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[26] Especially since the next planet Venus has an orbit that is

almost an exact circle, which makes it more difficult to locate

the perihelion with precision.

In point of fact, astronomers have found that the theory of

Newton does not suffice to calculate the observed motion of

Mercury with an exactness corresponding to that of the delicacy

of observation attainable at the present time. After taking

account of all the disturbing influences exerted on Mercury by

the remaining planets, it was found (Leverrier: 1859; and

Newcomb: 1895) that an unexplained perihelial movement of the

orbit of Mercury remained over, the amount of which does not

differ sensibly from the above mentioned +43 seconds of arc per

century. The uncertainty of the empirical result amounts to a few

seconds only.

(b) Deflection of Light by a Gravitational Field

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