Popular astronomy. A concise elementary treatise on the sun, planets, satellites and comets

Front matter

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Chapter I. The Sun, The Central Orb Op The Planetary System

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Discoveries of the Ancients. — The Source of Light and Heat and Life. — The Sun's apparent Motion. — Length of the Year. — The Sun's apparent Path among the Fixed Stars, a Circle. — His apparent Motion not Uniform. — The Explanation of Hipparchus. — Solar Eclipses. — Their Prediction.

Discoveries of tiie Moderns. — The Sun's Distance. — His Horizontal Parallax Importance of this Element. — Measured by the Transit of Venus. —

The Sun's real Magnitude, and Micrometrical Measure of his Diameter. — The Physical Constitution of the Sun. — Solar Spots. — Their Periodicity. — Hypotheses and Speculations 13

Chapter II. Mercury, The Ftrst Planet In The Order Of Distance From The Sun

Its Early Discovery. — Difficult to be distinguished from the Stars. — Elongations.— Motion Direct and Retrograde. — Sometimes Stationary. — Nature of the Orbit. — Variation in the Elongation Explained. — The Nodes. — Transit of Mercury. — Inclination of Mercury's Orbit.— Mean Distance from the Sun. — Conjunctions. — Phases.— Diameter and Volume 45

Chapter III. Venus, Tiie Second Planet In The Order Of Distance From The Sun

The First Planet Discovered. — Mode of its Discovery.— Her Elongations.- • Morning and Evening Star. — A Sattellite of the Sun. — Her superior and Inferior Conjunctions. — Her Stations. — Direct and Retrograde Motions. — These Phenomena indicate a Motion of the Earth. — Transits of Venus. — Inclination of the Orbit of Venus to the Ecliptic. — Her Nodes.— Intervals of her Transits. — Knowledge of the Ancients. — Phases oi Venus. — Her Elongations Unequal. — No Sattellite yet Discovered.— Sun's Light and Heat at Venus.— Her Atmosphere ; 62

Chapter IV

THE EARTH AND ITS SATTELLITE; TIIE THIRD PLANET IN THE ORDER OF DISTANCE FROM TIIE SUN.

The Earth the apparent Center of Motion.— To all the Senses it is at Rent— The Center of the Motions of the Sun and Moon.— Explanation o( the Acceleration of the Orbitual Motion of the Bun and Moon. — Ptolemy's Epicycles.— The Explanation of Copernicus. — The Sun the Center of Planetary Motion.— The Earth One of the Planets.— Objections to this Hypothesis.— The Answer. — System of Jupiter discovered by the Teleseope. — The Old System superseded by the New. — The Figure and Magnitude of the Earth — How determined.— The Earth's Motions. — Rotation and Revolution.- A Unit Of Time furnished by the Earth's Period of Rotation.— Earth's Orbitual Motion.— Vernal Equinox.— Perihelion o( Earth's Orbit.— its Period of Revolution.— Solar and Sidereal Time.

Tub Moon.— Revolution in her Orbit.— Her Phases.— Eccentricity of her Orbit—Revolution of her Apogee.— Inclination of her Orbit— Moon's Parallax and Distance.— llor Physical Constitution.— Center o( Gravity and Center of Figure ." $i

VI CONTENTS.

CHAPTER Y.

MARS, THE FOURTH PLANET IN THE ORDER OF DISTANCE FROM THE SUN.

PAGB

Phenomena of Mars difficult to explain with the Earth as the Center of Motion.— Copernican system applied. — Epicycle of Mars. — Better Instruments and more accurate Observations. — Tycho and Kepler. — Kepler's method of Investigation. — Circles and Epicycles exhausted. — The Ellipse.— Its Properties.— The Orbit of Mars an Ellipse. — Kepler's Laws. — Elliptical Orbits of the Planets. — The Elements of the Planetary Orbits explained — How these Elements are obtained. — Kepler's third Law. — Value of this Law. — The Physical Aspect of Mars. — Snow Zones. — Rotation of the Planet. — Diameter and Volume. — Speculation as to its Climate and Color. 102

Chapter VI

THE ASTEROIDS: A GROUP OF SMALL PLANETS, THE FIFTH IN THE ORDER OF DISTANCE FROM THE SUN.

