VOL. XXXIII.] PHILOSOPHICAL TRANSACTIONS. 77 



communicated in the words of the manuscript Journal of Observations of the 

 said Mr. Roemer. " Hodie sexto Maii (Anno 1707) hora matutina 4'' IQ"\ 

 spectabatur Mercurius in extremo margine solis jamjam exiturus; altus supra 

 imum solis marginem -f diametri Solaris, et ad sinistram in tubo (sc. invertente) 

 accuratius haec determinare non licuit ob moram nimis brevem." It was great 

 pity, that he did not at least estimate, how many diameters of his body he was 

 distant from the sun's limb, or what part of a diameter, if so near: but having 

 examined this observation, the sun, at that time was but just risen, or rather 

 rising, and soon after entered into a cloud, so that the sun's limb could not be 

 distinctly seen, it always undulating and sparkling much, when so near the 

 horizon; in which circumstance, a just observation could hardly be made. 



Let us now see how our numbers, corrected as above, will represent this 

 observation. Anno 1707, April 24^ 16'' ig'" at Copenhagen, is 15*^ 28'" at 

 Greenwich ; but 15'' 24"" 20' aeq. time. To this time, the sun's true place is 

 y 14° 50' 1", and his distance from the earth 101005. The correct epocha of 

 Mercury's mean motion, for the year 1707, is 3^ 13° 18' 45", to which adding, 

 for the rest of the time, 3' 19° Q' 28", we have his middle motion at the time 

 of the observation TT^ 2° 28' 13"; and taking his aphelion in | 12° 49' 49" 

 from it, we have his mean anomaly 10= 19° 38' 24", and thence the equation to 

 be added 12° 39' 41', and the place of Mercury in his orb tT]_ 15° 7' o4". But 

 the correct place of the descending node is TT]^ 14° 46' 25", and therefore Mer- 

 cury, being 2l' 29" past the node, had 2' 36" south latitude at the sun; and 

 his place, reduced to the ecliptic, was TT], 15° 7' 43", that is, 17' 44" past the 

 conjunction of the sun ; which diminished in the proportion of 5567 to 4533, 

 or of the distance of the planet from the earth to his distance from the sun, 

 becomes 14' 27"; and by so much was he past the conjunction as viewed from 

 the earth. Again, by the same proportion, his geocentric latitude at that 

 time, was 2' 7" south; and therefore his apparent distance from the sun's 

 centre, was 14' 37"; that is, but l' 18" from his western limb; so that he 

 might well be said to be, jamjam exiturus. 



But that Mercury should at that time be so far northerly, as Mr. Roemer's 

 words import, was absolutely impossible; and probably so acute an astronomer 

 as Mr. Roemer was, could not himself be the observer, but some person less 

 acquainted with these matters; which the words spectabatur Mercurius, instead 

 of Mercurium vidi, seem to import. If he had then had north latitude, he 

 must needs have been seen in the sun in April 1720, which we are assured 

 he was not. 



Lastly, it may not be amiss to advertise, that on the last day of Oct. 1736, 

 Mercury will again traverse the northern part of the sun's disk, both ingress 

 and egress being visible to all Europe. 



