Show I LECTURES ON astronomy my SY BY PROF PRATT LECTURE SEVENTH I 1 Is the sun in motion or at rest is a question frequently asked by the inquisitive mind of man we have already shown in our former lectures that the apparent diurnal and annual motions of the sun are not real but arise from the real motions of the earth upon its axis and in its orbit so far as we have been able to extend our observations the planetary bodies of this system have bave a rotation upon an axis this seems to be indispensably necessary in order that different skies sides of 0 those bodies may alternately be exposed to the heat beat and light of the sun but the sun bun being the great source of heat and light does not apparently stand iii need of either heat beat or light from any external source and therefore it would seem altogether unnecessary for it to have a rotation upon an axis if no beneficial object is perceived to result from a rotation of the sun we might he be led to con conclude clade in the absence of any direct evidence wr nce that the swa sun was really quiescent in the centre odthe of the system I 1 dot bat when we examine the sun by the aid of a I 1 telescope we discover upon its disc many large spots which appear dark and when watched from day to day they appear to move across the disc from east to west when first discovered near the eastern east eriv limb their velocity is very slaw but in creases creaser as they approach the middle of their paths at t which place the velocity arrives at its maximum value after which the velocity gradually decreases until near the western limb when the spots pear peal and aad remain invisible some 12 then them again reappear re appear near the eastern limb describing the same paths as before were these appearances produced by dark bodies intervening between the earth and sun san having a motion independent of that luminary then the velocity would be uniform but the variable velocity observed vedin in the description of their apparent paths ia is precisely imprecisely the same as it would be if those spots were a part of the murfa surface og of the sun itself rotating upon an axis from west th fe east with such a rotation it is evident that the side of the sun turned towards the earth would have a movement apparently from east to west while the most distant hemisphere which is turned from us and consequently invisible would have a motion from west to east if these spots appertain to the surface of the son sen they will each describe a circle parallel to the suns equator and if this eI equator were coincident with the plane of the ecliptic each of these circles would at all times appear like straight lines on the suns disc but if the plane of the suns equator be inclined to the plane of the ecliptic the circles described by these spots pots will by the effect of perspective appear elliptic one half of the year these apparent semi ellipses will present their convex sides towards the upper limb of the sun and the other half odthe of the year their convexity will be presented towards the lower limb these appearances are produced by the relative positions of the suns axis to the earth in different points of its orbit these ellipses will at certain seasons of the year appear much more elongated than at other seasons this also depends upon the relative position of the axis of the sun to 01 the earth in different portions of its orbit there are two days in a year when the axis of the sun is at right angles to the line of vision namely the of june and the of december at these tivo tiro periods period the spots on the sun appear to describe straight linee line after the of december the apparent paths odthe of the spots begin to deviate frona from a straight line the convexity being towards the north or towards the upper limb of the sun this deviation from a straight line will continue to increase more and more the I 1 oth of march when ib will attain to its greatest value from the of mire marchy the curvature of these ellipses will begin to decrease and continue decreasing until the alth of june Nine ahem the spots will again apparently de scrib a straight strai gM line after which they will again deviate but butin in a contrary direction the convexity being towards the southy south or er in other words towards the suns lower limby limb the thae curvature will i increase nerease in more 0 re and more until the of september when it will become greatest then again receding until the of december when their apparent paths will again be straight lines thus it will be seen that on the of december the earth passes from the northern to the southern side aide of the plante of the suns equator while a spot on the suns equator at the same time appears to ascend from the southern to the northern corthern side of the plane of the ecliptic this is called the ascending nodle node of the suns equator the heliocentric longitude of the ascending node is 80 deg 21 roin min on the bilth of june the earth passes from the southern to the northern side ide of the plane of the suns equator while a spot on that equator appears to descend from the northern to the southern side of the plane of the ecliptic this is called the descending sc sr nods node of the suns equator and is situated directly opposite the other node deg further east the