THE SUN.

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Of these one is the rotation of the Barth, by which any placeupon its surface is moved with a velocity whose magnitude anddirection at any time depend upon its latitude and longitude.This produces an apparent motion of Yenus in the reversedirection across the Sun’s disc, and may either tend to shorten,or to lengthen, the transit according as the place of observationis carried round upon the same side of the Barth’s axis asVenus , or upon the opposite side. An observer at either poleof the earth, who would be unmoved by its rotation, would ofcourse experience no such effect.

The other cause by which a difference of duration is producedis the difference of the directions in which, owing to their distanceapart upon the Earth , any two places, at any given moment, lookat the planet. The lines joining them to it will consequentlyneither meet the Sun’s edge at the same point of its circum-ference, nor at the same instant of time. Two such places,indeed, look at the planet in directions which differ justas do those iii which the surveyor, in the method previouslyindicated in Figs. I. and II., looks at a distant object from hisstations. It is when, and where, those directions intersect theedge of the solar disc that the transit is seen to begin, or toend. One place may from this cause see the planet enter uponthe disc sooner than another, and leave it later, or vice verm.And its path across the Sun seen from the one station willappear to be longer, or shorter, than that which is seen fromthe other; and will, in the former case, at the middle of thetransit, pass correspondingly nearer to the centre of the Sun’sdisc.

Nor is it hard to understand that the angle between the linesof sight in which Yenus is seen from the two places of observa-tion will be as much greater than the difference of the direc-tions in which they look at the Sun, as Venus , when seen intransit, is nearer than the Sun is to the Earth , i.e., about 3times. But even such a difference of direction as this is stillmuch too small for the application of the surveying method ; *

* We are, moreover, unable to use the actual displacement of theposition of Yenus (caused by the difference of the directions in which itis looked at), as we are considering its displacement upon or relatively to