
Fig1_4 The Sun's angular size and radius
Fig. 1.4 . The solar radius can be determined from the Suns angular size and distance. As long as this angle is small, the physical size is just a small arc of a large circle, denoted by the dashed line, and the angular size is the ratio of the physical size to the distance. Astronomers specify this angle as a partial arc of a full circle of 360 degrees, and for the Sun it is about 32 minutes of arc, where there are 60 minutes of arc in one degree. This angle has been enlarged to display it in this illustration. In mathematics, the radian is the standard unit of angular measure. It describes the angle subtended by a circular arc as the length of the arc divided by the radius of the arc. When the arc length is equal to the arc radius, the angle is one radian. You can convert between the two methods of describing angles by noting that the circumference of a circle is 2p times its radius, so 1 radian is equal to 360 degrees/(2p), or 57.2958 degrees. For the Sun, the angular size q = 2R/D radians, where R denotes the Suns radius and the mean distance of the Sun, D, is 1 AU. The observed angular size of the Sun corresponds to a radius of 695.5 million meters.
Copyright 2010, Professor Kenneth R. Lang, Tufts University