descending node therefore lies deg belio heliocentric centric longitude if the shorter semi axis of these elliptic paths be accurately measured with a micrometer say f for orin instance when the earth earths is 90 deg distant from either of the aforementioned nodes or on the of march or esth of september it will be an easy matter to determine from these measurements the inclination i of the plane of the suns equator to the plane of the ecliptic this is found to be about 7 deg 20 min and the period of rotation 25 days 7 hours and 48 minutes this period does not bring the same spots in same relative position in regard to the wa sun and earth that they occupied at the commencement men ment cement ement of such period for the earth during I 1 tins this period advances in its orbit over a space equal to about 1 14 part of the whole circumference consequently the sun has to perform more than one complete rotation before the same spots are brought round to the samp same relative posit position lon that they occupied at the commencement of the period this period which may be termed the apparent period is about 2 days longer than the real period of the rotation the distance around the sun being about miles a spot on the suns equator must move with a vel velocity of about miles per hour this is over 4 times swifter than the earths equator moves by its rotation the rotation of the sun generates a centrifugal force at its equator about 16 1 6 of the centrifugal tri fugal force generated at the equator of the earth by Us its rotation cn in 14 a former lecture we proved that the centrifugal force at the earths equator is about 1 29 part of the earth s gravity hence the centrifugal force at the suns equator is only about one sixth of this fraction or 1 1734 part odthe of the earths gravity but in the sixth lecture we proved that the earths eartha gravity is times less than the suns gravity therefore the centrifugal force at the suns equator is isa 1 part of the suns gravity A body will fall fail at the suns equator about 49 I 1 inches in hi a second and if the sun had no rotation it would fall about 19 1 9 of an inch further A clock pendulum which will vibrate seconds here on the earth would if carried to the surface of the sun vibrate over five times more rapidly that is a pendulum of the same length would make vibrations at the surface of the sun in the ahe same time that it would make MOO 1000 vibrations at the surface of the earth if the suns sons velocity should beco become me about times more rapid than it it is at the present bodies would have no weight at the sun s equator and a complete rotation would be accomplished in about 2 29 hours and 46 minutes it is probable that the sun is not a perfect sphere an a it would be had it no rotation but its deviation from the spherical form is very much less than that of the earth for the centrifugal force of the suns equator eau el pator ator com compared with the force of gravity at his stir surface flace is times less than the centrifugal force of the th earths equa equator ter compared with the force of gravity at its surface therefore the force exerted to alter the spherical form of the sun is times less than the force exerted upon the earth to alter its spherical form hence the form of the earth is much more oblate than that of the suns although the sun is by far the largest body of our system yet we are comparatively ignorant ignorant odthe of the nature of his vast luminary some have v e supposed it to be a tast avast globe of fire but when viewed through a telescope as we have already observed vedi large dark akots are seen upon the suns disc I 1 I 1 these spots are of different magnitudes some are over miles in diameter while others sw am barely visible being not over miles in im diameter the spots appear to be perfectly dark surrounded with a border 0 or penumbra of a fainter shade this penumbra is often of the same shape as asi the central dark spot and frequently embraces several thousand miles of area or space the shade of the penumbra wears a uniform aspect throughout its whole extent sometimes several dark spots of different sizes are embraced within the limits of the same penumbra pen umbra the number of spots w is continually varying sometimes there are none to be seen sometimes only two or ahme and at other times limes several hundreds daring the last century and a half more or less spots have been seen every year but they are far mow numerous some years than in others these spots are continually changing their aspects this arises from two causes one of which is real the other only apparent the real changes may be described as follows first the they yape sae observed to expand or contract in their dimensions when they first make their appearance pea pe arance ranee the dark nucleus and the penumbra surrounding it are so ao small a as to be barely visible these gradually expand from day to day some with much greater velocity than others until they attain to various degrees of magnitude from a thousand to forty or fifty thousand miles in diameter again they are observed to contract the sides approaching until from their smallness they are rendered invisible that these changes axe are not the effects of perspective occasioned by the relative positions of the spots on the surface of the