Simon Newcomb

“ The most interesting phenomenon connected with Mercury is that of its occasional transit over the disk of the sun at inferior conjunction. These occur only when the planet is near one of its nodes at the time. The earth, in its orbital revolution, passes through the line of the nodes of Mercury about the 8th of May and the 10th of November of each year. It is only near one of these times that a transit can occur. The periodic times of Mercury and the earth are such that the transits are generally repeated in a cycle of 46 years, during which 8 transits occur in May and 6 in November. ”

“ As the moon always presents the same face to the earth, so Mercury must, in this case, always present very nearly the same face to the sun. Schiaparelli also announced that the axis of rotation of the planet is nearly perpendicular to the plane of its orbit. The rotation being uniform, while the orbital motion, owing to the great eccentricity of the orbit, is affected by a very large inequality, it would follow that there is a libration in longitude of nearly 24° on each side of the mean position. ”

“ We may therefore regard it as certain that, if a group of intra-Mercurial planets exists, its members are too small to be seen when projected on the sun’s disk. During the eclipses of 1900 and 1905 the astronomers of the Harvard and Lick Observatories photographed the sky in the neighbourhood of the sun so fully that the stars down to the 7th or 8th magnitude were imprinted on the plates. ”