At one time of the year, our earth is on one side of the sun, over 92,000,000 miles distant. Six months later, the earth is on the other side of the sun, not quite 93,000,000 miles away from him in that opposite direction. Twice 93,000,000 comes to 186,000,000. This line, therefore--the diameter of the earth’s whole yearly orbit, may be roughly stated as about 185,000,000 of miles in length. Here surely was a base-line fit to give parallax to any star--or, rather, to make parallax visible in the case of any star. For it is, after all, a question, not of fact, but of visibility; not of whether the thing _is_, but of whether we are able to _see_ it. As the earth journeys on her annual tour round the sun, following a slightly elliptic pathway, the diameter of which is about 185,000,000 miles, each star in the sky must of necessity undergo a change of position, however minute, performing a tiny apparent annual journey in exact correspondence with the earth’s great annual journey. The question is not, Does the star do this? but, Can we _see_ the star do this? Its apparent change of position may be so infinitesimal, through enormous distance, that no telescopes or instruments yet made by man can possibly show it to us. The star-motion of which I am now speaking is purely a seeming movement, not real. Be very clear on this point in your mind. _Real_ star-movements, though suspected earlier, were not definitely surmised--one may even say “discovered”--until the year 1718, by Halley. And though Cassini in 1728 referred to this discovery of Halley’s, yet very little was heard about the matter until the days of Herschel. Only _apparent_ star-motions were generally understood and accepted. The first and simplest of such seeming star-motions is one which we can all see--the nightly journey of the whole host of stars, caused by our earth’s whirl upon her axis. The second is also simple, but by no means also easily seen. Astronomers reasoned out the logical necessity for such an apparent motion long before it could be perceived. As far back as the days of Copernicus it was felt that if the Copernican System were true, if the earth in very deed traveled round the sun, then the stars ought to change their positions in the sky when viewed from different parts of earth’s annual journey. Observations were taken, divided by six months of time and by one hundred and eighty-five millions of miles of space. And the stars stirred not! Stupendous as was this base-line, it proved insufficient. So much _more_ stupendous was the distance of the stars that the base-line sank to nothing, and once again parallax could not be detected. Not that the seeming change of position in the star did not take place, but that human eyes were unable to see it, human instruments were unable to register it.