rotating globe of the sun is demonstrated from the fact that in the same regions some spots are seen in the act of enlarging while others are diminishing this could not happen from a change of their relative positions sit ions for in such cases the relative positions remain the same therefore the changes must mast be real the time occupied in the expansions and contractions of these spots follows no regular law some have been seen to arise and vanish in less than one day others continue for six weeks but bat it seldom happens that they continue longer than this though in some rare ia stancA they have been known to continue for several months second it is also observed as a general thing that when the expansions are gradual the contractions are also gradual and when they enlarge suddenly they diminish suddenly these spots sometimes break in pieces and the fragments recede from each other with very great velocity the movements exhibited ty by the expansions contractions and the receding of oath the fragments in case the spots become broken are carried on upon the grandest scale A single second of singular measure upon the suns disc as seen from the earth would include an extent of miles A circular spot therefore of this diameter would embrace an area of about square miles which is the least space that is distinctly visible on that distant orb but spots have been observed embracing an area of about of miles whose diameters were over miles now such spots have been observed to close up and disappear in the short space of 2 or 3 weeks the borders therefore of such spots must approach each other with a velocity of between one and two thousand tho asand miles per day sometimes very large spots have closed up within the short period of one day their velocity therefore must have been equal to or a thousand mires miles hour third besides the changes observed in the dimensions men of the spots themselves it is also observed that they are not stationery station itry try on the suns disc but travel from one region to another with prodigious velocity as we have already remarked in in regard to the fragments of broken spots receding from each other in every variety of direction but unbroken spots instead of moving 1 indiscriminately in all directions seem to follow a more regular law I 1 in their movements those on OB each side of the s suns un s equator in most instances gradually move towards the nearest pole with a velocity of four or five thousand miles per day fourth these spots are subject to numerous changes of form as well as of magnitude these forms are constantly changing sometimes being nearly circular sometimes of an oval shape at other times presenting a variety of irre irregular 9 ular figures the exterior boundary of the penumbra always presents a appearance instead of sharp angles and straight lines while the borders I 1 of the dark nucleus assume every variety of shape when the penumbra is in the act of encroaching upon the nucleus it sometimes divides the ducle nucleus us into two or more parts and when the nucleus disappears the penumbra remains visible for a short period and then vanishes these spots do not appear on all parts of the globe of the sun but are limited generally speaking to a one ione extending from 30 deg to 40 deg each side of his equator spots are rarely seen beyond these limits in the polar regions they are never seen the equatorial belt or zone is less frequently visited by these spots than the adjacent zones situated a few degrees to the north and south of the equator the zone in the northern hemisphere comprised com prized between the and degrees is the most fertile in the production of spots their numbers and magnitudes being greater than in the southern hemisphere another circumstance worthy of remark is the arrangement of these spots it is observed that when the spots are numerous they frequently arrange themselves in belts or zones parallel to the dans equator we will next explain the apparent changes which these spots exhibit arising from the rotation of the sun if a large well defined spot he be observed throughout its passage across the suns disc it will exhibit the following phases first when it is in in the centre of the disc or in in the middle of its path the whole encircling penumbra b a and the central dark nucleus will be distinctly visible second as the spot approaches the western limb the penumbra on the side nearest to the e eye y e of the observer will gradually become more and more narrow in the direction of the line of vision while the penumbra on the opposite side aide of the nucleus from the eye will apparently grow broader and broader at length as the spot arrives near the western limb the eastern part of the penumbra as well as the dark spot entirely disappears while the western or more distant portion of the penumbra still remains remains visible until sear near the time of its pissing passing the edge of the limb I 1 third after the spot is ta ar arid ried d louvon the ID lop P cosite and invisible hemisphere by the suns rotation it will re appeal reappear near the edge of the eastern limb in this position the eastern portion of the penumbra will be seen first then the dark nucleus then the narrow portion of thew the western estem or nearest side of the penumbra which as the spot |