Mass is always positive or zero. Benjamin Franklin performed many experiments in electricity, and he discovered that electric charge can be positive, zero, or negative. The fundamental units are meter for length, second for time, gram for mass, and Ampere for electric current. Therefore, small angular sizes are very common. So, the Moon subtends 30'.
Stars are so far away that they have angular diameters of zero, i.e. they are point objects .
Angular measurement can be used to define yet another measure of linear distance, which arises naturally out of observational astronomy. When the Earth moves in its orbit, nearby stars can be observed to move against the static background of far-away stars. This apparent motion of an object due to the motion of the observer is known as parallax . Anyone is welcome to view this page in situ or print it, but any other copying is by permission only.