Astronomical distances are so great that use of terrestial units (miles and kilometers) makes calculations difficult and interferes with conceptual reasoning. Some units of distance used by astronomers are fairly simple to define, others require a little more explanation and are less commonly used.
The most common unit is the light-year. It is simply the distance light travels in one terrestial year or about 6 trillion (6,000,000,000,000) miles. Light year is commonly abbreviated as LY. A light-minute is occasionally used to express distance within the solar system. It is equivalent to about 11,415,500 miles and is abbreviated LM. The Sun is about 8 LM away and the next nearest star, Alpha Centauri, is 4 LY distant.
Astronomers use angular distance many times when referring to celestial objects. The concept is simple. There are two objects in the sky. Draw a straight line from the observer to each object and measure the included angle between the lines. Commonly, angular distance is measured in degrees with 360 degrees in a complete circle. Most astronomical angular measurements are much smaller than a degree. An arc-minute is 1/60 of a degree and an arc-second is 1/60 of an arc-minute. The full moon is about 1800 arc-seconds angular diameter.
A unit found in scientific literature is the parsec, abbreviated pc, is equivalent to 3.26 LY. Draw a straight line from the Sun's center to a celestial object. Draw a line perpendicular to the first intersecting the Earth's orbit twice and the center of the Sun. Note an object's position against the galactic background as the Earth passes through one of the two intersection points. Again record the object's position as the Earth passes through the other intersection point. If the object is located 3.26 LY from the Sun's center, it will appear to shift one arc-second against the galactic background between the two observations. This effect is called parallax, hence, the term parsec.
The astronical unit, abbreviated a.u., is the mean distance between Earth and Sun. It's equal to 93 million miles or about 8 LM.