Boys start to get acne at the age of approximately 12 years, with a range of 9 to 15 years old. As most girls will attest, boys on average mature later than girls. Early on in the onset of teen acne, the main lesions are blackheads and whiteheads.
The angular size (angular diameter, apparent size) of an object as seen from a given point is the visual diameter of the object measured as an angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its center that is perpendicular to the viewing direction.
Linear speed versus angular speed. Using approximate measurememts, the earth's equatorial circumference is 24,000 miles and the earth rotates once in 24 hours so the speed of rotation at the equator is. 24,000 mi./24 hours = 1,000 mi/hr. But we can also measure the angular speed of rotation. 360 degrees/24 hr = 15 degrees per hour.