The grandeur of the scale upon which the visible universe is fashioned lies almost beyond human comprehension. In measuring the vast extent of our own solar system, which is but a single unit in the system of the stars, we may have recourse to some earthly standard of measurement, such as the mile. But when we desire to express in terms of units that can be grasped by our imagination, the distances of the stars that lie far, far beyond, we find that all ordinary standards of measurement become utterly inadequate for our purpose. In the measurement of celestial distances within the solar system the unit employed is either the familiar mile or kilometer or the "astronomical unit," which is the mean distance from the earth to the sun (ninety-two million nine hundred thousand miles in round numbers). In the measurement of distances _beyond_ the solar system the unit employed is either the _light-year_ or more recently the _parsec_, which is rapidly replacing the light-year among astronomers. A "light-year" is the distance that light, with its finite but almost unimaginable velocity of one hundred and eighty-six thousand miles _per second, travels in a year_. It is equal in round numbers to sixty-three thousand times the distance from the earth to the sun or approximately six thousand billions of miles. The parsec is equal to three and twenty-six hundredths (3.26) light-years, and it is approximately two hundred thousand times the distance from the earth to the sun. It is "the distance of a star with the _parallax_ of a second," a fact which its name, parsec, conveys to us. In other words, at the distance of one parsec the distance from the earth to the sun, "the astronomical unit," would subtend an angle equal to one second of an arc. This angle is spoken of as the parallax of the star. The larger the parallax, that is, the larger the angle the astronomical unit or radius of the earth's orbit subtends, viewed from the star, the nearer the star is to us. The fact that there is no known star within one parsec, or three and twenty-six hundredths light-years, of the sun shows the immensity of the scale of the universe of stars. Before considering the distances of the stars and the extent of the sidereal system of which our sun and his satellites form a part, let us undertake to express the distance of the sun, moon and planets from the earth and the extent of the solar system in terms with which we are familiar. The nearest to the earth of all celestial bodies is its satellite, the moon. So near is the moon that if we should make on some great plain a model of the solar system in which the astronomical unit, the distance from earth to sun, would be four hundred feet, the distance between the earth and moon would be only one foot. On the same scale the most distant planet Neptune would be two and one-quarter miles away.