|ASTRONOMICAL MAGNITUDES AND DISTANCES.|
THE magnitudes and distances considered in physical astronomy are so immense that we cannot hope to reach even a faint conception of them except by illustration and comparison. If even then, with our best effort, we fail to measure up to the magnificent dimensions of the universe, the attempt will at least enlarge our intellectual conceptions, and lead us out mentally into a broader place.
The results reached by modern astronomy, respecting the dimensions and distances of the heavenly bodies, are based on two lines, the radius (or semi-diameter) of the earth and the radius of its orbit; the former is accurately known, the latter approximately. In modern times the highest refinements of engineering skill have been applied to the measurement of base-lines, which furnish through triangulation arcs of a meridian. So thoroughly has this work been done that, in the opinion of Prof. Young, the error in the ascertained length of the earth's equatorial radius cannot exceed 200 feet. This radius forms our base-line for broader operations. The equatorial, horizontal parallax of the moon, or the angle subtended at the moon by the earth's equatorial radius, is found to have an average value of 57' 2". Hence by plane trigonometry the moon's mean distance is 238,885 miles, or nearly ten times the circumference of the earth. Light, with a velocity of 186,500 miles a second, travels from the earth to the moon and back again in two and a half seconds, thus producing that faint illumination of the dark portion of the new moon turned toward us. Knowing the moon's distance, the measurement of its apparent diameter in minutes of arc furnishes immediately its absolute diameter in miles.
So, then, this queen of the night, once supposed to be a kind of lantern, fed by exhalations from the ocean, is a body 49 as large as the earth. It is our nearest celestial neighbor—in fact, a little out-lying, condensed nebulosity; and if we had a weather-station on the lunar mountain Tycho, connected by telegraph with Washington, General Myer would receive the lunar weather-reports in fifteen seconds by electricity.
Aristarchus, in the third century before the Christian era, attempted to use the moon's distance to compute the greater distance of the sun; but the method failed, and astronomers were compelled to fall back on the radius of the earth as a base-line for a still grander triangulation. The parallax of Mars, at opposition, gave us the first approximation to the sun's distance; then the transit of Venus furnished a nearer estimate; latterly, Le Verrier, who found Neptune by figures, has also determined the distance of the sun by means of planetary perturbations; still a fourth method combines the retardation of the eclipses