VOL.. LVm.J PHILOSOPHICAL TRANSACTIONS. 341 



will also offer when Venus is moving from her inferior conjunction with the sun, 

 and becomes a morning star. 



In regard to the observation of Venus, it is remarked by astronomers, that she 

 is to be seen with a good transit telescope, when she is within a few degrees of 

 the sun ; but, as she is 3 times nearer the earth, than the sun's mean distance, 

 when her elongation is 25° in the inferior part of her orbit, it is plain, that the 

 necessary observations may easily be made, when her menstrual parallax will be 

 at a medium, 3 times greater than the sun's ; and consequently amounting for 

 the whole difference to A.1". 



To avoid embarrassment in description, Mr. S. has hitherto supposed, that all 

 the observations are made in the meridian ; in which case the right ascensions will 

 be the same as they would appear from the centre of the earth ; and conse- 

 quently the planet's longitudes, thence deduced, nearly the same : but it is easy 

 to see, that if the quartile observations be made when the planets are consi- 

 derably to the east or west of the meridian, and so chosen, that the place of the 

 observer be further distant from the common centre of gravity, than the centre 

 of the earth is from that centre, that the base of the observations will be consi- 

 derably enlarged. Thus, in our latitude, supposing that the quartile observations 

 are made 4 hours before and 4 hours after the planet passes the meridian, this 

 will produce an enlargement of the basis by one of the earth's semidiameters : 

 and as the whole base or diameter of the epicycle comes out, according to Mr. 

 Maskelyne, no more than 1 .6 of the earth's semidiameters ; the base will, 

 according to this method, come out 2.6 ; and consequently, at the next oppo- 

 sition, the menstrual parallax of Mars will be thus enlarged to 50", the greatest 

 to 62" \, and that of Venus at a mean, to 74"-l. 



It must however be acknowledged, that no kind of observations of the places 

 of the planets are of equal validity with those taken with the best instruments in 

 the meridian ; those taken with micrometers perhaps not excepted : for however 

 accurately small distances can be measured by the micrometer of Mr. Dollond, 

 yet as these measures can hardly be reduced to the ecliptic, without having the 

 difference of declination or right ascension from other means (except two stars 

 making somewhat near a right angle with the planet should appear within the 

 field of view at once) ; and as in all these cases the rectification of the places of 

 the stars themselves ultimately depends on meridian observations, we may per- 

 haps be allowed to say, that in the most favourable cases of the micrometer, the 

 determinations thence to be drawn, are not superior to meridian observations, 

 and in less favourable cases, must be inferior : however, as the micrometer obser- 

 vations out of the meridian give an opportunity of repetition as often as we 

 please ; and the observations for rectification of the stars concerned, can be re- 

 peated in the meridian, as often as we please also ; it must be equally allowed, 



