THE SEASONS.

PARALLAX.

both the body giving forth light and that one which receives it be at rest, the former will be seen in its true Let S represent the sun, and A B C D the earth at place, at least in so far as aberration is concerned ; but various places of its annual circuit; when the earth is let either of the bodies move, and this will not be the at B or D, these are the periods of the equinox, when case. In order to render this plain, suppose a shower the line of the equator intersects or cuts through the of hail to fall perpendicularly upon a number of tubes line of the ecliptic. At this period, one-half of the globe-say the pipes of an organ; if the organ remain stais illuminated from pole to pole, or there is over all the tionary, the hailstones will descend sheer from the top earth an equal day and night of twelve hours. But to the bottom, without any deviation right or left; but when the earth has proceeded to A, the pole or axis move the organ in any direction, and they will strike still keeping the same position, or pointing to one par- the side opposite to the direction in which the motion ticular place in the starry heavens, it will be turned is made. Now, it is just in this way that the eye misses more directly from the sun; a greater proportion the perpendicular ray, and, meeting an oblique one, reof his rays will shine on any particular spot of the ceives an impression that the star lies in that direction. southern half of the globe, and the period of day, or The object thus appears displaced, and the amount of sunlight, will exceed that of darkness by the proportion displacement is aberration. The earth travels at the of the light and shade parted in the circle of the earth. rate of about nineteen miles per second, and therefore It will be observed, also, that within the circle of the is every instant changing its direction. Time is also south pole, the sun will shine continually as the earth occupied by light in traversing space, which it does at revolves on its axis, or, in short, to the inhabitants of the amazing rate of 192,000 miles per second; so that that part of the globe the sun will never set for several also requires to be calculated for by astronomers. The months. When the earth has proceeded on to D, one- effect of aberration is to make a star apparently dehalf of its annual course is finished, or this is the spring scribe a small ellipse in the heavens, in the centre of equinox, or equal day and night. At C, again, the which it would be seen if the earth were motionless. earth has arrived at our longest day in summer, when The reader must carefully distinguish between aberrathe axis is turned to the sun, and the regions around tion and refraction; their effects are the same-namely, the pole are in the light for a greater period, while to displace the ray-projecting object-but they proceed darkness, or night, prevails for a less. It will be seen, from very different causes. Besides these corrections too, that now the pole and circle around it revolve in which astronomers have to make in their calculations, perpetual light; or to the inhabitants of that region, there is another, resulting from what is called parallax, the sun never sets for some months, but they have one which may be as well introduced in this place. continued and uninterrupted day. At the other, or south pole, the same changes take place, only matters are reversed there it is summer while we have winter, The word parallax, in its general signification, deand the winter of the north pole is the summer of the notes change of place; but in astronomical books it south. In the middle regions of the earth, or around has a conventional meaning, and implies the difference the equator, the sun's place does not suffer a very great of apparent positions of any heavenly luminary when change; and, accordingly, there the heat is nearly of viewed from the surface of the earth and from its the same intensity all the year through; and the length centre. The centre of the earth is the general station of their days and nights is nearly equal, or nearly the to which all astronomical observations are referred; same as at the periods of the equinoxes. But the orbit the situation of a heavenly body, observed from the in which the earth travels round the sun is not an surface of the earth, is called the apparent place; and exact circle; it is, as we have already mentioned, an that at which it would be seen from the imaginary place ellipse, and the sun is placed near one end of it, as at of observation at the centre of the earth, the true or the small circle and letter S. In consequence of this mean place. Hence the altitudes of the heavenly bodies circumstance, the sun is much nearer us at one period are depressed by parallax, which is greatest at the of the year than another, and this happens in our win-horizon, and decreases as the altitude of the object inter; accordingly, the sun appears about one-thirtieth creases. This may be rendered very plain, by suppos part larger in January than in June. But in propor- ing that two persons placed individually at the end of a tion as the earth approaches in her orbit to the sun, her straight line, look at a candle removed at, say, 100 motion is quickened, and she passes over the winter yards' distance from them. It is evident that the burnhalf year in nearly eight days' less time than the sum- ing body will appear to be projected upon the wall of mer. It is principally from this circumstance, as well an apartment, or any other background, at very diffeas the shorter period of the day, that although the sun rent positions to each of the spectators. The angle be nearer us in winter, and consequently his power of which this difference of position makes is similar to imparting heat greater, yet the actual quantity imparted parallax. The farther they remove from the light, is, on the whole, much less in the one season than the allowing them still to remain at the same distance from other. We have said that the north pole of the earth each other, the more obtuse the angle would become, always points to a particular spot in the heavens; this and the less the parallax. Thus, the fixed stars, being is not, strictly speaking, correct; the pole or axis makes so far removed from us, when viewed from any two a circle round the centre of the axis of the ecliptic in a positions upon the earth's surface, are seen at the same long period of years, and it is this motion that gives place upon the celestial sphere, and hence have no perrise to the precession of the equinoxes, which will be ceptible parallax. It is different, however, with the afterwards described under that title. luminaries belonging to our system; and by this means astronomers have been enabled to estimate the quantity of space which separates us from them. For a complete account of the means by which this is accomplished, we must refer the reader to more elaborate treatises than the present. A general and correct enough idea of it may be formed from the familiar example we have given. In the same manner, suppose two observers, one in the northern the other in the southern hemisphere, at stations on the same meridian, observe on the same day the meridian altitudes of the sun's centre. Having thence derived the apparent zenith distances," says Sir J. Herschel, whose language would be deprived of clearness were it abridged, and cleared them of the effects of refraction, if the distance of the sun were equal to that of the fixed stars,

