SCIENCE- GOSSIP. 



CIIAI'TERS rOR \OUNC; ASTRONOMKKS 



Bv FHA^fK C. Dknsett. 



i^CoHtinutd/roiu fragt aS^.) 



MERCURY. 



TlIK innermost of the Sun's planets has a diameter 

 of only 3,ooS miles, so that no less than eighteen 

 such glohcs would he neu(le<l to make up one the 

 size of (tur own world. It would, however, onl)' 

 want fifteen such glohes toweigh down our earth ; yet 

 from the snudlness of Mercury, a stone dropped on 

 its surface would only fall 7 feet in a second instead 

 of 16 feet on our earth. Krom its proximity to the 

 Sun, heing never more than 29° distant, it is not so 

 cisily seen as the other members of our system. The 

 liest opportunities are when it reaches greatest elonga- 

 tion east in May or June, or greatest elongation west 

 in July or August. The writer has, at these periods, 

 readily seen Mercury even in smok)' I^ondon, shining 

 as a bright ros\"-tinled star. Knt^wing where to look 

 is the secret of successful search. The beautifully 

 clear skies in the ICast doubtless account for the 

 detection of Mercury by the ancients. The oldest 

 record we have dates back to 265 B.C. On the other 

 hand, the va]Xiurs near the horizon on the banks of 

 the Vistula, as was long since pointed out by n.assendi, 

 were the probable reason for Copernicus failing. 



Its greatest apparent diameter, when it is in transit 

 over the sun's disc, is about il"-5; whilst at the 

 time of superior conjunction, when it shines like a 

 little full moon, its diameter dwindles down to 4" '5, 

 the difference being shown by the diagram. When 

 in the best position for observation it appears very 

 like a half-moon. 



It is not a satisfactory object to observe when near 

 the horizon, as the atmos|ihere is seldom steady 

 enough or sufficiently clear to permit of good observa- 

 tions being made. Observations should be attempted 

 when it is high in the sky during d.iytime. This is 

 not really so difficult a matter as may at first appear, 

 even with the ordinary altazimuth stand, whilst with 

 an ei|uatorial stand and circles it is comparatively easy. 

 To find an inferior planet in daytime with an altazi- 

 muth telescope, I choose the time when it is near the 

 meridian, and then set the telescope pointing south. 

 Of course ever)- telescopist knows the south point 

 from his place of observation. Then, by the help of 

 an ordinary protr.actor, raise it to the approximate 

 altitude. A large field eye-|)iece of low power, say 

 30 to 45, must be put on the telescope. .Slightly 

 raise the instrument three or four degrees, and sweep 

 it along east and west a few degrees. Then gradually 

 lower it, sweeping at each lowering, and very soon 

 the planet will be seeii in the field of view, and higher 

 powers may be sulislituted. 



An ordinary fine day is, however, often very un- 

 satisfactory for such a search, because the amount of 

 vapour in the atmosphere reflects such a quantity of 

 sunlight as to almost or quite blot out the planet front 

 view. Days should be selected when the sky is deep 

 blue. 



Sometimes the planets draw so near to each other 

 as to be l)Oth seen together in the same telescopic 

 field. Once in May, 1737, Bevis, at Greenwich, 

 actually witnessed the occultation of Mercury by the 

 planet Venus. When thus near each other, the 

 relative brilliance of the planets can be compared. 

 On Septeml>er 26th, 1S7S, Xasmyth compared these 

 two. Whilst Venus was said to be equal to clean 

 silver, Mercury was nothing better than le.id or zinc. 

 This indicates that the former must have much the 



highest reflecting power, since, area for area. Mercury 

 receives about 34 times greater light. 



Very little has been revealed concerning this 

 planet even by the most [xiwerful telescopes, s<> that, 

 m fact, little more is known now than when .Schriiter 

 observed it about a century ago. Its most marked 

 characteristics are the phases. One peculiarity has 

 been noticed about these which is a puzzle to a.stro. 

 nomers, the bright portion appearing narrower than 

 calculation wnuld lead them to expect. This pheno- 

 menon has been seen by many observers, Ixith in the 

 cases of Mercury and \ enus ; but up to the present 

 time no satisfactory explanation has been given. 



So far as .surface detail is concerned, we know very 

 little. Schriiter and his as.sistant, Harding, con- 

 .sidered that the surface was nunintainous, because 

 from time 10 time the southern horn appeared blunted 

 ofl^. Likew ise projections were seen, and irregularity 

 of the terminator. lie inferred that some of the 

 mountains must have a height of at least eleven 

 miles. This blunting of the horn h.a.s been confirmed 

 by many observers since the days of Schroter. From 

 various observations a rotation period of 24h. 5m. 28s. 

 Wi\s obtained. An atmosphere was inferred from the 

 occ.asional appearance of dusky streaks and spoLs. 

 Prince and Noble have noted the appearance of dark 

 spots, and Birmingham of a bright one. Schiaparelli 



Mercuuv at Gre.vtest and Least Diameters. 



has fancied brownish streaks on the planet, perma- 

 nent in their positions. He arrived at the .startling 

 conclusion that Mercury only turns once on its axis 

 during about 88 days, the time occupied in travelling 

 round the sun. 



From time to time Mercury passes between the 

 earth and sun. Then he appears as a circular black 

 spot on the disc. Thus seen, some observers have 

 fancied that a greyish border surrounded the planet, 

 others, at the same time, thinking that a bright 

 bordering was to be seen. Another peculiarity 

 noticed is that when in transit many observers fancy 

 the presence of one, and some two, bright spots near 

 the centre of the planet's black disc. Whether the.se 

 appearances are real or only optical illusions seems 

 for the present to remain uncertain. One optical 

 illusion generally obsei-ved at the time of transits is 

 that known as the "black-drop," the planet appear- 

 ing lengthened out to the sun's limb at the times of 

 ingress and egress. This is due to irradiation, the 

 bright .sun appearing too large and the black planet 

 too small in contrast. The speclro.scope of \'ogcl 

 seemed to demonstrate the presence of an atmosphere 

 containing water)- vapour. \'et there are those who 

 would have us believe that, owing to the small mass 

 of Mercur)', his gravitation power is so slight that he 

 cannot retain even water on his surface. 



( To he continue J. ) 



