This knowledge is all‐important in a scientific point of view. From it we can deduce the distance of every other planet of the solar system. With it we can carry our survey beyond that system into the stellar world. The distance of our earth from the sun—the orbital radius of the earth, is, for the astronomer, his unit of measure—his yard‐stick, as it has been termed—when he would estimate or measure stellar distances or velocities. Any error in it is multiplied millions of times in such surveys. Any uncertainty or reasonable apprehension of error about it casts a cloud of embarrassment over almost every portion of the newly acquired domain of astronomy. No wonder, then, that no effort is spared to secure as soon as possible, and in the easiest and most certain way we know of, an accurate solution of the question. This, more than anything else, is the spring of the whole movement. The earth, as all know, revolves, as do the other planets, round the sun, not precisely in a circle, but in an oval or ellipse not differing much from a circle. The length of our year, or time of one complete revolution of the earth around the sun, is 365 days, 5 hours, 48 minutes, 49.657 seconds. Inside the earth, and next to us, among the planets, comes Venus, revolving around the sun in her elliptical orbit in 224 days, 16 hours, 48 minutes, and 42 seconds. Were both orbits on the same level, in the same plane, Venus and the earth would come to be in the same direction or line from the sun as often as Venus, moving on her inner and shorter course, and more rapidly, would overtake the more sluggish earth. Such conjunctions would happen once in every 584 days nearly; and every such conjunction would show a transit, and Venus could be seen between the earth and the sun. But the orbits, though both around the same sun, are not on the same level. That of Venus is somewhat tilted up or inclined, so that one‐half of it lies above the level of the earth’s orbit, and the other half sinks correspondingly below. The line where the orbits cross or intersect each other is the nodal diameter, the only one common to both orbits. Venus overtakes the earth regularly, but ordinarily elsewhere than on or in the immediate vicinity of this nodal line. The planet then, in her apparent journeying from one side of the sun to the other, generally seems to pass near that luminary, either to the north or the south of it. But whenever, as sometimes happens, Venus overtakes the planet on the line of the nodes, either as she is descending on her orbit on one side, or ascending on the other, then the planet is seen to pass across the sun, and there is a transit. It is not necessary that Venus should be precisely on the line uniting the earth’s centre to the sun’s centre. The apparent size of the sun, 32’ in diameter, and the size of the earth, and the smallness of the angle of inclination between the orbits, all combine to give a little latitude in the matter.