Show a I 1 HM I 1 RES ON oar ASTRONOMY I 1 BY ORSON PRATT I 1 I 1 LECTURE L EIGHTH I 1 IHfe we shall next proceed to give a brief I 1 sketch of the primary planets of the solar byg system ra the names of which we here give in id the order of their distances from the sun namely mercury venus earth mars mare iris s metis hebe As ju no ceres pallas jupiter saturn uranus and neptune of these tile the earth lidi has already been under our investigation and the principal phenomena exhibited in relation tp to its motions ofis the form of its orbit the time of its revolution ac have been explained and as I 1 there alk rc are many of these characteristics ac acte teri sties similarly manifested by all the rest of the planets it it will be unnecessary to rep repeat eat the ill illustrations us trat ions in iii each individual case of th se bodies the planet mercury ai ahe firmun order tilis this planet performs a revolution arand the sun at the mean distance of about of miles its periodic time lime is about 83 layt days its diameter diamente r is about miles its bulk is about 16 times less than alian our globe this planet is supposed to be placed the re nearest arest to the sun yet it is possible that there may be several planetary bodies stin nearer as it would bever bs very y difficult to observe such bodies even though we were assured of then their existence the bri bright I tit glare glare of the sun suit would probably shield them arom from our view view it is very likely that if such bodies existed we chuld I 1 occasionally behold them crossing like small black dots the suns dise disc but no such phenomena have been observed vee it is also als overy very probable that if such bodies existed their perturbations upon the planet mercury would be rendered visible the gravitating force of each planet affects very every other planet more or less according to the distance and quantity of matter in the disturbing planet these mutual disturbances are such as to c cause ause the planets to deviate from the true elliptic paths which they would pursue iano if no such disturbance ivance existed now the planet mercury does no not appear to be disturbed by any any planetary body within its orbit and therefore from these nega negative eive evidence s we have I 1 grounds to suppose mercury to be the nearest planet aanet to the sun the orbit of bf mercury is enclosed within that tile the earths being about 59 of miles from the path pursued by wr our globe it 11 follows lohowa therefore that the illuminated hemisphere of mercury will assume every variety of position in relation to i tile earth for mercury is an opaque tody body and only shines as it is shone ahone upon that hemisphere ivsich is turned towards the sun will be highly ill illuminated while the opposite in the dark and consequently invisible to us plow now the illuminated hemisphere will be till turned ned froin us when mercury is in that portion its orbit between the sun and earth consequently fi it will invisible be in this position the planet is r said aid to be in its inferior conjunction when the planet is in that portion of its orbit most distant from the earth eaith or when it is nearly on the opposite side odthe of the sun from us it is said to be in its superior conjunction it is s then that the sun illuminates the hemisphere of the planet which is turned towar towards ds the earth and if the bri bright ig bt glare of the sun san did not prevent us from seeing the planet its disc would appear round and full at all intermediate positions between the inferior and superior conjunctions the planet will assume every pha phase se exhibited by our moon between I 1 her conjunction and opposition ifft if it were possible bossle to perceive through tile the telescope the planet when near its inferior conjunction it would appear like a crescent becoming more and more slender the nearer it ap pro preaches aches the line between the carthand eart earth hand and sun this crescent would have precisely the shape of the new moon As the plank planet recedes from its inferior conjunction june tion so as to be in a line drawn from the th sun at alright right angles to the line of vision by which we see the planet it will then appear like a hal half filloon moon one half or of the alie enlightened hemisphere being visible and the other half invisible invisibly because it is turned from us prom from this point as it still recedes from us towards its superior conjunction a greater portion of tile the enlightened lightened e il hemisphere gradually heaves in view exhibiting a gibbous phase precisely of the shape of the moon between the full and second and 1 third quarters but as it now approaches its superior conjunction the great intensity of the suns rays will overpower the feeble rays jays of reflected light from the planet and it will be hirl bid in the gloriosa glori oua out splendor of day it is therefore only for a few days when near its greatest eastern and western elon elongation ration that it ean be easily seen with the naked eye what we mean by the planets greatest eastern and western elongation is that position in its 0 orbit abi rbi when just one half of the enlightened hemisphere is is turned towards the earth this happens happen a on an average age about six or seven times during durina the