ABERRATION OF LIGHT.

Although the most convincing proof of the earth's orbitual motion is not to be found in any circumstance of which the senses can take immediate cognisance, but is afforded by the full development of the planetary system, there is, however, one direct proof of it in a phenomenon discovered by Bradley, an illustrious astroIt is called the aberration of light, and is manifested by a small difference between the apparent and true places of a star, occasioned by the motion of light combined with that of the earth in its orbit. Vision, it is well known, arises from rays of light proceeding from any object, and entering the eye; and we see the object in the direction in which the rays have come. If

nomer.

an anomalistic year, it must describe a farther arc of 11" 8 to arrive at its original position in perihelion, the latter having moved forward to that amount. In so doing it occupies 4' 39" 7, which must be added to the sidereal period, making the anomalistic year 365 days, 6 hours, 13 minutes, 49 seconds, 3, in length. All these periods have their uses in astronomy; but the one in which mankind are most particularly interested is the tropical year, or that on which the seasons depend, and which is a compound phenomenon, depending chiefly and directly on the annual revolution of the earth round the sun, but subordinately also, and indirectly, on its rotation round its own axis.

MEASUREMENT OF TIME.

the sum of the zenith distances thus found would be precisely equal to the sum of the latitudes north and south of the places of observation; for the sum in question would then be equal to the meridional distance of the stations across the equator. But the effect of the parallax being in both cases to increase the apparent zenith distances, their observed sum will be greater than the sum of the latitudes by the whole amount of the two parallaxes. This angle, then, is obtained by subducting the sum of the latitudes from that of the zenith distance; and this once determined, the horizontal parallax is easily found, by dividing the angle so determined by the sum of the signs of the two latitudes." It may be observed, that the angles are determined by means of very nice instruments. The parallax thus obtained is called the daily or geocentric, in Although the sidereal day, from its uniformity, is well contradistinction to the annual or heliocentric, by adapted for astronomical purposes, yet it is scarcely which, in general, is understood the difference of place sufficiently marked for the ordinary wants of life. No of a heavenly body, as seen from the earth and from person but an astronomer ever attends to the culminathe sun; in particular, however, it denotes the angle tion of a star; on this account, the diurnal return of formed by two lines from the ends of the diameter of the sun to the same meridian has been universally the earth's orbit to a fixed star, which, as we have adopted as the measure of time; and this is called a already observed, from the immense distance of the civil day. Most nations reckon the beginning of their latter, is inappreciable. Some idea of the importance day from midnight, but astronomers count from noon of parallax may be obtained from the fact, that before to noon. The day thus determined is called the astrothe sun's was determined, the distance of that luminary nomical or solar day, and, being regulated by the true from us was not estimated at within 13,000,000 of miles motion of the sun, the time which is measured by it is of its true amount. Its parallax is, of course, a very called true or apparent time. Two causes conspire to minute quantity, only 8" 6. render astronomical days unequal; first, the variable velocity of the sun in his orbit, and, second, the obliquity of the ecliptic. A mean astronomical day, which is independent of any causes of inequality, has been obtained by astronomers introducing into the system two imaginary suns. These two fictitious bodies are supposed to move uniformly, the first in the ecliptic, the second in the equator; and as the circles are both equal, the actual motion of each of the bodies is equal. To those desirous of studying this part of the subject, we would recommend a perusal of the article Astronomy in the seventh edition of the Encyclopædia Britannica, page 778, where it is well illustrated. The correction or equa tion, by which apparent time is reduced to mean time, is technically called the equation of time. There are only four days in the year when the apparent and mean time are the same, and the equator of time nothing. In the interval between the first and second of these, that is, December 24th and April 15th, and, again, in that between the third and fourth, that is, June 15th and September 1st, the apparent is always later than the mean time, or the clock is before the sun; in the other intervals which complete the year, the reverse is the case, and the clock is after the sun. The greatest difference between solar and true time amounts to between fifteen and sixteen minutes. Tables of equation are constructed for the purpose of correcting the differences.

