What, then, is that little black spot which they are so anxious to examine as it passes across the sun next December? How comes it to be of such importance that all these mighty efforts are made to have it fully and correctly observed? To what great results, scientific or other, will a correct knowledge of everything about it lead the world?
That little black spot is the planet Venus, then passing directly between the earth and the sun, and producing an homœopathic solar eclipse, just as, under similar circumstances, the moon might produce an annular or a total solar eclipse. As ordinarily seen in her [pg 147] character of morning or evening star, Venus shines more brightly and joyously in the heavens than any other star. But on this occasion the whole of her illuminated half is turned towards the sun. Towards the earth she shows only her dark, unillumined half, which even looks darker by contrast with the bright face of the sun, on which it is projected. This passage across the sun is called the transit of Venus. If the observations are successfully made, they will give us the means of ascertaining with sufficient precision what as yet is not so known—the actual distance of the earth from the sun.
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 [pg 148] 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. The earth arrives punctually every year at one end of this line in June, and at the other in December. The astronomical question is, When will Venus be there also at the same time? To answer requires a calculation which appalls. First, there is the planetary velocity proper of Venus, varying according as in the various parts of her elliptical orbit she is nearer to or further from the sun. Then there are the influences of planetary attraction—the earth and the other planets acting on Venus, accelerating or retarding her movements, and tending sometimes to draw her to one side of her orbit. Then there is or may be question of that nodal diameter shifting its position, and trying, as it were, to swing round the circle of the earth's orbit. When all these calculations have been made, the diurnal movement of the earth must be taken into account, and the geography of her surface must be duly studied, to determine finally when the transit will take place, across what portion of the sun's face the planet will be seen to travel, and from what portion of the earth's surface that transit can be seen, and where in that portion stations for observing it can be placed with the greatest probability of success.
It is a fearful sight even to look over a seemingly endless series of pages all bristling with serried columns of figures, broken every now and then by mysterious formulas of higher calculus, like a group of officers commanding a brigade. Mathematicians and astronomers may delight in them; we shall be satisfied to take the results.
The transits of Venus go in pairs eight years apart. There can be only one pair to a century; some centuries will have none. The pairs occur alternately in June, as Venus descends from the upper to the lower half of her orbit, and in December, as she ascends again from it. Thus there were transits in December, 1631, and December, 1639. A second pair occurred in June, 1761, and June, 1769. A third pair is near at hand, in December, 1874, and December, 1882. The next century will have none. The fourth pair will appear in June, 2004, and June, 2012.
So much on the character of that dark little round spot, the passage of which across the sun hundreds of astronomers, with all manner of telescopes, spectroscopes, and photographic instruments, will watch, examine, measure, and record, as they see it sweeping on in its course on the 8th of next December. What will be the special purpose animating observers as they view the transits of 2004 and 2012—if, despite the prophetic and apocalyptic Dr. Cumming, the world lasts till then—no one can now tell. Astronomy by that time may be advanced as far beyond the present state of the science as the present state surpasses the state of two centuries ago. It is probable that new and, to that generation, most interesting questions may have then arisen, which they will strive to solve by their observations of the transits—questions now perhaps undreamed of. But at present our astronomical world is deeply impressed with the advantage and necessity of definitely ascertaining the distance of the earth from the [pg 149] sun. This is the paramount, though by no means the only, purpose of all this expenditure of time and skill and money in preparing for, in making the observations, and afterwards in laboriously working out the results.
How, by merely looking never so attentively at an object whose distance you do not know, as it stands in a line with, and perhaps far in front of, another, likewise of unknown distance, you can tell how far off that second object is, may seem as difficult as the king's requirement of the prophet first to tell him the dream he had forgotten, and then to explain its meaning. It might seem almost an impossibility; but a few words will explain how the difficulty is turned by availing ourselves of other data.