year alternately on the east and then on afie alie west of the sun the greatest distances at which the planet plane fis is sven seen on the east and wet west odthe of the sun varies front from 16 deg 12 min to 28 deg 48 min the principal cause of this variation is the great eccentricity the elliptic orbit of mercury mere ury which is nearly one fourth of the planets mean distance from the sun and as the sun is situated in one focus of alie ellipse it is evident that when tile earth is in a line at right angles to the fijor axis of mere Merc orbit tile the elongations which happen at that time near tile the perihelion will be only a little over one half what they would be at or near the aphelion point of the orbit I 1 when the planet is at its greatest eastern elonga tion it will be seen in the west just after sunset ana iiii is at its grea greatest western elongation it will be seen in the jn orning just before su sunrise hrize when the planet is see seen i n with the naked eye it exhibits a 4 very tery brilliant white light like venus only much smaller the bet best or most favorable seasons of tile the year to view this thin planet is when its greatest elongations happen in the months of march or april or in august or september the further you recede from f ro in the orbit of mercury the more difficult will it lie be to perceive this planet it is extremely probable that the inhabitants on the most distant planets of our system have never been f favored with a view of mercury and are it is presumed altogether ignorant of its existence unless they have seen it apparently crossing the suns disc like a small black point nt for in al all other lother positions it would be overwhelmed in the brightness of the solar rays we have already observed that mercury revolves revolver around arnd the chesun sun in alouise ah ouISE day therefore is the length of one year to the inhabitants of that planet each of the four seasons will alternately take place in thi the short period of 22 days during the time that mercury performs one com complete revolution around the sun the earth performs about 0 one ne quarter I 1 of its revolution hence if they both set out together from the inferior conjunction that planet has to describe one complete revolution and about ore third of another in in order to bring itself again into conjunction with the sun and earth this requires a period of about dalti and is called the period in this period happens all the phases which have been d described the real time of one complete revolution is is called the sidereal period and nd is exactly equal to 87 days 23 hours I 1 15 mm min 43 9 sec if the orbit of mercury were in tu the same plane as that of the earth at each inferior conjunction that planet would through the telescope be observed to enter the suns disc on the eastern limb and pass across the same to the western limb being observed as a small email dark spot from five to seven seve n hours to traverse tT averse the disc but as the orbit of f mercury instead of being coincident with the plane of tile the faiths orbit is considerably inclined to that plane namely 7 deg om oin 91 s the transits transita tran sits do not happen only at intervals of several years the transits transita tran I 1 sits of f mercury that will harpen happen curing the present century I 1 will take place as follows I 1 HOURS I 1 I 1 vir visible sible 1861 nov 12 11 68 52 A M in vi sae 1868 nov 4 11 KF 16 P M visible 1878 may 6 1 I 11 I 1 10 A M visible tra for a short time 1881 nov 7 5 12 P F M 7 visible for a short shori time timea 1891 may aly 9 7 1 17 P F M vi visible I 1 i si ble 1894 N nov ov 10 10 49 AM I 1 the time of them the transits traits tran sits here e r e expressed Js is the mean civil time at t great salt lake city which place we have assur assumed qed to be 7 hours and 2 minutes west of Greenw greenwich ieh we have only given the I 1 I 1 time in in which they will be seen ill in the middle of thear paths path at greenwich they will be seen nearly 1 in in the same position on the suns disc from this w city as from Gre greenwich enwia the few seconds deviation I 1 willbe principally owing to the parallax these transita transits will require the telescope peto to render them visible owin owing ij to Merc dazzling appearance it is is ve very ry d it to discover any pron lluent marks upon its surface yet schroeter a german astronomer has been enabled not only to discern spots but mountains also upon its its disc tile the height of vivo two of these mountains lie he has calculated one of which is about 1 14 1 4 milos high the other about 10 31 34 3 1 miles or about 8 tunes times higher than the twin peas peaks on the east valley The highest mountains on mercury are said to be situated in in the southern b hemisphere emi sphere of that planet when the horns or cusps of this planet are carl carefully observed at the time when it appears of a crescent crecent form it is is that they vary from day to day by these variations the time of the rotation of the planet is is determined to be 24 houi bourse 5 minutes and 28 seconds henre i nce the length of days and nights upon that planet do not nal dier anly by a very small liall si frac fraction tiolA from those enjoyed laipoo our gl globe obe I 1 z tile the intensity of light which mercury