The Interplanetary Spaces. — Kepler's Speculations.— Great Interval between Mars and Jupiter. — Bode's Empirical Law. — Conviction that a Planet existed between Mars and Jupiter. — Congress of Astronomers. — An Association Organized to Search for the Planet.— Discovery of Ceres. — Lost in the Solar Beams. — Rediscovered by Gauss. — The New Order Disturbed by the Discovery of Pallas. — Olber's Hypothesis. — Discovery of Juno and Vesta — The Search Ceases. — Renewed in 1845. — Many Asteroids discovered. — Their Magnitude, Size, and probable N umber „ 125

Chapter VII

JUPITER, ATTENDED BY FOUR. MOONS, THE SIXTH PLANET LN THE ORDER OF DISTANCE FROM THE SUN.

Arc of Retrogradation.— Stationary Point.— Distance of the Planet Determined. — Periodic" Time. — Synodical Revolution gives the Sidereal. — Surface of Jupiter as given by the Telescope — Period of Rotation— Diameter.— Volume. — Mean Distance. — Amount of Light and Heat. — Figure of Jupiter. — Equatorial and Polar Diameters. — Discovery of the Four Moons by Galileo. — Effect on the Copernican Theory —Jupiter's Nocturnal Heavens.

The Sattellitesof Jupiter. — How Discovered. — Their Magnitude. — Form of their Orbits. — Period of Revolution.— Eclipses.— Transits. — Occultations. — Velocity of Light Discovered. — Terrestrial Longitude. — Rotation of these Moons on an Axis . 134

Chapter VIII

SATURN, THE SEVENTH PLANET IN THE ORDER OF DISTANCE FROM THE SUN, SURROUNDED BY CONCENTRIC RINGS, AKD ATTENDED BY EIGHT SATELLITES.

The most Distant of the Old Planets.— Its Light Faint, but Steady.— Synodical Revolution.— The Sidereal Revolution.— Advances in Telescopic Discovery.—Galileo announces Saturn to be Triple.— Hnygens Discovers the Ring. — Division of the Ring into Two. — Cassini announces the Outer Ring the Brighter.— Multiple Division— Shadow of the Planet on the Ring— Belts and Spots.— Period of Rotation of the Planet and the Ring.— Disappearanceof the Ring Explained. — The Dusky Ring.

Satellites of Saturn. — By whom Discovered. — Eight in Number. — Their Distances and Periods.— Saturn's Orbit the Boundary of the Planetary System, as known to the Ancients 156

CONTENTS. Vll

Chapter IX. The Laws Of Motion And Gravitation

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The Demands of Formal Astronomy.— Those of Physical Astronomy.— Synopsis of the Discoveries already made. — Questions remaining to be Answered. — Inquiry into Causes. — The Laws of Motion demanded. — Rectilineal Motion.— Falling Bodies. — Law of Descent. — Motion of Projectiles.— Curvilinear Motion. — First Law of Motion. — Second Law of Motion. — Momentum of Moving Bodies. — Motion on an Inclined Plane. — The Centrifugal Force. — Central Attraction. — Gravitation. — Laws of Motion applied to the Planets. — Questions Propounded in Physical. Astronomy. — Newton's Order of Investigation.— His assumed Law of Gravitation. — Outline of his Demonstration. — Its Importance aud Consequences. — The Law of Gravitation embraces all the Planets and their Satellites.— Gravitation Resides in every Particle of Matter 166

Chapter X

THE LAWS OF MOTION AND GRAVITATION APPLIED TO A SYSTEM OF THREE REVOLVING BODIES.

A System of two Bodies.— Quantities Required in its Investigation.— Five in Number. — Sun and Earth. — Sun, Earth and Moon, as Systems of Three Bodies. — The Sun supposed Stationary.— Changed Figure of the Moon's Orbit.— Sun Revolving Changes the Position of the Moon's Orbit. — Solar Orbit Elliptical. — Effects Resulting from the Inclination of the Moon's Orbit. — Moon's Motion above and below the Plane of the Ecliptic. — Revolution of the Line of Nodes. — Sun, Earth and Planet, as the Three Bodies. — Perturbations Destroy the Rigor of Kepler's Laws. — Complexity thus Introduced. — Infinitesimal Analysis. — Difference between Geometrical and Analytical Reasoning 194

Chapter XI. Instrumental Astronomy

Method for Obtaining the Mass of the Sun.— For getting the Mass of a Planet with aSatellite. — For weighing a Planet having no Satellite. — For weighing the Satellites. — Planetary Distances to be Measured. — Intervals between Primaries and their Satellites to be Obtained. — Intensity and Direction of the Impulsive Forces to be Determined — -These Problems all Demand Instrumental Measures.— Differential Places.— Absolute Places.— The Transit Instrument.— Adjustments. — Instrumental Errors — Corrections Due to Various Causes. — American Method of Transits. — Meridian Circle. — The Declinometer '210

