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near to, sucli a position as p or p', the planet will either passexactly behind the Sun , as seen from the Earth , or else it willpass in transit across the Sun ’s disc as a small black spot; inthe former case it might be at p' when the Earth is at e, or atp when the Earth is at e' ; in the latter, at p when the Earth isat e, or at p' when the Earth is at e'. For the planet Mer cury the positions e and e' of the Earth are in the months ofMay and November ; for Yenus in June and December.
At all other times, the tilt of the planet’s orbit carries itsomewhat above or below the Sun ’s disc, as it passes over orunder a straight line, joining the places of the Earth and theSun ; just as in Lecture IY. we explained that the tilt of theMoon ’s orbit prevents Solar or Lunar eclipses from occurring,
' Pig. XXXVII.—The curve e e' is the Earth ’s orbit lying in the plane of the Ecliptic.
The curve p p' is a planet’s orbit in a plane inclined or tilted to the former.
as they otherwise would, upon the occasion of every new andfull Moon .
We shall make further reference to the transits of Mercury in Lecture YI. At present we will only remark that Mercury ,when in transit, is too small to be seen by the naked eye, sothat we are not surprised that no very ancient observations ofits transits are recorded. But we think it somewhat strangethat the astronomers of early days do not seem to haveobserved any Transit of Yenus ; in fact, that none was noticed,as we have stated in our first lecture, until the year 1639.
Such Sun -spots, however, as are visible to the naked eye, wereobserved in very ancient times. It is therefore just possiblethat a Transit of Yenus, the disc of which is quite largeenough to be distinguished upon the Sun without any tele-