OF SOLAR, SIDEREAL, AND ANOMALISTIC YEARS. There are three different periods at which the sun may, in different senses, be said to return to the same position-when he returns to the same equinox at which he was before; when he returns to the same point in his orbit, or the ecliptic; and when, being in perigee (least distance from the earth), or apogee (farthest distance from the earth), he comes back to either again; or, which is the same thing, when, having been at a given distance from any of these points, he returns to the same point with respect to them. Each of these may be said to be a completion of the revolution of the sun (strictly speaking, it is a revolution of our own earth round him), and a revolution thus performed is called a year. The first and shortest is the equinoctial, solar, or tropical year; for his time of returning from tropic to tropic, they being situations holding the same relation to the equinox for the time being, is obviously the same as that from equinox to equinox. The value of this year is 365 days, 5 hours, 49 minutes, nearly. But although the earth has thus returned to the same equinox, it has not made the entire circuit of its orbit, but must travel a little farther to arrive at the same point it was in a year before. This arises from a backward movement of the equinoctial point. (See "Precession of the Equinoxes.") The second is the sidereal year, which consists, as we said before, of 365 days, 6 hours, 9 minutes, 9 seconds, 6, reckoned in mean solar time, or a day more, reckoned in sidereal time. Here, then, there is a remarkable difference between solar and sidereal time, which requires explanation. If the reader will recollect what was said with regard to a solar and sidereal day, the discrepancy between the times of the years will become apparent. In the course of twelve months, all the little daily deficiencies, as it were, amount to twentyfour hours, which constitutes the difference between the two years. The sun's apparent annual motion among the stars is performed contrary to the apparent diurnal motion of the sun and stars; hence the stars gain every day three minutes fifty-six seconds on the sun, which makes them rise that portion of time earlier every day. In the course of a year, the sun will fall behind the stars a whole circumference of the heavens, or one revolution, which deficiency he must make up to complete the number of days in a year. It is evident, then, that the sun apparently, or the earth really, turns 366 times round upon its axis; and had it no other motion, there would be as many days in a year. After the earth or sun has completed a sidereal year, before it can finish

THE MOON.

Next to the sun, the moon is to the inhabitants of the earth the most remarkable and important of all the heavenly bodies. The mean horizontal parallax of the moon is 57′ 48′′; and her mean distance from the earth 236,847 miles. Like the sun, the moon advances in the heavens in a motion contrary to that of the stars. Notwithstanding the vast distance she is from us, it is little more than one-fourth of the sun's diameter, and the globe of that magnificent luminary would nearly twice include the whole orbit of the moon! It has various motions; as a secondary planet, it revolves round the earth, which is its primary. Along with the latter, it revolves round the sun, and it has a rotatory motion upon its own axis. Owing to the sun's apparent movement in the heavens being in the same direction with that of the moon, only slower, the latter has to make up for that slowness in the same way as we have mentioned with regard to the earth, and the time it takes constitutes the difference between the sidereal and synodic month or lunation. The sidereal month is 27 days, 7 hours, 43 minutes, 11 seconds, 5, in which time

the consequence is, that, in general, the moon, when she is in conjunction with the sun, either passes on one side or the other, and therefore does not intercept the sun's rays, or produce an eclipse. An eclipse of this kind can only take place when the earth and moon are in conjunction in that part of their orbits which cross each other (called the nodes), because it is then only that they are both in a right line with the sun. If the orbit of the moon were parallel to that of the earth, an eclipse would happen every month. Partial eclipses, again, are caused when the moon, in passing the earth, is not directly in a line with the sun, but a little on either side; the consequence of which is, the edge of one side of the moon only dips into the sun's disc. When the sun is eclipsed, the total darkness is confined to one be seen from every part of the earth, when the moon is above the horizon; and both circumstances prove that the earth is a good deal larger than the moon. The moon arrives very nearly at the same situation with respect to the earth, after making 223 revolutions, which are performed in eighteen years, of 365 days, 15 hours, 7 minutes, and 43 seconds, each; so that, after a period of about eighteen years, the series of eclipses recommences nearly in the same order, a circumstance observed by the ancients. The mean number of eclipses which occur in a year is about four, and there are sometimes as many as seven. There must necessarily be two solar eclipses, but it is possible that there may not be even one lunar. A remarkable eclipse, called an annular (or circular) solar eclipse, happens when the moon being in conjunction with the sun, the edge of the latter appears for a few minutes as a narrow ring of light encircling all round the dark disc of the moon. A great solar eclipse, visible in England, will take place in March 15, 1858, and a still more remarkable one, when the whole disc will be nearly covered, in August 19, 1887.

