To find star-distance through star-parallax sounds quite simple, when one thinks of the general principle of it. Just merely the question of a base-line, accurately measured; and of two angles, accurately observed; and of another angle, a good way off, accurately calculated,--all resting on the slight seeming change of position in a certain distant object, watched from two different positions, about 185,000,000 miles apart. Quite simple, is it not? Only, when one comes to realize that the “change of position” is about equal to the change of position in a penny piece, miles away, looked at from two windows in one house,--then the difficulty grows. Besides this, one has to remember all the “corrections” necessary, before any true result can be reached. A penny piece, miles away, seen from two windows, would be difficult enough as a subject for measurement. But, at least, the penny would be at rest; and you yourself would be at rest; and light would pass instantaneously from it to you; and there would be no wobbling and nodding motions of everything around to add to your perplexities. In the measurement of star-parallax, all these things have to be considered and allowed for; all have to be put out of the question, as it were, before any correct answer is obtained. The refraction of light must be considered; because that displaces the star, and makes it seem to us to be where it is not. And the aberration of light must be considered; because, in a different way, that does the same thing, making the star seem to take a little journey in the course of the year. And the precession, or forward motion, of the equinoxes, and nutation, or vibratory motion, have both to be separately considered; because they, too, affect the apparent position of every star in the sky. To watch the star in comparison with another star is easier than merely to note its exact position in the sky; for the other star--the “companion-star”--is equally with itself affected by refraction of light and by aberration of light. But, then, another question comes in seriously: whether or no the companion-star shows any parallax also; since, if it does, that parallax must be carefully calculated and allowed for. So the measurement of star-distance, even when parallax can be detected, is by no means a light or easy matter. On the contrary, it bristles with difficulties. It is a most complicated operation, needing profound knowledge, accurate observation, trained powers of reasoning and calculation.