Understanding Cosmic Scale
Miles and kilometers are useless here. Learn the true language of the universe.
The Astronomical Unit (AU)
An Astronomical Unit represents the average distance from the Earth to the Sun—about 93 million miles (150 million kilometers). We use AU to measure distances within our own solar system, such as the orbit of Jupiter or the distance to distant comets.
The Light Year (ly)
Despite the name, a light year is a measure of distance, not time. It is the distance that light travels in a vacuum in one Earth year. Since light moves at a staggering 299,792 kilometers per second, one light year is approximately 5.88 trillion miles (9.46 trillion kilometers).
The Parsec (pc)
A portmanteau of "parallax of one second," the parsec is the unit preferred by professional astronomers for measuring distances outside our solar system. One parsec is equivalent to about 3.26 light years. It is defined as the distance at which one AU subtends an angle of one arcsecond.