the moon performs a complete revolution round her | exactly coincide, but cross or intersect each other; and primary; and the other is 29 days, 12 hours, 44 minutes, 2 seconds, 87, the time which elapses between two new moons, or two conjunctions of the sun with the moon. It happens that its revolution upon its axis is performed in the same time as its revolution round the earth, so that the same side of her orb is always presented to the latter planet. Although the moon's rotation on her axis is uniform, her motion in her orbit is not so, and we are by this means enabled at times to obtain a peep of the equatorial portions of her eastern and western sides. Her axis, also, is not perpendicular to her orbit, and a small part of each of her poles alternately becomes visible. These phenomena are known by the name of librations of the moon, and they are of two distinct kinds, the result of different causes. The wisdom and beneficence of the Deity are strik-particular part of the earth, but the lunar eclipses can ingly displayed in the economy of moonlight, as distributed to our globe during various seasons of the year. The remarkable phenomenon of the harvest moon is familiar to every one. During the time that our satellite is full, and for a few days before and after, in all about a week, there is less difference between the time of her rising on any two successive nights, than when she is full in any other month of the year. By this means, an immediate supply of light is obtained after sunset, so beneficial for gathering in the fruits of the seasons. To conceive of this phenomenon, it must be recollected that the moon is always opposite to the sun when she is full; that she is full in the signs Pisces and Aries, these being the signs opposite to Virgo and Libra, which the sun passes through in September and October, our harvest months. Thus, although, whenever the moon enters the two former signs (and she does so twelve times in a year), the same circumstance takes place with regard to the time of her rising; yet it is not observed on these other occasions, just because she is not full at the time. The reason of there being little difference in the time at which she rises on several consecutive nights, is, that at these periods her orbit is nearly parallel to the horizon. The harvest moons are as regular in south latitude as with us in north latitude, only they happen at different periods of the year.

ECLIPSES.

Eclipses are caused by the positions of the earth and moon with respect to each other and to the sun. An eclipse of the sun takes place when the moon is between the sun and earth; and an eclipse of the moon is the result of the earth being between the sun and moon. In other terms, the shadow of the earth cast upon the moon causes a lunar eclipse, and that of the moon upon the earth causes a solar eclipse.

The following figure explains an eclipse of the sun. A B is the sun, M the moon, and CD the earth. The

THE SATELLITES.

The earth, we have seen, is attended in her annual circuit round the sun by one satellite, the moon, which revolves round her as a centre. Strictly speaking, both move round a common centre of gravity in an elliptic orbit, the regularity of which is disturbed by their mutual attractions, so that it is undulated or waved, thus, The number of undulations in a whole revolution is, however, only thirteen, so that the deviation from the ellipse is exceedingly trifling. Jupiter, Saturn, and Uranus, are all attended by satellites, as we have seen; and they form, as it were, each of the primaries with its attendant moons, a sort of miniature system, entirely similar in the laws by which they aro governed to the great system to which they all belong, where the sun may be termed the primary planet, and the primary planets the satellites. Their orbits are circles or ellipses of small eccentricity, the primary occupying one focus. Of these systems, that of which Jupiter forms the head, has been studied with the greatest attention. The discovery of Jupiter's satellites by Galileo, was one of the first fruits of the invention of the telescope, and forms a remarkable era in the history of astronomy. From it resulted a solution of the great problem of the longitude, and the grand discovery of the aberration of light. It also established completely the Copernican system, and confirmed the laws of Kepler. The satellites of Jupiter revolve from west to east like our moon, but they are much less in comparison with their primary than it, whilst their orbits are of smaller dimensions, and less inclined to the ecliptic of their primary than that of our satellite. The largest of them is about 3377 miles, and the least about 2068 miles in diameter. The satellites of Saturn have been much less studied, and have fewer peculiarities. Those of Uranus, however, are remarkable, inasmuch as their orbits are nearly perpendicular to the ecliptic, and in these orbits they are supposed to have a retrograde motion-that is, from cast to west, instead of froin west to cast, like

the other planetary bodies. No satisfactory cause for | nox now happens in the constellation Pisces; the this departure (if it be one) from the general rule can be given. It is by accurate observation of the satellites that the densities of the planets, or their weight as proportioned to their bulks, have been ascertained; as also, by watching their frequent eclipses, that the velocity with which light travels from the heavenly bodies to the earth has been brought within our calculation.

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