A circle, placed on the reticle of a transit or theodolite, with an angular radius of 15' 45" and centred on the intersection of the cross hairs. The Sun's radius on 1 July is 15'45" and is approximately the minimum for the year; there is a slight, noticeable overlap of the image from about September to April. In making observations, the image and the circle are taken concentrically to result in horizontal and vertical angles to the Sun's center.