452 



KNOWLEDGE 



[Dec. 8, 



lines <i ti' aiid h i', aloDg which her centre is travelling, as 

 wat«.-hcd by these two observers, is known quite certainly 

 to bo IS.dOO miles, if the observers' stations arc 7,000 

 miles apart in a north-and-south direction (measured in a 

 straight line). Thence the diameter, S S', of the sun is 

 determined, l>ecause it is observed tliat the known distance, 

 <i ft, is such and such a part of it. And the real diameter 

 in miles being known, the distance must be 107 times as 

 great, because the sun looks as large as any globe would look 

 which is removed to a distance exceeding its own diameter 

 (great or small) 10" times. 



But unfortunately it is no easy matter to get the dis- 

 tance, <i l), Fig. C, determinetl in this simple manner. 

 The distance, If.OOO miles, is known ; but the ditticiilty 

 is to determine what proportion the distance bears to 

 the diameter of the sun, S S'. All that we have heard 

 about llalley's method and Delisle's method relates only 

 to the contri\nnccs devised by astronomers to get over 

 this dillk-ulty. It is manifest that the difficulty is very 



For, first, the observers would be several thousand miles 

 apart How, then, are tliey to ensure that their observa- 

 tions shall be made simultaneously 1 Again, the distance 

 a l> is really a very minute quantity, and a very slight 

 mistake in observation would cause a very great mistake 

 in the measurenient of the sun's distance. Accordingly, 

 Halhy de\-ised a plan by which one observer in the north 

 (orasat A, Fig. o) would watch Venus as she traversed 

 the sun's face along a lower path, as a n'. Fig. 7 ; while 

 another in the south (or as at 15, Fig. 5) would watch her 

 U.S she traversed a higher path, as l> U, Fig. 7. I!y timing 

 her they could tell how long these paths were, and there- 

 fore how placid on the suns face, a.s in Fig. 7 ; that is, 

 how far apart, which is the same thing as determining // o, 

 Fig. C. This was Halley's plan, and as it requires that 

 the duration of the transit should be timed, it is called 

 the method of duration.<L Dcli.sle proposed another 

 method— viz., that one observer ihould time the e.\utt 

 moment when Venus, seen from one station, iiija,, to 

 tra\ erw^ the path " "', while another should time the exai t 

 moment when she t,.,,,n, to traverse the path h h' ; this 

 would thow how nmch I, is in advance of ri, and thence 

 the position of the two paths can be determined. Or two 

 obvrveni might not« the fnd of the transit, thus finding 

 how much «' is in advance of //'. This ia Delisle's method, 

 and it has this advantage over Halley's— that an observer 

 in only rfv|uind to 8wj cit/..r the Ijeginning or the end of 

 the trannit, not lio//i. 



I hhall not here consider, except in a general way, the 

 various astronomical conditions wliich affect the applica- 

 tion of thr«e two methods. Of course, all the time that 

 a tran*it laitt*, the earth is turning on her axis; and as a 

 transit may lost as long as eight hours, and generally lasts 



from four to si.v hours, it is clear that the face of tlio 

 earth turned towards the sun must change considerably 

 between the beginning and end of a transit So that 

 llalley's method, which requires that the whole duration 

 of a transit should bo scon, is hampered with the difliculty 

 arising from the fact that a station exceedingly well placed 

 for observing the beginning of the transit might bo very ill 

 placed for observing the end, and riv vrnd 



Delisle's method is free from this objection, because an 

 observer has only to note tho beginning or the end, not 

 both. But it is hampered by another. Two observers 

 who employ llalley's method have each of them only to 

 consider how long the passage of Venus across tho sun's 

 face lantn : and they are so free from all occasion to know 

 the exact time at which the transit begins and ends, that 

 theoretically each observer might use such an instrument 

 as a stop-watch, setting it going (right or wrong as to thi- 

 time it showed) when the transit began, and stopping it 

 when the transit was over. ISut for Delisle's method this, 

 rough-and-ready metliod would not serve. The two ob 

 servers have to compare the two moments at which they 

 severally saw the transit begin— and to do this, being many 

 thousand miles apart, they must know tho exact time. 

 Suppose they each had a chrononietor which had originally 

 been set to Greenwich time, and which, being excellently 

 constructed and carefully watched, might be trusted to 

 show exact Greenwich time, even though several months 

 had elapsed since it was set Then all tho requirements of 

 the method would lie quite as well satisfied as those of thr 

 other method would be, if the stopwatches just spoken of 

 went at a perfectly true rate during the hours that the 

 transit lasted. But it is one thing to construct a time- 

 measure which will not lose or gain a few seconds in a few 

 hours, and quite another to construct one which will not 

 lose or gain a few seconds in a journey of many thou.sancl 

 miles, followed, perhaps, by two or three months' stay at 

 tho selected station. An error of five seconds would bo 

 perfectly fatal in applying Delisle's method, and no chro- 

 nometer could be trusted under the conditions described to 

 show true time within ton or twelve seconds. Hcnc(f 

 astronomers had to provide for other methods of getting 

 true time (say GrecMiwich time) than the use of chrono 

 meters ; and on tho accuracy of these astronomical method."* 

 of getting true time depended the successful use of Delislc'."- 

 method. 



,g. y. 



Then another difficulty had to bo considered, which 

 affected both methods. ]t was agreed by both Halley and 

 Delisle that the proper moment to tin)e the beginning or 

 end of transit was the instant when Venus was just 

 within the sun's disc, as in P'ig. >*, either having just com- 

 plet<'d her entry, or being just about to begin to pass ofi" 

 the sun's face. If at this moment Venus presented ft 

 neatly-defined round disc, exactly touching the edge of the 

 sun, also neatly defined, this j)lan would be perfect. At 

 the very instant when the contact ceased at the entry of 

 Venus, the sun's light would break thron;(h between the 



