Parsec Parsec The parsec (symbol: pc) is a unit of length used to measure large distances to astronomical objects outside the Solar System. A parsec is defined as the distance at which one astronomical unit subtends an angle of one arcsecond, which corresponds to astronomical units. One parsec is equal to about 3.26 light-years (30 trillion km or 19 trillion miles) in length. The nearest star, Proxima Centauri, is about from the Sun. Most of the stars visible to the unaided eye in the night sky are within 500 parsecs of the Sun. The parsec unit was probably first suggested in
Pearson symbol more commonly used hexagonal setting has 3 translation equivalent points per unit cell. The Pearson symbol refers to the hexagonal setting in its letter code ("hR") but the following figure gives the number of translation equivalent points in the primitive rhombohedral setting. Examples: "hR"1 and "hR"2 are used to designate the Hg and Bi structure, respectively. The Pearson symbol should only be used to designate simple structures (elements, some binary compound) where the number of atoms per unit cell equals, ideally, the number of translation equivalent points. Pearson symbol The Pearson symbol, or Pearson notation, is used in crystallography as