Benjamin Franklin performed many experiments in electricity, and he discovered that electric charge can be positive, zero, or negative. We will also be using many other quantities which depend on the above, e. 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 . So, if a star exhibits a maximum parallax of one second of arc when the Earth has moved through 1 AU, it is one parsec away. Used under license.