Show U SIT sli Y H HURE urby ni malli i vu t s itna P n yeb wa f r i i I 1 ft ASTRONOMY ir i r i T C byn BYr pROF ORSON SEN y a m a 2 E 1 i TURID IV f tf seasons ns of 1714 cipho precession bession of the egui Equi Equinox noa now es w iW sidwell Sid ercal weal dru cre and tropical year idt r tian ibn 0 of 39 tle the e elii elit elliptic ati orbit anomalistic lid ira r geocentric and awl places p 1 ma gfa ge a body bartis barths mean vean and jacus atrus longitude mean and apparent atme irme I 1 Min distributions buttons 0 of temperature attest ablest fy hottest Bottest ane and coldest days of tie the vear year perma Berma permanency ncncy nency of the mean annual temperature invariability of the earths diurnal a ani and nd a annual inua I 1 periods stability oath p of motion motton Inte interior temperature of the earth as affected by the sun ocean temperature cause of the great our cur vents ants in the ocean atmospheric phenomena enzena reflections on the orl origin of ranc Rane planetary tanj motion notion tre THE first subject which we propose to investigate vesti gate in this lecture is the seasons during the time that the earth performs one annual revolution the inhabitants experience a variety of seasons those who live in the southern hemisphere have their seasons in the reverse order of those in the northern december january and february are their summer months while here they are our winter months their spring corresponds to our autumn their winter to our summer their autumn to our spring when the days in the northern hemisphere are the longest in the southern they are the shortest ana and vice ties versa when they are the shortest here they are the longest there from the of march to the of september the sun shines without any intermission on an our north pole while the south pole during that time is enveloped in darkness from the of september to the of march the south pole is constantly enlightened by the sun while our north pole is left in darkness the whole order of the seasons in the northern hemisphere is repeated inthe in the southern but during the opposite time of year if the earth revolved around the sun directly from west to east that is if the plane of the earths orbit coincided with the plane of the equator there would be no variety of seasons and also the days and nights over the whole earth would be of equal length if the earth revolved around chesun the sun from south to north and back again to the south then our seasons would have the greatest possible change that could be given to them the difference between the length of days and nights would increase with much greater rapidity and the extremes of temperature beskeen between summer and aud winter would also be far greater on the of march the days and nights would be equal in all latitudes from that time until about the loth of may the days would iii iri our latitude increase from 12 hours to 24 while lethe the nights would decrease from to nothing from the loth of may to the 2nd and of August the sun would not set to u amt he would be seen among our circumpolar stars exhibiting the same apparent phenomena manifested by those stars about the 2nd and of august ni night lit would again set in the length of which would now increase until the of september when the days and nights would again be equal from the aist of september the length of the ul nights would increase until about the lith of november when the sun would set and alid remain below the southern horizon about eighty days or until about the of january when the day would set in being only a few minutes long at first but increasing rapidly in len ien length th until the of march when day any anT and night would again be equal thus thui if the earth revolved in an orbit whose plane was perpendicular to the plane of the equator the vicissitudes of the seasons and the variations of day and night would be such as to render our globe unfit for the habitation of man at one season of the year he wo boulde fildbe scorched not only with a vertical sun aun but edth an accumulation of heat arising from the great leng ieng length of the day while whilo at another season beason he would be exposed eto to all the severity of coia cola experienced in the polar regions I 1 if the earth should revolve around the sun biffi einy tiny other direction except the two that we have already mentioned mention edthe the of and of day ana and ana night would be proportional to the inclination of the ecliptic plane piane of the equator as the angle of increases so would the duren durer differences e n e 3 in the seve sove severity ity of the seasons increase anis inclination of the two planes is called the tho obliquity of the ecliptic which is ls is 14 about adiz 2 cosec rosec of an arc are lve kwe ive hall shall next point out some interesting phan PhIn phenomena omena connected with the earths an lotion motion or explain i what is meant by the signs of the zodiac the precession on of axes the tropical sidereal and anomalistic to years the mean and true places of the earth ear things in 16 orbit and inea mean lland iland liand and ap parent time after which ve we hail hali hall aa again recur to the phenomena of khe the season and their essect effect upon p on the earths temperature or climatic changes from the of december to the