GNOMON, nemon, an astronomical instru ment for measuring the altitudes and declina tions of the sun and stars. It is usually a pillar or pyramid, erected upon level ground or on a pavement and is especially used for making the more important observations. Many have pre ferred it to the smallest quadrants, both as more accurate and more easily made and applied. The most ancient observation of this kind extant is that made by Pytheas, in the time of Alex ander the Great, at Marseilles, where he found the height of the gnomon was in proportion to the meridian shadow at the summer solstice, as 213% to 600. This method of observation was by no means accurate in ancient times, since observers did not take into account the sun's parallax, which makes his apparent altitude less than it would be if the imomon were placed at the centre of the earth; they also neglected refraction, by which the apparent height of the sun is somewhat increased; and made their cal culations as if the shadows were terminated by a ray coming from the sun's centre; whereas it is bounded by one coming from the upper edge of his limb. These errors, however, may be
easily allowed for; and, when this has been done, the ancient observations are generally found to coincide nearly with those of the moderns.
Gnomon, in geometry, is the space included between the lines forming two similar parallelo grams, usually squares, of which the smaller is inscribed within the larger, so as to have one angle in each common to both. The word gnomon is also used to designate an odd num ber ; one of the terms of a series in arithmetic for the finding of polygonal numbers. See