Junk 1. 1900.] 



KNOWLEDGE. 



133 



had their attcutiou drawn to the fact that as seen 

 from some given station the suu rose and set behind 

 different portions of the horizon at different times of the 

 yeai'. In an open country free from mist« aiid ground 

 fogs, this observation would be one of great<?r delicacy 

 than might be expected — a delicacy the greater accord- 

 ing to the distance of the horizon and to the number 

 and distinctness of the objects which could be recognised 

 upon it. They would serve the purpose — so to speak — 

 of the divisions of a gigantic azimuth circle, and a few 

 years" cai'eful and sedulous record of the exact position 

 of the sun at rising and setting would give an ex- 

 ceedingly close determination of the true length of the 

 tropicaJ year. 



They would do more than this. They would give the 

 means of determining the south point of the horizon — in 

 other words of the meridian line. A lino drawn at right 

 angles to the line joining the point of rising and setting, 

 would be roiif/Iih/ but not precisely the meridian line. 

 But the mean of all the points this indicated as due 

 south would, unlsss the horizon were much more ob- 

 structed on one side than the other, approximate very 

 closely indeed to the true south point. 



The conditions for different observei-s will vary so 

 widely that it would be useless to give detailed directions 

 as to making this observation, and it would be u.5eless 

 for another reason. It is most important that those 

 who take up the pursuit of naked eye Astronomy should 

 make their observations independently, and too detailed 

 instruction beforehand would defeat the very object 

 for which those observations were made. 



It would soon be felt that the natural horizon was a 

 rough and inconvenient instrument to work with. The 

 objects ranged along it which serve as division marks 

 are apt to be irregular, the horizon itself to deviate 

 very considerably from an ideal plane. So perhaps the 

 next step in the obsei-vation of the sun would be the 

 erection of some means of observing the shadow it casts 

 — in other words a simple sun-dial. 



It is probable that the earliest sun-dial was simply the 

 spear of some nomad chief, stuck upright in the ground 

 before his tent. Amongst those desert wanderers, keen 

 to observe their surroundings, it would not be a difScult 

 thing to notice that the shadow shortened as the sun 

 rose higher in the sky, and that the shortest shadow 

 always pointed in the same direction — north. Ths re- 

 cognition would have followed very soon that this noon- 

 day shadow changed in its length from day to day. 

 A sLx-foot spear would give a shadow at noonday in 

 latitude 40° of 12 feet at one time of the year, of less 

 than 2 feet at another. This instniment, so simple, 

 so easily carried, so easily set up, may well have begun 

 the scientific study of Astronomy, for it lent itself to 

 measurement, and science is measurement; and pro- 

 bably we see it expressed in permanent form in the 

 obelisks of Egyptian solar temples, though these no 

 doubt were retained merely as solar emblems ages after 

 their use as actual instruments of observation had ceased. 

 An upright stick, carefully plumbed, standing on some 

 level surface, may therefore well make the fii'st advance 

 upon the natural horizon. A knob at the top of the 

 stick will be found to render the shadow more easily 

 observed. 



The careful study of this instrument will enable the 

 meridian line to be marked with some considerable 

 exactness. This should be done by taking an observation 

 at some time in the morning, a good while before noon, 

 drawing a circle with the base of the stick as centre, 

 and the length of the shadow as radius, and then in the 



afternoon watching till the tip of the shadow again 

 lengthens itself to exactly reach the circle. We shall 

 lind the north point lie midway between the two 

 positions of the shadow. Here again we must trust not 

 one observation but many, and the mean will give us a 

 very close approximation to the true meridian. 



The date of the summer or of the winter solstice would 

 not be very readily ascertained from such an instrument 

 — the very word solstice intimating that the change in the 

 sun's }K)sition at that season is scarcely perceptible. But 

 the time of the equinoxes can be fixed with sufficient 

 exactness, since the length of tho noonday shadow of a 

 six-foot rod will vary in our latitude more than an inch a 

 day at that time of the year. 



A far cxacter instrument for the observation of the 

 sun can bo made with the very slightest trouble; a 

 light tube, f) feet 4 inches long, made cither of tin or of 

 piusteboard, and covered at one end wit,h a cardboard 

 disc, with a pinhole one-sixteenth of an inch in diameter, 

 carefully perforaUxl in its centre, and at the other with 

 a cap of oiled paper, will enable the sun to be observed 

 with great ease. If this tube is directed t,o the sun an 

 image of the sun will be formed by the pinhole on the 

 oiled paper some six-tenths of an inch in diameter, and 

 if a cardboard disc some ten or twelve inches in diameter 

 is fixed to the tube— the tube passing through the centre 

 of it — so as to screen the observer from the rays of the 

 sun, he will find the sun's image on the oiled paper quite 

 bright enough to observe, and much better defined than 

 the shadow given him by tho rod. 



The next step would be to fit the tube with a 

 graduated circle. The mateinal of which tho circle 

 should be made and the manner in which it should be 

 graduated may be left to the ingenuity of the student. 

 Protractors of horn, metal, glass, or card can be very 

 easily purchased and may well serve the jourpose. The 

 reading of the circle may be accomplished in one or two 

 ways ; the circle may be fixed firmly to the telescope 

 so as to turn with it, and the altitude of tho tube 

 may then be read by a plumb-line dropped from 

 the centre of the circle across its circumference; or the 

 circle may itself be fixed in one position with respect to 

 the vertical, and the tube may be turned round upon 

 the same centre as that of the circle. In this case the 

 tube should be supplied with pointers to read on the 

 circle. 



The tube being provided by a vertical circle and con- 

 structed so as to turn in a vertical plane, should also 

 have its stand so arranged that it may turn iu a hori- 

 zontal plane also, and it should be fitted with a second 

 ciixle, the centre of which is the pivot on which it turns. 

 This circle must be fixed iu the horizontal plane, and 

 our instiTjment will then be a rough model of an 

 altazimuth. 



Its fir.st u.sc will be to determine the meridian — by 

 taking an observation in the morning reading botli 

 circles — then in the afternoon, waiting until the sua 

 had descended to tho same altitude a second time, and 

 then reading the azimuth circle again. To set the 

 telescope to the azimuth midway between these two 

 azimuths would be to set it roughly in tho meridian. 

 Here again the observations should be repeated many 

 times, and the mean should bo taken as the true south 

 point. 



The south point once found, the observation of the 

 varying altitude of the sun at noon from day to day 

 throughout the year would be a simple and easy matter. 

 At midsummer and midwinter the meridian altitude of 

 the sun will not vary perceptibly for a fortuigu* or 



