cutting the two lines, AB and CB in any two points, as E and F, the number of degrees, minutes, and seconds contained in the part of the circumference, EF, included between the two lines, AB and CB, gives the value of the angle, ABC. For example, if the length of the circumference, DEF, was 360 inches, and the part, EF, contained 40 inches and nine-sixtieths (40^-) of an inch, ABC would be an angle of forty degrees and nine minutes (40° 9'.) Degrees are denoted by the following character, ° ; minutes thus, ' ; seconds thus, ".

12. A Right Angle. A right angle contains 90°, and can be thus constructed. Draw two diameters through any circle, dividing the circumference into

FIG. 3.

RIGHT ANGLE.

How are degrees, minutes, and seconds denoted? What is a right angle, and how it it constructed ?

four equal parts, and each of the angles at the centre will be a right angle, for since the whole circumference contains three hundred and sixty degrees, onefourth of it contains ninety degrees. Thus, in Fig. 3, the two diameters, AB and DE, dividing the circumference of the circle, A, D, B, E, into four equal parts, make each of the angles at the centre, right angles, viz., ACD, DCB, BCE, andECA.

13. Triangles. A triangle is a figure that is bounded by three lines, either curved or straight, and contains three angles : hence its name, derived in part from a Latin word, tres, meaning three. The sum of the

FIG. 4. FIG. 5.

A RECTILINEAR TRIANGLE. A RECTILINEAR RIGHT-ANGLED TRIANGLE.

three angles of any rectilinear 1 triangle is always equal to 180°. Fig. 4 represents any such triangle, and the sum of its angles is equal to 180°. A right-angled rectilinear triangle is one that contains one right angle. Thus, Fig. 5 is a right-angled triangle, since it contains a right angle, viz., ABO.

14. Similar Triangles. Similar triangles are those which have all the angles of one triangle equal to

1. Rectilinear, from rectus, straight, and linea, a line, (Latin,)

STRAIGHT-LINED.

What is a triangle ? How many degrees does it contain ? Refer to figure. What is a right-angled triangle? Refer to figure. What are similar triangles? Explain from figure.

those of the other, each to each, and the sides forming the equal angles proportional ; thus, in Figs. 6 and 7, the triangles, A^C 1 and ABC, are similar, because

FIG. 6. FIG. 7.

SIMILAR TRIANGLES.

the angle B 1 equals B, A 1 equals A, C 1 equals C, and the side A'B 1 : AB : : A l C l : AC\ and so of the other sides.

15. A Plane Surface. A plane surface is such that if any two points in the surface are connected by a straight line, every part of that straight line touches the surface. To illustrate : The surface of tranquil water in a pond is a plane surface, because if any two points on the surface are taken, and are connected by a perfectly straight rod, every part of the lower side of the rod touches the water. Such a surface is sometimes termed a plane. Thus, the surface of this page, when pressed flat, is a plane surface, or plane.

16. A Plane Figure. A plane figure is one whose bounding line or lines are situated in the same plane. The flat cover of this book is a plane figure.

17. Ellipse. An ellipse is a plane figure, bounded by a curved line, and so constructed that if two straight lines are drawn from two points within, called the foci, to any point in the curve, the sum of

What is a plane surface ? What a plane figure ? What is an ellipse ? Describe it from figure.

these lines is invariably the same for the same ellipse. Thus, the annexed figure is an ellipse. F and F 1 , the foci, and if straight lines are drawn to any points, as

FIG. 8.

AN ELLIPSE.

M and M 1 ; FM added to F X M equals FM 1 added to F l M*, and so of lines drawn to any other point. The line DD 1 , drawn through the foci and terminated by the curve, is called the major axis of the ellipse. The line PP 1 , drawn through C, the middle of DD 1 , or the centre of the figure, is the conjugate axis.

18. To Construct an Ellipse. Stick two pins into a piece of paper, at a short distance from each other, as at F and F 3 , and pass over them a loop of thread, place a pencil in the loop, and keeping the thread tight, a triangle will be formed like FMF 1 , the pencil being at M. Passing the pencil completely round F and F 1 , its point will mark out an ellipse. For since in making the circuit, the length of the loop does not change, neither the distance between F and F 1 , it necessarily follows that the sum of the distances from the pins to the pencil: viz., F 1 ]^, FM, &c, is invariable.

Construct an ellipse.

19. Eccentricity. Ellipses differ among themselves. If the foci are near the centre of the ellipse, the ellipse approaches the form of a circle ; but if the foci depart widely from it the length of the conjugate axis is small in proportion to that of the major axis, and the ellipse is said to be very eccentric. 1

The distance from the centre to either focus, viz., FC, or F X C, is termed the eccentricity of the ellipse.

In Figs. 9 and 10, two ellipses are exhibited which differ greatly in their eccentricity — one being almost a circle, and the other very oval.

20. A Sphere. A sphere is a solid, bounded by a curved surface, every point of which is equally distant from a point within^ called the centre / every line passing through this centre, and limited by the surface, is a diameter. The half of this line is a radius of the sphere. Thus, in Fig. 11, representing a

1. Eccentric, from ex, out of and centrum, centre, (Latin) out of the centre.

What is meant by the term eccentricity 1

What is a sphere ? Describe it from the figure with its lines and sections.

FIG. 11

A SPHERE WITH iTS SECTIONS.

sphere, the points D, O, L, A, E, H, N, &c., are all equally distant from the centre C. DE and AB are diameters, and CP, CL, CA, CB, &c, are each a radius.

If a plane passes through a sphere, any section it makes with the sphere is a circle. A great circle passes through the centre of the sphere, all other circles are small circles. Thus, in the figure, AFB is a great circle, and LHN and OP small circles.

21. Poles of a Circle of a Sphere. The poles of a circle of a sphere are points on the surface of a sphere, equally distant from every point in the circumference of that circle. Thus, D is the pole of the circle LHN, because the curved lines DH, DN, and

What are the poles of a circle of a sphere ? Explain from figure.

DOL, and all others drawn to the circumference LHN, are equal to one another. For the same reason D is the pole of the circles OP and AB. It will also be seen that the point E is situated like D, with respect to these circles, since the curved lines EBN and EAL are equal, as likewise ELO and EOT\ Each circle of a sphere has therefore two poles.

In a great circle the poles are each ninety degrees distant from the circumference of the circle ; thus, in the great circle AB, the poles E and D are each ninety degrees from its circumference AFB.