## Distances in Space

Space is vast and to try and measure it we must use distance measurement units unfamiliar on Earth.  Astronomers decided to use the speed of light as the basic unit of measure since it is a universal constant: 186,282 miles (~300,000 km) per second in a vacuum.  That would put the Moon at 1.3 light seconds away from Earth.  Light travels 11-million miles in one minute.  The Sun is 8.45 light minutes from Earth.  Neptune is 4.26 light hours away from the Sun.  Light travels 6-trillion miles in one year.  Proxima Centauri, the closest star is 4.22 light years away (~25-trillion miles).  The Milky Way galaxy has a diameter between 100,000 and 130,000 light years.  The M31 Andromeda Galaxy is 2.54 million light years away.   The Virgo Cluster of galaxies are about 54 million light years away.

## Space Units of Measure

Miles and Kilometers are so tiny in space that we have crafted better units based on observed phenomena.  The Earth’s Orbital Radius (Semi-Major axis of the elliptical orbit of Earth and Sun) is called one Astronomical Unit (1 AU = 93 million miles).  Using that distance as the base line to measure parallax to a star results in another empirically measured unit, the parsec.  1 parsec = 3.26 light years.  A star at 3.26 light years distance would appear to move 1 arc-second in position for the Earth’s movement of 1 AU as we orbit the Sun.  Proxima Centauri is 1.29 parsecs from the Sun.  We multiply the parsec by 1,000 for the kiloparsec, Kpc (3,260 light years) and by 1 million for the megaparsec, Mpc (3,260,000 light years).  The M31 Andromeda Galaxy is 0.78 Mpc from the Milky Way.  Gigaparsec is 3,260,000,000 light years, Gpc. These units are derived directly from parallax measurements of stars and galaxies based on the Earth’s orbital motion about the Sun.  They can be converted easily to light years and miles, but are a shorthand for these otherwise huge resulting numbers.