*THE POPULAR SCIENCE MONTHLY.*

court of Rome saw how unwisely she had acted in deciding a question beyond her competence, thus laying herself open to the danger of being the next day convicted of error, it became her interest, no less than the interest of Science, to distinguish clearly between the two domains, Science and Faith. If, nowadays, she avoids entering into scientific controversies, it is because she has been taught by experience that a decision might compromise her. Her authority could hardly stand after a second edition of the sentence in which she once forbade the sun to stand still and the earth to revolve.

DISTANCE AND DIMENSIONS OF THE SUN. |

OF DARTMOUTH COLLEGE.

THE problem of finding the distance of the sun is one of the most important and difficult presented by astronomy. Its importance lies in this, that this distance—the radius of the earth's orbit—is the base-line by means of which we measure every other celestial distance, excepting only that of the moon; so that error in this base propagates itself in all directions through all space, affecting with a corresponding proportion of falsehood every measured line—the distance of every star, the radius of every orbit, the diameter of every planet.

Our estimates of the masses of the heavenly bodies also depend upon a knowledge of the sun's distance from the earth. The quantity of matter in a star or planet is determined by calculations whose fundamental data include the distance between the investigated body and some other body whose motion is controlled or modified by it; and this distance generally enters into the computation by its cube, so that any error in it involves a more than threefold error in the resulting mass. An uncertainty of one per cent, in the sun's distance implies an uncertainty of more than three per cent, in every celestial mass and every cosmical force.

Error in this fundamental element propagates itself in time also, as well as in space and mass. That is to say, our calculations of the mutual effects of the planets upon each other's motions depend upon an accurate knowledge of their masses and distances. By these calculations, were our data perfect, we could predict for all futurity, or reproduce for any given epoch of the past, the configurations of the planets and the conditions of their orbits, and many interesting problems in geology and natural history seem to require for their solution just such determinations of the form and position of the earth's orbit in by-gone ages.