Chapter XII. Uranus, The Eighth Planet In The Order Of Distance From The Sun

Accidentally Discovered by Sir William llerseliell. — Announced as a Comet. — Its Orbit proved it tu be a Superior Planet. — The Kletnents of its Orbit Obtained. — Are of Uetrogradatiou. — Period of Revolution. — Figure of the Planet.— Inclination nf its Orbit.— Six Satellites Announced by the Elder

llerseliell. — Pour of these now Reoogntied, — Their Orbital Planes and 1M-

rections of Revolution Anomalous. — Efforts made to Tabulate the Plaoes of

Uranus Unsuccessful, — This Leads to the Discovery of a New Exterior Planet , Mfl

Vlll CONTENTS.

Chapter XIII

NEPTUNE, THE NINTH AND LAST KNOWN PLANET IN THE ORDER OF DISTANCE FROM THE SUN.

PAGE Uranus Discovered by Accident. — Ceres by Research with the Telescope. — Rediscovered by Mathematical Computation. — The Perturbations of Uranus. — Not due to any known Cause. — Assumed to Arise from an Exterior Planet. — Nature of the Examination to find the Unknown Planet.— Undertaken at the 6ame time by two Computers. — Computation Assigns a Place to the Unknown Planet. — Discovered by the Telescope. — Discoveries Resulting. — A Satellite Detected.— The Mass of Neptune thus Determined.— Neptune's Orbit the Circumscribing Boundary of the Planetary System 266

Chapter XIV. The Comets

Objects of Dread in the Early Ages. — Comets Obey the Law of Gravitation and Revolve in some one of the Conic Sections. — Characteristics of these Curves.— Comet of 1680 Studied by Newton. — Comet of 16S2 named " Halley's Comet." — Its History. — Its Return Predicted. — Perihelion Passage Computed. — Passes its Perihelion 13th April, 1759 — Elements of its Orbit. — Physical Constitution. — Nucleus. — Envelopes. — Tail. — Intense Heat Suffered'by some Comets in Perihelio. — Dissipation of the Cometic Matter. — Encke's Comet. — A Resisting Medium. — Deductions from Observation. — Biela's Comet.— Divided.— Number of Comets

Chapter XV. The Sun And Planets As Ponderable Bodies

General Circumstances of the System. — The Sun. — His Diameter and Mass. — Gravity at the Surface. — Mercury. — His Mass and Perturbations. — Venus as a Ponderable Body. — Long Equation of Venus and the Earth. — The Earth and Moon as Heavy Bodies. — Figure and Mass of the Earth. — Precession. — Aberration. — Nutation.— Mars. — His Mass and Density. — Gravity at His Surface.— The Asteroids. — Jupiter's System. — Saturn. — His Moons and Rings as Ponderable Bodies. — Uranus. — Neptune. — Stability of the whole System... .

Chapter XVI. The Nebular Hypothesis

The Arrangement of the Solar System.— The Phenomena for which Gravitation is Responsible. — The Phenomena Remaining to be Accounted for. — Nebulous Matter as found in Comets. — Nebulous Matter Possibly in the Heavens. — The Entire Solar System once a Globe of Nebulous Matter. — Motion of Rotation. — Radiation of Heat. — Condensation and its Effects. — Rings disengaged from the Equator of the Revolving Mass. — Formation of Planets and of Satellites 349

Introduction

The great dome of the heavens, filled with a countless multitude of stars, is beyond a doubt the most amazing spectacle revealed by the sense of sight. It has excited the admiration and curiosity of mankind in all ages of the world. The study of the stars is therefore coeval with our race, and hence we find many discoveries in the heavens of whose origin neither history nor tradition can give any account. The science of Astronomy, embracing, as it does, all the phenomena of the celestial orbs, has furnished in all ages the grandest problems for the exercise of human genius. In the primitive ages its advances were slow, but by patient watching, and by diligent and faithful records transmitted to posterity from generation to generation, the mysteries which fill the heavens were one by one mastered, until at length, in our own age, there remains no phenomenon of motion unexplained, while the distances, magnitudes, masses, reciprocal influences, and physical constitution of the celestial orbs have been approximately revealed. In a. former volume an attempt was made to trace the career of discovery among the stars, and to exhibit the successive steps by which the genius of man finally reached the solution of the great problem of the universe.

