The two other planets interior to the earth, namely, Mercury and Venus, are occasionally seen in the act of transit; and there cannot be a doubt that if Vulcan exists, its transits across the sun must be more numerous than those of Mercury, and far more numerous than those of Venus.
"The Story of the Heavens"
Robert Stawell Ball
The prediction of those transits by Kepler, some years before they occurred, was justly regarded at the time as a most remarkable achievement.
"The Story of the Heavens"
Robert Stawell Ball
The transits may be all grouped together into pairs, the two transits of any single pair being separated by an interval of eight years.
"The Story of the Heavens"
Robert Stawell Ball