enjoys anjo ys is is far greater than alian what is is enjoyed on the surface of the earth earli it can easily be demonstrated I 1 that the intensity of light varies inverse inversely ay as the square 0 Df o the distance therefore if we divide the square of the earths earth s distance from the sun by the square of Merc distance the quotient m will ill be about 6 23 don consequently the intensity of light upon that planet is is about 6 23 2 3 times greater th than an upon le aarti ear ti As we the apparent disc of the he sun san is assolin proportion to the square of the distance it foll follows that the orb of day will appear nearly 7 times greater to the inhabitants of mercury than what he appears to us CAn consider isider for a moments moment the mountains and vallies and al all I the objects wite with which w we are surrounded 1 with a seven fold fold splendor such a brilli brilliancy aney would be far too great for the present constitution of our eyes if the he pupils pupil of our eyes were contracted to about one s seventh part of their present dimensions we should still beable be able under tile the influence of a seven fold intensity of f light to perceive every object with the same ease and distinctness that we do now tie the splendor odthe scenery upon mercury must bo be magnificently grand the vividness of colors radiated from surrounding objects must be exquisitely beautiful and the whole landscape must be adorned as if with a gorgeous robe of light while mercury enjoys 6 23 2 3 times more light than the earth the planet neptune only enjoys the 1900 1 part as much as the earth for Nep neptune tulie is about 30 times limes the distance of the earth from the sun the thea square of 30 is hence the intensity of the suns light on neptune tune isoo times less than what we receive multiplied by 6 28 20 is equal to therefore the lighton light on mercury is is times greater than the light on neptune Nep fune the sun at the distance of the outermost planet discovered in the system will subtend an angle of I 1 deg 4 see sec and conse i quent lj will only appear about tile size of the planet venus when n nearest to the earth or in other words the suns apparent diameter will be only about one half greater than the pl plane amet Ju when in opposition many have heat to klow follow the same law as light ir if so ne neptune lilune would have times less heat from fram the su sun n than what we experience and the n bathe of the suns suns heating power at the extremities es of our system would be as I 1 to but independent of f the heating power althe of the sun the planetary spaces and worlds no doubt have a natural temperature of their own modified more or less by the heat odthe sun according to their proximity to that great luminary one cause of the common temperature which we have great reason to believe e exists in the celestial regions is the combination or united heat emanating from the fixed stars which are known to be great suns similar to our own I 1 heat is no doubt generated or rather set free by the chemical action of the materials of which the planets consist prom from these two sources it is very probable that the planets are maintained at a temperature pera ture far greater than what they would enjoy if they were depe dependent n dent upon pon u the sun alone tle the amount of common temperature existing in the celestial re giona g bolls is very likely in proportion to the alno amount un t of star light if if so it is comparatively easy to calculate the amount or degreeff degree of this temperature pera ture this has been calculated by a great number of different methods and they all concur in showing that it does not differ much from 56 58 degrees below the zero of scale or about 90 degrees below the freezing point of water now this is a degree of cold much less than what we are capable of producing artificially such a common temperature would operate to greatly diminish the cold that would otherwise exist in the more distant extreme extremities ext remi ties of our salol solar ar system As mercury is alie nearest ne rest planet to the sun its velocity according carding ac to the law of mechanics governing centrifugal c P forces is greater than that of any other planet pla net its average a v velocity is equal to about miles every hour but as its orbit is much more cc eccentric centric than the earth s its velocity I 1 varies adrie from the mean to a much greater extent jhan than that of the earths ats average velocity is 1830 miles every and over 30 miles ev every ery second the density of mercury is about 6 times greater than water or about tile the density of lead ore that is a globe of lead ore miles in in diameter i would just balance the planet mercury this is is considerably greater than the density of the je bearth arthmand arth and jand greater than the density of any other planet in the system the mass of tifis planet is times less than that of the suns but as the materials of which mercury consists is much heavier than tle the suns stills materials it its bulk is about 4 1 2 times less les s than it would be if composed of materials si similar milar to the suns consequently it would require about 23 22 of globes of the size of mercury to com compose liose one as large as the sun the weight of bodi bodies on the surf |