Do not misunderstand me, when I speak of a “more rapid motion,” and of “rather near neighborhood.” The real rate of 61 Cygni’s rush through space is believed to be about forty miles each second, or one thousand four hundred and fifty millions of miles each year. All we can perceive of this quick motion is that, in the course of three hundred and fifty years, 61 Cygni travels over a space in the sky about as long as the breadth of the full moon. Little enough, yet very far beyond what can be detected in the greater number of even the brightest stars. Then, again, as to the near neighborhood of this star, 61 Cygni is near enough to have his distance measured, and that is saying a good deal. Alpha Centauri, the nearest star of which we know in the southern heavens, is two hundred and seventy-five thousand times as far distant as the sun. But 61 Cygni, the nearest star of which we know in the northern heavens, is nearly twice as far away as Alpha Centauri. [Illustration: CYGNUS.] We call 61 Cygni a star, for so he appears to common observers. In reality, instead of being only one star, the speck of light which we call 61 Cygni consists of _two stars_. The two are separated by a gap about half as wide again as the wide gap between the sun and Neptune. Yet so great is their distance from us that to the naked eye the two seem to be one. These two suns would together make a sun only about one-third as large as our sun. They differ in size, the quicker movements of one showing it to be the smaller; and it is by means of their known distance from one another, and their known rate of motion, that their size, or rather their weight, can be roughly calculated. Of course neither of the two shows any actual measurable disk. So much and so little is with tolerable certainty known about this particular pair of suns. Next let us turn to Alpha Centauri--named _Alpha_, the first letter of the Greek alphabet, because it is the brightest star in the constellation of the Centaur. The second brightest star in a constellation is generally called Beta, the third Gamma, the fourth Delta, and so on; just as if we were to name them A, B, C, D, in order of brightness. The constellation Centaur lies in the southern heavens, close to the beautiful constellation called the Southern Cross, and is invisible in the northern hemisphere. Of all the stars shining in the heavens round our earth, two only--Sirius and Canopus--show greater brilliancy than Alpha Centauri. As in the case of 61 Cygni, astronomers were led to attempt the measurement of Alpha Centauri’s distance, by noticing how much more distinct were his movements than the movements of other stars, though less rapid both to the eye and in reality than those of 61 Cygni. Alpha Centauri’s rate of motion is some thirteen miles each second. And this, so far as we yet know, is our sun’s nearest neighbor in the heavens outside his own family circle.