alst of june the earth pursues a direction not due east bast but nearly east oum oath east from tho the gist of june to the alst of december its direction Is nearly east north horth northeast east in december the earth as seen from the sun is in cancer while the stin sun a appears TP ears eans in capricorn it is evi evl evident dont that while the earth goes east southeast south bouth east tast from cancer to capricorn it must pass frem from the north through the equinoctial plane to 16 the south the earth crosses the equinoctial about the slat of ot march it is then in the first point of libra while the sun appears in the first point of aries when the earth goes east cast northeast north east from capricorn to cancer it the equi from south to north about the bist of september when it is as seen from the sun in the first point of aries and the sun as seen from the earth appears in the first r t point of libra As the earth goes round its annual circuit it maintains its axis parallel to itself th that t is ls Is 19 the angle of its inclination to the pine plane piane of its orbit remans the same through out an entire revolution consequently the axis will be directed towards one part particular len leu point in the infinite sphere ef f the heavens in other words if the parallel lines represented by the parallel position of the axis in every point orits of its orbit were produced to the immense distance of the starry sphere they would seem to coalesce in one point therefore the stars because of their great distance would not exhibit any ap appreciable pr parallax or displacement b by az the e earths r th ga annual annu 1 motion that is the whole w ole oie orbit of the earth if seen from the distance of the fixed stars would appear like a mere mero point subtending sub tending no apparent angle now if aline be drawn from the sun to the earth it will be perpendicular to the axis of rotation when the earth is in the vernal and autumnal equinoxes equinox es hence the days and nights will be equal at all other seasons of the year the angle augle which the radius vector makes with the axis of rotation deviates from the tho perpendicular cular this deviation on either side of the perpendicular is equal to the suns buns north or south declination when the sun is in either of the tropics the deviation i is s at its maximum and is aen then equal to the tha obliquity of the ecliptic all the variety of seasons together with the differences of the length of day and night are the results of the continual variation of this angle and the variation of the angle which the radius vector makes with the earths axis esthe is the resulted result ef the obliquity of the ecliptic combined with the parallelism of atthe the axis in different points of the orbit the ecliptic is divided into twelve parts called signs each sign therefore contains SO 30 degrees these signs are reckoned from the mhd vernal equinox and are called aries taurus gemini cancer LeoVIr leovin leo neo virgo goLIb libra scorpio sagittarius acx aquarius pisces these signs are merely names given to the subdivisions of the ecliptic commencing from the actual equinox which Is constantly shit ting its ita position in respect to the fixed stars retreating upon the ecliptic westward at the rate of about aoi sec see of an arc are per annum the signs of the ecliptic therefore must not be confounded with the constellations or certain clusters of stars which are called by the same names A little over two thousand years ago the signs of the ecliptic were coincident with the constellations that bear the same names cambs bat but during that period the actual equinox has haa receded upon the ecliptic about one full fall sign consequently the constellations of df the zodiac are about one sign in lit advance of the signs of the barne earne name marked on the eclid ecliptic to it was formerly the practice practice to reckon the longitude of the heavenly bodies by signs degrees minutes and seconds but the practice of using wing signs in the reckoning of longitudes is now being abandoned in consequence of the misunderstanding liable table to arise from confounding these signs with the constellations longitudes are now reckoned by degrees ac from 0 or the vernal equinox tb th degrees As the equinox equinoxes es recede upon the eblin ecliptic i it is evident that the earth will not perform one complete revolution as indicated by the stars when it arrives at the same equinox again ibn the amount which it clacks of one complete revolution is as before stated about sec see of a degree over this distance therefore the earth must move in order to complete one sidereal year the time of describing this are arc is hence the sidereal year is so much longer than the tropical year ear the former Is equal to d ah am ggs while the latter is equal to d ah it is during th tb e arpp ical leal and not hot the sidereal year that our seasons come round in tho the same order the longer axis I 1 of the tho elliptic orbit of the earth has bas a slow motion per annum in advance that is the peri peni perihelion belion advances eastward upon the ecliptic that much in a sidereal year this small arc are which i so much over a complete revolution must be d described bed before the earth can again reach the perihelion point of its orbit the via