SCIENCE- G OSS /P. 



27 



CHAPTERS FOE YOUNG ASTRONOMERS. 

 By Frank C. Dennett. 

 VENUS. 

 (.Continued from Vol. VI. page 377.) 



Many observers never see spots on Venus which 

 they can delineate, whilst others have drawn a great 

 amount of detail. That markings really do exist 

 there is not any real doubt ; but whether the mark- 

 ings are of a stable character or only atmospheric 

 is questionable. Observers for the past 230 years 

 have from time to time seen and drawn such spots. 

 Yet among the drawings there is little, if any. of 

 that similarity which one meets with in the case 

 of Mars. Most of those who have studied the 

 planet agree that whilst they have no doubt of the 

 reality of the markings, there is a strange want 

 of definiteness which makes them difficult to 

 secure. The size of the telescope seems to be less 

 necessary in this work than is usually the case. A 

 good, rather than a large, telescope appears to be 

 the important item, combined with persistent 

 observation, so that the transient times may not ln- 

 missed when our own atmosphere is in good condi- 

 tion and the gaseous envelope of Venus itself 

 equally transparent. Excellent observations have 

 been made when the planet has been seen through 

 a cloud of Aurora, the detail being then remark- 

 ably visible. 



The spots were carefully observed and charted 

 by Bianchini at Rome in 1726 and 1727 with a 

 non-achromatic telescope 66 feet long and just 

 over 2^ inches aperture, bearing a power of 112. 

 He, however, made the mistake of supposing that 

 the rotation period was 24d. 8h. More than a 

 hundred years later, 1839 to 1841, De Vico and six 

 assistants, with the 6^-inch Cauchoix achromatic 

 instrument then at the Roman observatory, con- 

 firmed the work of Bianchini so far as the accu- 

 racy of his chart is concerned, save that he added 

 one new spot. An important fact noted was that 

 those who were most successful in seeing the spots 

 were those astronomers who were least successful 

 in detecting faint close companions of brilliant 

 stars. Amongst the dark markings brilliant spots 

 and patches have sometimes been seen. Huggins 

 repeatedly saw a round bright spot. With and 

 Key, in April 1868, observed a small brilliant spot 

 on the limb which appeared as a projection. 

 Browning has also seen a bright patch on the disc 

 With frequently observed with an unsilvered glass-' 

 mirror, which reduced the glare of the planet. 

 A similar effect may also be obtained by using a 

 solar diagonal on an achromatic. In using a 

 diagonal, solar or star, it must be kept in mind 

 that the object viewed is inverted, but not re- 

 versed. 



The accompanying drawings of Venus were made 

 by the writer in February 1881. The one marked 

 A was made on the 11th at 6.45 p.m. The most 

 striking object was the little oval "sea " oear the 

 limb, reminding one of the Mare Crisium on the 

 moon when of similar phase. At 7.5 p.m. it had 

 moved perceptibly nearer to the limb. On the 

 15th, at 6.12 p.m. and 7.27 p.m.. the drawings B and 

 C were made, the brightish patch near the centre 

 of the disc having much increased in size in the 

 latter. The instrument employed was a 9^-inch 

 Calver Newtonian. 



In 1890 the astronomical world was startled by 



an announcement by Schiaparelli that the revolu- 

 tion iif Venus, instead of taking just less than a 

 day. really occupied between six and nine months. 

 ( Observations made at Lowell's Flagstaff Observatory, 

 Arizona, confirmed this opinion. Drawings of the 

 planet which were made give it the appearance 

 almost of a cart wheel. The markings were said 

 to be very evident, though not with the large 

 instruments, but with the smaller, and best of all 

 with the 3-inch. There has been much discussion 

 on the subject, and tried observers, such as Xiesten 

 of Brussels, Stuyvaert, and Trouvelot, have con- 

 sidered their observations and supported the short- 

 time rotation period. Xow, apparently, the matter 

 has been set at rest by the spectroscope. Dr. 

 Belopolsky of Moscow finds that the amount of 

 displacement of the lines indicates a short period 

 of rotation. There can be little doubt that the 

 surface we see consists of something analogous to 

 clouds, and is not the real surface of the planet. 



Vexus. — As seen on February 11th and 15th, 1881. 



The spectroscope makes it evident that aqueous 

 vapour is certainly present in its atmosphere. 



Respecting the transits of Venus across the sun, 

 little need be said, as anyone now living is never 

 likely to see another. The next transit will occur 

 on the morning of June 8th, 2004. followed by one 

 on June 5th, 2012. The "black drop,"' due to 

 irradiation, i- a very striking object at the internal 

 contacts, when the planet, instead of appearing 

 round, becomes lengthened out into the shape of 

 a pear. Venus has been seen projected on the 

 bright, yet invisible, corona of the sun. When on 

 the disc many observers have seen this planet sur- 

 rounded by a bright aureole, and having one or two 

 bright spots near its centre. 



(7'o be continued.) 