The performance of that task did not permit the special study of any one object, except so far as ii was required in the march of the general investigation. It is our

X INTRODUCTION.

object now to execute what was then promised, and to examine in detail the various bodies which are allied to the sun, constituting (as we shall find) a delicately organized system of revolving worlds, a complex mechanical structure, whose stability has challenged the admiration of all thinking minds, and whose organization has furnished the most profound themes of human investigation.

The plan adopted will lead us to present clearly all the facts and phenomena resulting from observation; with these facts the student may exercise his own genius in attempting to account for the phenomena, before proceeding to accept the explanation laid down in the text.

To aid the memory and to present a systematic investigation, we shall adopt the simple order of distance from the solar orb, commencing with that grand central luminary, and proceeding outward from planet to planet, until we shall develop all the phenomena employed in the discovery of the great law of universal gravitation. With a knowledge of this law the worlds already examined cease to be isolated, and arrange themselves under the empire of gravitation into a complex system, the delicate relations of whose parts, leads to new discovery and to the final perfection of the system of solar satellites.

Having closed our investigation of the planets and their tributary worlds, we shall render an account of those anomalous bodies called comets, which, by the suddenness of their appearance, their rapid and eccentric motions, and the brilliant trains of light which sometimes attend them, have excited universal interest, not unattended with alarm in all ages of the world.

Before passing to the execution of this plan, we must examine, to some extent, the phenomena of the nocturnal

INTRODUCTION. XI

heavens, as the stars furnish the fixed points to which all moving bodies are referred.

To the eye the heavens rise as a mighty dome, a vast hollow hemisphere, on whose internal surface the glittering stars remain forever fixed. In case we watch through an entire night, we find the groupings of stars slowly rising from the east, gradually reaching their culmination, and then gently sinking in the west. A more attentive examination enables the eye to detect some of these groups of stars toward the north which ever remain visible, rising, culminating, and descending, but never sinking below the horizon. Every star in this diurnai revolution, as it is called, is found to describe a circle, precisely as if the concave heavens were a hollow sphere to which the stars were attached, and that this hollow globe, were made to revolve about a fixed axis, passing through its center. Indeed, we find by attentively watching, that this hypothesis of a spherical heavens accounts for all the phenomena already presented. As the stars are situated nearer to the extremity of the axis of revolution the circles they describe grow smaller and smaller, until, finally, we find one star which remains fixed, and this one must be at the point where the axis of the heavens pierces the celestial sphere. This is called the north star ; and the point in which the axis pierces the heavens is called the north pote. The opposite point is called the south pole.

Only one half of the celestial sphere is visible at one time above the horizon, but this spherical surface extends beneath the horizon, and forms a complete sphere, encompassing us on all sides, while its center seems to be occupied by the earth. It is true that, in the day-time, the stars fade from the sight in the solar blaze, but they

Xll INTRODUCTION.

are not lost ; they still fill the heavens, as we shall see hereafter, and the starry sphere sweeps unbroken entirely round the earth.

These great truths, the diurnal revolution of the heavens, its spherical form, the central position of the earth, the north polar star, the axis of the heavens, the circles described by the stars, were among the discoveries of primitive antiquity, and are matters of the most simple observation.

The spherical form of the heavens was soon imitated, and the artificial globe became one of the first astronomical instruments. On this artificial globe certain lines were drawn to imitate those described in the heavens by the celestial orbs, and as these lines must henceforth form a part of our language we proceed to give the following Definitions : —

A great circle is one whose plane passes through the center of the sphere.

A small circle is one whose plane does not pass through the center of the sphere.

The axis of the heavens is an imaginary line passing through the center of the earth, and about which the heavens appear to revolve once in twenty-four hours.

A meridian is a great circle passing through the highest point of the celestial sphere (called the zenith) and the axis of the heavens.

The equator or equinoctial is a great circle, perpendicular to the axis of the heavens, and half-way between the north and south polar points.

These important lines have been employed from the earliest ages in the study of the heavenly bodies, and having thoroughly mastered their meaning and position we are prepared to examine any changes of location which

INTRODUCTION. Xlll

may be discovered among the vast multitude of shining bodies which go to fill up the concave of the celestial sphere.

We shall proceed, then, without further delay, to the execution of the plan already laid down.

Chapter I . The Sun, The Central Orb Of The Planetary System

DISCOVERIES OF THE ANCIENTS— The Source of Life and Light and Heat. — The Sun's Motion among the Stars. — His Orbit circular. — Length of the Year.— Inequality of the Sun's Motion. — Explained by Hipparchus.— Solar Eclipses. — Their First Prediction.

