It is not necessary for two observers to actually station themselves at two distant parts of the earth in order to determine a parallax.  If an observer could go from one end of the base-line to the other, he could determine both angles.  Every observer is actually carried along through space by two motions:  one is that of the earth’s revolution of one thousand miles an hour around the axis; and the other is the movement of the earth around the sun of one thousand miles in a minute.  Hence we can have the diameter not only of [Page 70] the earth (eight thousand miles) for a base-line, but the diameter of the earth’s orbit (184,000,000 miles), or any part of it, for such a base.  Two observers at the ends of the earth’s diameter, looking at a star at the same instant, would find that it made the same angle at both ends; it has no parallax on so short a base.  We must seek a longer one.  Observe a certain star on the 21st of March; then let us traverse the realms of space for six months, at one thousand miles a minute.  We come round in our orbit to a point opposite where we were six months ago, with 184,000,000 of miles between the points.  Now, with this for a base-line, measure the angles of the same stars:  it is the same angle.  Sitting in my study here, I glance out of the window and discern separate bricks, in houses five hundred feet away, with my unaided eye; they subtend a discernible angle.  But one thousand feet away I cannot distinguish individual bricks; their width, being only two inches, does not subtend an angle apprehensible to my vision.  So at these distant stars the earth’s enormous orbit, if lying like a blazing ring in space, with the world set on its edge like a pearl, and the sun blazing like a diamond in the centre, would all shrink to a mere point.  Not quite to a point from the nearest stars, or we should never be able to measure the distance of any of them.  Professor Airy says that our orbit, seen from the nearest star, would be the same as a circle six-tenths of an inch in diameter seen at the distance of a mile:  it would all be hidden by a thread one-twenty-fifth of an inch in diameter, held six hundred and fifty feet from the eye.  If a straight line could be drawn from a star, Sirius in the east to the star Vega in the west, touching our [Page 71] earth’s orbit on one side, as T R A (Fig. 28), and a line were to be drawn six months later from the same stars, touching our earth’s orbit on the other side, as R B T, such a line would not diverge sufficiently from a straight line for us to detect its divergence.  Numerous vain attempts had been made, up to the year 1835, to detect and measure the angle of parallax by which we could rescue some one or more of the stars from the inconceivable depths of space, and ascertain their distance from us.  We are ever impelled to triumph over what is declared to be unconquerable.  There are peaks in the Alps no man has ever climbed.  They are assaulted every year by men zealous of more worlds to conquer.  So these greater heights of the heavens have been assaulted, till some ambitious spirits have outsoared even imagination by the certainties of mathematics.