390 KANSAS CITY REVIEW OF SCIENCE. 



the transit of Venus over the face of the Sun is certainly the most celebrated, 

 and, while lacking some elements of accuracy, and requiring a considerable ex- 

 penditure of time and money, is a method not to be despised. 



Such a transit will take place on the 6th day of next December, and it may 

 be interesting to know beforehand what to expect and when. This phenomenon 

 recurs regularly four times every 243 years, at intervals of 8 years, 105 ^^ years, 

 8 years, and 121^ years. Here are the dates for four centuries: 



1761 June 5th. 2004 June 8th. 



1769 June 3rd. 2012 June 6th. 



1874 . . December 9th. 2117 . . December nth. 



1882 . . December 6th. 2125 . , December 8th. 



From this we see that, if the coming transit is not watched, it will be as im- 

 possible for one of us to see the phenomenon again as it is to examine the geolog- 

 ical structure of the other side of the moon. The next transit will be reserved for 

 the third generation to come. 



It is not a difficult matter to understand the outline of the method that de- 

 duces the distance between the Earth and Sun from the observations of a transit. 

 Suppose two observers, one at New York and the other at Cape Horn, and each 

 provided with the necessary instruments. Since Venus is between the Sun and 

 the Earth, it is evident that the path of the planet across the Sun's disc, as seen 

 by the observer in New York, will be nearer the southern edge of the Sun, and 

 therefore shorter, than the similar path as seen by the observer at Cape Horn. 

 Knowing the time that it takes Venus to travel each of these apparent paths, the 

 rate with which the planet moves with reference to the Sun, and the apparent 

 size of the Sun, the lengths of these two apparent paths and their distance apart 

 are readily found. All these measurements are, of course, in seconds of arc. 

 Now the distance between the two paths is, also, 2.61 times the distance be- 

 tween New York and Cape Horn, since Venus is 0.723 of the Earth's distance 

 from the Sun. Knowing, then, the length of a certain line on the Sun both in 

 seconds of arc and in miles, the distance between the Sun and the Earth is the 

 immediate result. Previous to 1874 but three transits had been observed, and 

 the results were far from satisfactory. The uncertainty of the result, as estimated 

 at the time, was only nine-tenths of one per cent, and though this amount of 

 error is comparatively small, it really means about 840,000 miles. Combining 

 the results from the transit of Venus in 1874 with the best results from the other 

 methods the probable error is reduced to one tenth of one per cent, but even 

 this small fraction amounts to more than 90,000 miles. This amount of uncer- 

 tainty will, for instance, produce an uncertainty of 20,000,000 miles in our estimate 

 of the distance of the nearest fixed star. With this comparatively small but es- 

 sentially great error still connected with the fundamental unit, it behooves the as- 

 tronomer to improve his methods and make the determination more exact. 



In 1874, neither labor nor expense were spared to make the work as thorough 

 as possible. In that year the transit could be seen only from places in the eastern 