DISCOVERIES OF THE MODERNS.— The Sun's Distance.— His Horizontal Parallax. — Importance of this Element. — Measured by the Transit of Venus. — The Sun's actual Diameter and real Magnitude. — His Rotation.— The Solar Spots. — Their Periodicity. — Speculations as to the Physical Constitution of the Sun.

The sun is beyond comparison the grandest of all the celestial orbs, of -which we have any positive knowledge. The inexhaustible source of the heat which warms and vivifies the earth, and the origin of a perpetual flood of light, which, flying with incredible velocity in all directions, illumines the planets and their satellites, lights up the eccentric comets, and penetrates even to the region of the fixed stars ; it is not surprising that in the early ages of the world, this mighty orb should have been regarded as the visible emblem of the Omnipotent, and as such should have received divine honors.

On the approach of the sun to the horizon in the early dawn, his coming is announced by the gray eastern twilight, before whose gradual increase the brightest stars and even the planets fade and disappear. The coming splendor grows and expands, rising higher and yet higher, until, as the first beam of sunlight darts on the world, not a star or planet remains visible in the whole heavens, and even the moon, under this flood of sunlight, shines only as a faint silver cloud.

This magnificent spectacle of the sunrise, together with the equally imposing scenes which sometimes accompany the setting sun, must have excited the curiosity of the very first inhabitants of the earth. This curiosity led to a more careful examination of the phenomena attending the rising and setting sun, when it was discovered that the point at which this great orb made his appearance was not fixed, but was slowly shifting on the horizon, the change being easily detected by the observation of a few days. Hence was discovered, in the primitive ages —

The sun's apparent motion. — In case the sun is observed attentively from month to month it will be found that the point of sunrise on the horizon moves slowly, for a certain length of time, toward the south. While this motion continues, the sun, at noon, when culminating on the meridian, reaches each day a point less elevated above the horizon, and the diurnal arc or daily path described by the sun grows shorter and shorter. At length a limit is reached ; the point of sunrise ceases to advance toward the south, remaining stationary a day or two, and then slowly commences its return toward the north. This northern movement continues ; each day the sun mounts higher at his meridian passage, the diurnal arc above the horizon grows longer and longer, until, again, a northern limit is reached, beyond which the sun never passes. Here he becomes stationary for one or two days, and then commences his return toward the south. Thus does the sun appear to vibrate backward and forward between his southern and northern limits, marking to man a period of the highest interest, for within its limits the spring, the summer, the autumn, and the \Dinter, have run their cycles, and by their union have wrought out the changes of the year.

The length of this important period was, doubtless, first determined by counting the days which elapsed from the time when the sun rose behind some well-defined natural object in the horizon until his return in the same direction to the same point of rising. Of course, these changes in the sun's place were studied with profound attention. They were among the first celestial phenomena discovered, and among the first demanding explanation. The stars were found never to change their points of rising, culmination, and setting. Their diurnal arc remained forever the same, and the amount of time they remained above the horizon depended on their distance from the north polar point.

Observation having thus revealed the fact that the sun was undoubtedly moving alternately north and south, a more critical research showed the equally important truth, that this great luminary was slowly shifting its place among the fixed stars. This was not so readily determined ; but by noting the brilliant stars which first appeared in the evening twilight, after sunset, it was soon discovered that these stars did not long remain visible. Indeed, the whole starry heavens seemed, from night to night, to be plunging downward to overtake the setting sun, or rather, that the sun himself was mounting upward to meet the stars, and thus was discovered a solar motion in a direction opposed to the diurnal revolution of the heavens.

From month to month the sun was soon to advance among the stars, and at the end of an entire year, after all the former changes of northern and southern motion had been accomplished, the sun was found to return to the same group of fixed stars from whence he set out ; and thus it became manifest that this revolution among the stars was identical in period with the changes from north to south, and hence these phenomena had, in all probability, a common origin.

Here was the first great problem offered for solution to the old astronomers. The facts and phenomena were carefully studied, and the reader may now exercise his own power of thought in an effort to explain the facts recorded, before accepting the solution we are about to present.

An examination of the points of rising, of culmination, and of setting, of the fixed stars, showed them to be absolutely invariable, and in case these glittering points could leave behind them, in their diurnal revolution, lines of silver light, sweeping upward from their point of rising to the meridian, and downward to the point of setting, these lines of light would be seen to be parallel circles. All the stars north of the equinoctial (in the region of the earth we inhabit) describe diurnal circles, of which more than one half is above the horizon, while all the stars south of the same line sweep round in circles, of which less than half lies above the same plane. Any star, precisely on the equinoctial, half way between the north and south poles, passes one half its revolution above and the other half below the horizon.

