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ASTRONOMY (from dvrpov, a heavenly body, and vtyog, law) is the science of the heavenly bodies and their motions. It is founded on observation, but it receives its last perfection from calculation. What an interval from the imperfect notions of the Chaldean shepherd and the Phoenician mariner to the Celestial Mechanics of a Laplace ! How many centuries of observations were necessary to render the motion of the earth suspected! How slow the progress to the laws of planetary motion, and from those laws to a universal principle of gravitation! Founded on geometrical considerations, this great principle explains all the celestial phenomena in their minutest details : there is not a single seeming irregularity which does not necessarily result from it. Outrunning the cautious advances of observation, it descends from causes to phenomena, and renders astronomy a great me chanical problem, of which the only data necessary are the motions, figures and magnitudesof the heavenly bodies. That part of the science which relates to their motions, magnitudes and periods of revolution, is called descriptive astronomy; that part which explains the causes of their motions, and demonstrates the laws by which those causes operate, is called physical astronomy. From a simple view of the heavens, we see stars, with which the blue vault above us is sprinkled, appear regularly in a certain point, rise with a uniform motion to a certain elevation, and then descend, and disappear in the opposite quarter of the heavens. This motion is common to all the heavenly bodies, and is performed in equal times, though they appear to pass through arcs of very different magnitudes. At a certain point, this motion appears to cease : this point is called the pole, which signifies a pivot, on which the heavens appeal* to turn. The celestial vault being conceived as forming a sphere, there are two of these points: that which is visible in our hemisphere is the north celestial pole ; and that which is visible in the opposite hemisphere is the south celestial pole. The circle which bounds our view on all sides is called the horizon, or boundary: its plane passes through the centre of the earth: it is also called the celestial or rational horizon, to distinguish it from the sensible horizon, which limits the view of objects on the surface of the earth. A circle perpendicular to the horizon, passing through the poles, is called the meridian. It divides the celestial hemisphere into two equal parts, so that the heavenly bodies, at the moment they arrive at this circle, are at the middle of their apparent course: the passage of the sun over this circle determines the instant of noon. The period occupied by the stars in passing from this circle through the celestial sphere, and returning to the same point, is called a siderial day, and is a little less than 24 hours. As we remove from the poles, the arcs described by the stars gradually increase, and at an equal distance between them, we find the largest, which, dividing the celestial sphere into two equal parts, is called the celestial equator. A line drawn from the centre of the globe, through the place of the observer, ascertains a point in the heavens, perpendicularly over his head, which is called the zenith: the same line produced in the opposite direction determines a point in the opposite part of the heavens, which is called the nadir. We have thus far spoken of the ascend ¦nig and descending of the heavenly bodies in the heavenly vault. But does all this train of worlds actually move round the earth daily? Or can it be proved that our senses deceive us, and that this apparent motion is an illusion ? The true cause of these appearances is the motion of the earth round its own axis, from W. to E., in the space of nearly 24 hours. A moment's reflection will convince us that the horizon of the observer, as it turns along with him during the rotation of the earth, must advance towards the stars successively, so as to give them the ap? pearance o^ gradually approaching the horizon; as a vessel leaving the shore causes it to appear to recede to a person on board. As the meridian turns at the same time, it must arrive successively at the same stars, which will then appear to have ascended to the middle part of the course they describe above the horizon. As soota as the star touches the western verge of the horizon, it appears to set, and ceases to be visible until the motion of the earth again brings it back on the eastern boundary of the same circle. But has the earth no other motion ? Eveiy one must have observed that the sun, besides its apparent diurnal motion, which it has in common with all the stars* appears in the course of a year to change its place in a twofold manner. First, it appears to rise and sink alternately towards one or other of the pcies,; and, secondly, if we observe its place dmong the stars, it appears either that the sun recedes daily towards the east, or that the stars advance daily towards the west; for the stars, which we see at one time set immediately after the sun, are, on the following evening, lost in his rays: some days after, they reappear in the east, and their rising precedes daily more and more that of the sun. At last, after a year, or about 365 days, the sun and stars are again seen in the same relative position. The complexity of these motions is increased by the confUsion presented by the apparent motion of the other planets: sometimes they seem to be hurried along with great rapidity; at other times they appear stationary, and, at still others, retrograde. All this seeming chaos of motions is reduced to order by a knowledge of the fact, that, while the earth turns on its axis, it advances, at the same time, in absolute space from west to east, and performs an entire revolution round the sun in the course of a year, in a plane inclined to the equator. The circle which the centre of the earth describes in this revolution, and which is the apparent path of the sun. is called the ecliptic. The axis of the earth remaining always parallel to itself, the opposite poles will be directed towards the sun once in each revolution. When a pole is directed towards the sun, it receives more light and heat, and for a longer period, than at any other portion of the revolution. It is then the summer solstice in that hemisphere; the days are longest, the nights shortest, and the heat greatest. Six months, or, rather, half a year from that period, every thing is reversed ; the same pole is turned from the sun ; the light and heat is received in small quantities, and for a short period; the days are short, the nights long; the cold intense: it is the winter solstice. At two other points of the orbit, equidistant from each other and from the solstices, the poles are equally inclined to the sun; they receive an equal supply of heat and light, and during equal periods; the days an I nights are equal all over the globe: it is the vernal or autumnal equinox. The diurnal rotation of the earth on its own axis produces, therefore, the alternation of day and night. The annual revolution round the sun, and the obliquity of the ecliptic to the equator, causes* the changes of the seasons. The