Professor Michelson has now invented a new method of measuring the angular diameter of the more distant stars. This is by means of an instrument called the "interferometer" which is attached to the 100-inch reflector of the Mount Wilson Observatory. Measurements of the angular diameter of Arcturus with the interferometer combined with the knowledge of its distance give us the astounding size of 19,000,000 miles for the diameter of this huge sun. Henderson, in 1840, was the first astronomer to make a successful measurement of the distance to a star. The trigonometric method such as surveyors use to find the distance to the far side of a river was employed for this and with a baseline and angles he reached out and touched the nearest stars. The baseline of a triangle whose apex touched a star which lay millions of millions of miles away would have to be very long indeed--and a very delicate and difficult task to handle it. Where in the world could one find such a baseline? Certainly not on the earth! A line extending from one side to the other of our earth would not even be as large as a point of light as seen from a star. Indeed when the moon was measured, and the moon lies only 240,000 miles away, astronomers used a baseline which extended from America all the way to France. The exceedingly clever idea of taking the earth's orbit as a baseline then suggested itself, and focussing upon a star at an interval of six months when the earth is at the two ends of the diameter of its immense orbit around the sun. [Figure 58] The angle between the baseline and the line of sight to the star may be noted in the summertime, for instance, and then again in the winter. The shift of the star on the heavens caused by the observer changing his place from one side of the sun to the other gives the parallax angle of the star, which is the angle between the two sight lines of the observer where they meet at the star. If the star is distant more than 500 billion miles this shift cannot be measured with any confidence for at 900 or 1000 billion miles, the star ceases to show any displacement. When a star is near enough to be measured by this trigonometric method it seems to describe a minute ellipse "like a reflection of the ellipse of the earth's orbit." If it lies so far in space that it does not seem to change its position even when viewed from stations on either side of the earth's orbit, its distance may never be measured by using the diameter of the earth's orbit as a baseline. Even the near stars are so very distant that their displacement is almost imperceptible. The parallax (or the apparent angular shifting caused by the changed position of the observer) of Alpha Centauri, our nearest star, would have, says Professor Moulton, the same difference in direction as a point of light 11 miles from the observer viewed with one eye and then the other, with the difficulties of observation extended over several months.