That the stars are situated at great distances from our system is well known; but few, even for a moment, imagine how enormous those distances in reality are. Previous to the year 1838, astronomers even were uncertain as to the immensity of the spaces lying between us and many of the orbs composing the sidereal universe. In that year, the astronomer Bessel, from numerous delicate observations of the small star 61 of Cygnus, was enabled to announce that he had determined the distance of that orb; and had discovered something of the magnitude of the scale on which the great starry system was built. A short time afterwards, the distance of another star was determined. Henderson, from observations made in the Southern Hemisphere, found that the bright star α Centauri was situated from us at a distance of about half that of 61 Cygni, or about eighteen billions of miles. Since then the distances of quite a number of stars in both hemispheres have been very fairly determined; but in every case it has been found that these orbs are removed much further from us than α Centauri, which, therefore, still remains the nearest known star to our system.
The problem of determining the distances of the stars is one of the most difficult that the astronomer has to solve. It requires the highest observational skill that the human intellect is at present capable of, combined with the most accurate instruments that it is possible for human hands to make; and in the great majority of cases, the distances of those orbs are so enormous that it is impossible to discover them. In attempting the solution of this grand problem — viz., to apply the sounding line to the depths of space, the astronomer employs the same method as a surveyor when measuring the distance of some inaccessible object. A base line at right angles to the object is carefully measured, and from each end of it the angle between the object and the opposite end of the base line is determined, from which data, by means of the properties of triangles, the distance of the object can be calculated. In determining the moon’s distance, a chord of the terrestrial surface can successfully be employed as a base line. The distance of the sun, however, is so great that even the whole diameter of our globe, or 7918 miles, is a base line too insignificant to measure it; while the stars are so far away that a base line even a hundred times longer would still be perfectly useless. Fortunately for astronomical science, a longer base line than this can be had, for in attempting the measurement of a star’s distance, the astronomer employs one whose length is twenty-three thousand times greater than the diameter of the earth, for he employs the whole diameter of the orbit of our globe, a length of over 185 millions of miles. From one end of this enormous base line, the observer from the earth at A (see Figure), carefully measures the angular distance of the star S, from some small and more distant star, as M, or he determines the very minute angle C A M. Six months afterwards, when our globe has completed one-half of her annual revolution, and has carried the observer to the other end of his base line, at B, the star S will no longer appear at C, as when viewed from A, but at D, from the parallactic displacement of the star, brought about by the change of the observer’s position. The astronomer, therefore, from his new position at B, determines the new angle M B D, and from these angles he can get the angle A S B, or the annual parallax of S, from which he can calculate the distance of the star.
It will be noticed from the Figure that the further S is from A and B, or the more distant a star is from the earth, the smaller becomes its apparent angular displacement or its annual parallax. In the case of the nearest star, this minute angle is less than one second of arc, or about the same angle as that under which one quarter of an inch would appear as viewed from a distance of a mile; for the nearest star is no less than 275 thousand times more distant than the sun! This distance is so great, that an object, travelling continuously at the velocity of an express train, would only journey over it in about sixty millions of years. Even light, which in each second travels over a space of 186 thousand miles — thus reaching us from the moon in about one second and a-quarter, and from the sun in seven and a-half minutes — actually requires four years to reach our earth from α Centauri. Such is the distance of the nearest star! The star next in point of distance, or the small star 61 of Cygnus, is situated at a distance of over 469 thousand times greater than that of the sun, and thus its light in journeying to us requires seven years. From the brilliant star Sirius light occupies ten years in travelling; from Aldebaran, fourteen years; from Vega, twenty-two years; and from Polaris, no less than thirty-six years [Note: the actual distance from Polaris is over ten times larger].