These facts being carefully noted, it was seen that in case the sun, on any day of its annual journey, chanced to coincide with a fixed star, that for that day the sun and star would describe the same diurnal circle, and would remain above the horizon an equal length of time. Thus along the sun's path it became possible to select a number of stars over which the sun passed, and which would by their position mark his route in the heavens. To aid in this investigation, as well as for some other purposes, the ancients erected a vertical staff on a level plane, and then noted where the shadow of the top of the staff fell at noon each day throughout the year. This instrument was called a gnomon, and its use revealed many important facts in the solar motion, and detected others hitherto overlooked. If on the same day we note carefully the length of the shadow of the gnomon a little while before and after noon, we shall find the shadow slowly decrease in length as the sun rises to its culmination; and immediately after passing the meridian the shadow commences to increase in length. Mark the point where the shortest shadow fell, and the line joining this point with the foot of the gnomon is a north and south line, and on this line all the noon shadows will fall throughout the entire year.

By a careful examination it was discovered that the noon shadow on the day of the winter solstice or southern limit always fell on the same point. The same was true of the noon shadow on the day of the summer solstice or northern limit. These points were exactly opposite each other on the sun's apparent orbit (or path among the stars). It was further discovered, that selecting any day in the year, the noon shadow for that day invariably fell on the same point as it had done on preceding years ; and hence it became manifest that the sun's track among the stars did not change from year to year.

The question now arose as to the figure of the sun's path: was that figure a circle? ami did the sun move with uniform velooity? As all the stars described diurnal circles ; as this curve was the simplest as well as the most beautiful o\' curves ; as its curvature was every where the same ; as it had neither beginning nor ending, it was earl j adopted as the celestial curve, shadowing forth, even in its form, the ceaseless journeys of the revolving worlds. It was assumed then that the sun swept round the sphere of the heavens once a year, with uniform volocity, in a circular orbit, of which the earth was in the center.

This hypothesis accounted fully for all the discovered phenomena, and justly ranks among the most important of the primitive discoveries.

The gnomon gave to the old astronomers a ready means not only of tracing the sun's path among the stars, but also of measuring the inclination of the plane of the ecliptic to the plane of the equinoctial. This is readily seen from the subjoined figure.

Let AB represent the gnomon, A' the shadow of the vertex at noon on the day of the summer solstice, and A." the shadow at noon on the day of the winter solstice. Then will the angle A 'A A" measure the entire motion of the sun from north to south ; and as one half of this motion lies north and the other half south of the equinoctial, it follows that half the angle, A' A A/' measures the inclination of the ecliptic to the equinoctial.

In the earliest ages it was assumed that the sun's orbit was absolutely fixed among the stars, and that the points in which this circle crossed the equinoctial were in like manner invariable. These points of intersection are of the highest importance. That one through which the sun passes in going from south to north, is called the Vernal Equinox, while the opposite point, through which the solar orb passes in going from north to south, is called the Autumnal Equinox. On the day of the equinoxes, as the sun's center was then on the equinoctial, the diurnal arc described by the sun would lie one half above, and the other half below the horizon, making the length of day and night precisely equal.

Among the ancient nations the day of the vernal equinox was an object of especial interest, as it heralded the coming of spring, and its approach was marked by the rising of a certain bright star in the early dawn of the morning. Now, in case the vernal and autumnal equinoxes were invariable, the same star by its heliacal rising, (as it was called,) would mark the crossing of the equinoctial by the sun in the spring and the equality of day and night. After the lapse of few centuries it was discovered, by the length of the noon shadow of the gnomon, that the sun had reached the equinoctial point, and yet the sentinel star did not make its appearance. Either the equinox or the star was in motion. It was soon decided that the vernal and autumnal equinoxes are both slowly moving backwards along the equinoctial and thus the sun crosses this celestial circle each year a little behind the point of the preceding year.

The ancient nations all seem to have attained to a

22 THE SCX.

knowledge of this great truth, and some of them are said to have fixed the period in which the vernal equinox retrogrades around the entire heavens, a period of nearly twenty-six thousand years ; as this is a matter of simple observation, and as the rate of motion can be obtained by comparing recorded observations, made at intervals of four hundred or five hundred years, we may readily credit the statement that this period became known even anterior to the commencement of authentic history.