time occupied in so doing is aoa this added to the sidereal year gives the interval between to wo consecutive returns to the perihelion this interval is equal to ch 0 ma i t v i and la Is called the anomalistic year yean the receding of the equinoxes equinox es and advance of the perihelion upon the ecliptic are results flowing from the action or of the forces existing in the solar system and which we probably shall more fully explain lain iain should we hereafter lecture upon the law faw law of those forces we shall next explain what is meant by the geocentric and Hello reilo heliocentric centric places of a heavenly body the geocentric place Is its position as it would be seen from the centre of the earth the centre contre of the earth is chosen as a convenient point of reference because it is not affected by the diurnal rotation the Hello heliocentric centric place of a body la Is its position as seen from the sun or rather from irom the centre centro of gravity of the solar system which is situated near the centre of the S sun un this point is chosen as a convenient point of reference because it is not I 1 affected by the nor the orbi motions of or the system the geocentric position refers the situation of bodies to the great sphere of the heavens concentric with the centre of the earth the Hello heliocentric centric position refers them to the sphere of the heavens concentric to an eye situated in the centre of gravity ol 01 the system the heliocentric longitude of the earth is its angular distance subtended at the sun from the first point of aries reckoned eastward on the great circle ot of the heavens formed by the infinite prolongation of the plane of the ecliptic the hello heliocentric centric latitude of a heavenly body is its angular distance subtended at the sun reckoned either north or south of the ecliptic on circles perpendicular to that plane As the earth is situated in the plane of the ecliptic its hello helio heliocentric centric latitude is generally nothing we shall bhail now explain what is meant hy by the mean mead and true places of the earth in its orbit the mean place is the position it would occupy if it revolved with a uniform motion in a circular orbit with the sun in its centre contre then its true longitude could be calculated by the following simple proportion one year the time elapsed deg the arc are of longitude passed over from the vernal equinox but as the orbit is not circular and tua is pot described wita a uniform motion this rule will not glye give the true longitude the longitude thus obtained is called the mean longitude As the th earths orbit does not deviate to any great extent from a circle the true longitude does not differ to any great degree from irom the mean the former formen may be calculated from the latter by applying plying applying to it a correction which will bead he additive al tive tivo or subtractive subtract ve according as the earth id iii iu advance or behind its mean place the amount of or this correction is computed upon the principle of the equable He of areas about tho the sun suii in e equal ual times the area swept over b by the radius vector ector V 0 in any given time may be ascertained by the following proportion L one year the time elapsed the whole area of the ellipse the area of the sector swept over in that time and having thus obtained obtained the area of the sector bector there are various methods of obtaining the angle about the sun which this fractional area would subtend in any given en position of the ellipse by the prin principles ples pies of geometry the true longitude 0 of the barth earth could to be calculated for any given moment to save the labor of calculating tables have been formed expressing tive the difference between the tho true and mean longitude for any given time throughout the year this difference Is called the equation of the centre at the tha perihelion and aphelion points of the or orbit it the tho mean and true places will coincide from the perihelion lo 10 the ap aphelion aph eliod hellod the true place will be in advance of the mean and from the aphelion to the tho perihelion the true place will be behind the mean the greatest difference between the true and mean places amounts to I 1 deg 55 in sec see from this the difference diminished diminishes to nothing and is additive to the mean place while the earth passes from the perihelion to td the aphelion and subtractive from the mean while the earth passes from the aphelion to the perihelion it is a well known fact that the sun comes to the meridian at different different seasons of the year not exactly as indicated by a well regulated clock but some times before and at other times after janean noon if the earth revolved with a uniform motion in a circular orbit with the sun in its centre and also in a place coincident with the equator the sun would always como come to the meridian precisely at 12 but as the earths orbit is elliptical with the sun in one focus and is described with velocities lo not uniform the sun will not on account of thesa causes coincide with a truo true clock but independent e n dent of these there is another cause which wh iti idi c produces an inequality in the time of the suns coming to the meridian and that Is s the obliquity of abo ecliptic I 1 even if |