154 



KNOWLEDGE. 



[July 1, 1895. 



\^ 



iK 





Tonigbt the first two are, comparatively speaking, close 

 together and Venus a long way off." 



In Figs. 1 and 2 are shown configurations of Venus, 

 ■Jupiter, and Saturn seen on the evenings of February 23rd 

 and March 2nd, 1881, adjusted for N. latitude 36°. This 

 configuration was very noticeable on account 

 of the brightness of the planets. 



In 1885 a fine conjunction was witnessed ^ 



in Bechuanaland, viz., Mercury, Venus, \ 



Jupiter, and Regiilus. The scale is too 

 large to bring in Jupiter, but Fig. 3 shows 

 the other three objects visible on July 

 16th. 



Fig. 4 shows the same three planets and 

 star (for the same latitude) as visible on 

 July Slst, 1826, the positions being laid 

 down from data in the Xautictil Ahiianin- 

 for that year. 



1882, September 20Lh, there was a pretty 

 combination of Spica, Mercury, and Mars. '■ 

 The first two were in the field of the 

 telescope (a 2| inch refractor) and presented 

 a beautiful contrast : Spica being a steely 

 blue and INlercury a rich orange-red colour. 

 Mercury by far the brighter, both in the 

 telescope and as seen with naked eye. 

 Mars a deeper red than IMercury. 



1882, October 16th. Venus and Antares 

 were visible together in the telescope. 

 Fine contrast between the ruddy hue of the 

 star and the whiteness of the planet. 



Many English readers will probably remember the con- 

 junction of November 3rd, 1877, when Mars and Saturn 

 were in the same field ; also that interesting spectacle of 

 Jupiter and Venus seen so close together in February, 

 1892. 



Suppose the five principal planets. Mercury, \'enus. 



Mars, Jupiter, and Saturn, arranged in a straight line, 



each being 1° (say), from its neighbour. These can be 



arranged in one hundred and twenty different ways. 



Kow vary the distance in R.A. and introduce tlie element 



of declination, i.e., extend the line into a belt 47° wide, 



and we increase enormously the possible combinations. 



Throw in one fixed star, such as Antares or Eegulus, and 



leave out one or two planets, and 



^k. it is evident that we only require 



-<"'" sufticient time to get myriads of 



tnura configurations — no two of which 



would ever be exactly alike. 



Of course, the conjunction or 



near approach of five principal 



planets, as seen from the earth, 



is a rare event in the history of 



the solar system. 



Tliis subject is not usually 

 much dwelt on in text-books, 

 but Chambers gives some in- 

 teresting examples, from which 

 the following are taken. 



From the Chinese records it 

 is slated that Mars, Jupiter, 

 Saturn, and Mercury were in ccnjuuction, in tlie constella- 

 tion Sbi, for which the dates February 28th. 2446 u c, and 

 Ftbruaiy [Ith, 2141 is.c. have been assigned by two 

 autliorities. 



On September 15th, lls6. Mercury, Venus, Mars, Jupiter, 

 and Saturn were in conjunciion between the wheat tar of 

 Virgo and Libia. This must have b^en a wonderful sigLt, 

 if It occurred at a sufficient distance from the sun to enable 



IkcTXury 





all these planets to have been seen at once. It would 

 apparently have been an evening 

 configuration, and any historical -ijt- 



references to this, if they exist, would *"«» 



be very interesting. 



On November 11th, 1524, Venus, 

 Jupiter, Mars, and Saturn were very 

 close to each other, and Mercury only ♦ 

 16= distant. 



Exactly twenty years later, on 

 November 11th, 1544, Venus, Jupiter, 

 Mercury, and Saturn were included in 

 a space of 10°. 



On March 17th, 1725, \'euus, 

 Jupiter, Mars, and Mercury were in 

 the same field of the telescope — a truly 

 remarkable assemblage. 



It will be seen that I have touched 

 but lightly on a wide subject. For 

 the mathematician and computer 

 there is a great field in accurately 

 calculating all the interesting conjunctions of historical 

 times. Then there must be many references in histories 

 and chronicles to the subject, the hunting up of which 

 would give work to the literary student. 



For myself, I am content in simply pointing out the 

 interest of the subject to anyone who is fond of surveying 

 the evening or early morning sky, with the only instru- 

 ment Nature has given him, his eyes. If such an one has 

 the means or the good fortune to get to latitude 86°, or 

 nearer the equator still, he will then enjoy, with a zest 

 increased by comparison with his poor English views, the 

 splendid sights of planetary groupings he is sure to be 

 favoured with. 



igjf^ 



THE DIAMETER OF THE FIELD OF VIEW 

 OF A TELESCOPE. 



By Thomas H. Jilakesley. 



IN the last edition of Mr. Webb's most excellent book. 

 " Celestial Objects for Common Telescopes," and 

 presumably in earlier editions of the work, the 

 author, admitting the usefulness of the knowledge of 

 the diameter of the field of view of a telescope, gives, 

 as a rule for finding this value, to time the passage of an 

 equatorial star centrally across the field, and miTltiply the 

 time so occupied in minutes and seconds by fifteen to 

 obtain the angular value in minutes and seconds of arc. 



The author of the book recommends several trials of this 

 sort, and the mean value of the angles so obtained is to be 

 taken as the truest. Now, in the first place, it is obvious 

 that the menu of such trials is not the best value, but apart 

 from this consideration, it is by no means an easy matter 

 to arrange a common telescope so that a star shall pass 

 through the centre of an open field, and even if the star 

 has so passed there is no proof of the fact in anything ot 

 the nature of contact or iuters( ction. The method, indeed, 

 is eminently of a theoretical character for any but a 

 well-divided field. In Godlray's "Treatise on Astronomy" 

 directions of the same general character are given for 

 finding the diameter of a Eing-micrometer. 



The method suggested in this paper is free from the 

 objections indicated above, especially in the important fact 

 that the results may properly be meaned. Tlie observed 

 times involved begin and end wiili definite contacts, and 

 central passage is not needed. The sun is the heavenly 

 body employed, so that the observations may be made on 