daily rotation of the earth produces, also, the phenomena of tides in the ocean and the atmosphere. (See Tides.)Let us now take a more genera) view of the celestial phenomena. The discovery of peculiar qualities common tc a number of heavenly bodies, has led to the formation of classes (see Planets, Satellites, Comets, fflxed Stars); or convenience of description has clustered them into groups with fanciful names (see Constellations) ; or their peculiar influence on human affairs has given a name to individuals (see Sun, Moon, Earth, &c). At first view, the stars in general do not seem to change their relative positions; and, if they have particular motions, a long series of observations is necessary to render them sensible. But, by continuing to compare the heavens at different epochs, we perceive that some of them are distinguished by relative motions, and by the nature of the light which they transmit to us. These we call planets, that is, wandering stars, in distinction from those, which, maintaining always the same relative positions, are called fixed stars. The planets transmit to us a soft, mild, steady light, never exhibiting any change of color. They are opaque bodies, and their light is only a reflection of that which they receive from the sun, around which they revolve in regular but unequal periods,turning at the same time on their axes. Their number now known is 11. We mention them in the order of their distances from the sunMercury, Venus, Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn, and Uranus, or Herschel. Five of these are visible to the naked eye, and were known to the ancients ; five have been discovered in modern times by the aid of the telescope. Some of these bodies have smaller ones in their neighborhood, which revolve round them at the same time that they accompany them in their orbits of revolution round the sun, and turn on their own axes. The former are called primary, to distinguish them from these attendants, which are called the secondary planets, or satellites. T 'ae latter are opaque, like the former. Th'; earth is accompanied by one, which is ^aTled the moon, Jupiter by four, Saturp bv seven, with his remarkable ring, and Horschel by six. The interposition of ore of thf; planets between the sun and an observer stationed on another planet, producer an eclipse, (q. v.) From time to time, small specks appear in the heavens, of a feeble lustre, moving slowly in the miast of tae other stars. Gradually, as they approach nearer, their velocity increases; their light is more brilliant; and, after passing into the immediate vicinity of the earth and sun, they recede again, and disappear in the distance. These are called comets (which signifies hairy bodies) from the peculiar luminous train by which they are attended, and which rhe ancients called hair, and the moderns, mil. These bodies, long the objects of oerror to man, as the harbingers of pestitence and war, are now known to be subject to the common laws of nature, and to revolve round the sun in regular periods. The s'?n, the 11 primary und 18 secondary pL lets, and the comets, constitute the *olar system. Far beyond these limits, at in immeasurable distance, lie the fixed itars, infinite in number, of a brilliant lus¦ re, and constantly changing color. Their iistance, and the brilliancy of their light, with the fact that their magnitudes remain always the same, render it probable that ihey are luminous bodies, like so many suns. They have been formed into groups of arbitrary extent, under the names of gods, men, beasts, &c, which are called constellations. Of these the ancients knew 48; the moderns have increased the number to more than 100. It should be understood, that the stars, thus grouped together under one name, have no connexion with each other, but VOL. i. 37 are so arranged for convenience of description. The first astronomers, in order to estimate better the apparent motion of the sun, referred it to those constellations through which it appeared to pass, and which are 12 in number. They are, in Latin, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius Capricornus, Aquarius and Pisces. Th . zone, or band, which contains them, is called the zodiac (q. v.), and each constellation is called a sign of the zodiac. In consequence of a motion of the earth's axis, the constellations no longer correspond to the same points of the orbit; but as we confine the name signs to the 12 divisions of the circumference of the circle, which measures the whole revolution of the earth, and as these divisions do not change, the vernal equinox always corresponds to the first point of the sign Aries, the summer solstice to the first point of the sign Cancer, the autumnal equinox to that of Libra, and the winter solstice to that of Capricorn, although the constellations, which bear these names, have ceased to be connected with these seasons. (See Precession of the Equinoxes.) To penetrate yet farther into the heavens, it is necessary to aid the imperfection of vision by the telescope, which discovers to us millions of stars in the infinity of space. In a clear night, turn your eyes to the irregular zone of whitish light: it is the milky way (q. v.): you will find it to consist of an infinite number of stars, whose inconceivable distance renders their light too feeble to make a distinct impression on the naked eye. Continue your examination, and you will observe luminous spots of an undefined shape: these are nebulce, some of which a further observation will show you to be assemblages of stars, like the milky way, while others will appear to consist of an unbroken mass of whitish light. You will find, also, some stars to be variable, undergoing a periodical change of brightness: some, which appear single to the naked eye, will be found to be double, triple, &c, and to revolve round a common centre of gravity by twos, threes, &e. Compare your observations with those of your predecessors, and you will find that new stars have appeared at different times, and afterwards disappeared, and that others have experienced a change in the intensity of their light. Of the actual magnitude and distance of the stars we know nothing. The diameter of the earth's orbit is 200,000,000 miles; yet we can detect no difference in their ap parent places, viewed from the opposite points of this diameter: a change of place amounting only to a second would be detected by the accuracy of modern observations : geometrical considerations, therefore, prove that the nearest star cannot be less than 20 billions of miles distant from us. After considering the apparent motions of the heavenly bodies, and the real motions which give rise to these appearances, physical astronomy rises to the explanation of the cause, and the investigation of the laws, of the celestial phenomena. Applying the laws of motion to the heavenly bodies, it discovers a force operating throughout, which is called attraction, the amount of which is directly as the quantity of matter, and inversely as the squares of the distances. By the application of this general principle, it descends to those more refined inequalities, which, owing to their minuteness, or the length of their periods, would escape or mislead the observer unassisted by theory.