SIDEREAL OBSERVATIONS

...viewed as if it were scarcely two Earth diameters away. After this, I frequently observed the stars, both fixed and wandering planets, with incredible delight of mind; and when I saw such a vast multitude of them, I began to devise a method by which I could measure the distances between them, and at last I found it. Regarding this matter, it is proper that everyone who wishes to undertake observations of this kind be forewarned. For first, it is necessary that they prepare for themselves a most accurate Spyglass original: Perspicillum; Galileo's term for the telescope before the word 'telescope' was coined that represents objects clearly, distinctly, and obscured by no haze; and it must magnify those same objects at least four hundredfold; for then it will show them twenty times closer. Unless the instrument is of such quality, it will be in vain to attempt to see all those things which have been observed by us in the heavens, or which will be enumerated below. Moreover, so that anyone may be made certain of the magnification of the instrument with little trouble, they shall outline two circles or two squares on paper, one of which is four hundred times larger than the other; this will be the case when the diameter of the larger is twenty times the length of the diameter of the other. Then, with both surfaces fixed upon the same wall, he shall look at them both at the same time from a distance—the smaller one with one eye applied to the Spyglass, and the larger one with the other eye free. This can be done conveniently at one and the same time with both eyes open; for then both figures will appear to be of the same size, provided the Instrument original: Organum has magnified the objects according to the desired proportion. Once such an instrument is prepared, one must inquire into the method of measuring distances; which we shall achieve by the following artifice. For the sake of easier understanding, let the Tube original: Tubus be ABCD. Let the eye of the observer be E. The rays, while no lenses were present in the tube, would be carried toward the object FG along the straight lines ECF and EDG; but with the lenses placed...