Beyond Earth orbit, the units of measurement we use on the surface of our planet rapidly become quite inadequate to deal with the vast distances of stellar and interstellar space. Our nearest planetary neighbour, Mars, is “just” 50 million miles away – the equivalent of a little under nine thousand road trips from New York to San Francisco and back again. However, the most distant planet, Neptune, is an intimidating 2.7 billion miles. The nearest star system to our own, Alpha Centauri, is over 25 trillion miles away. Beyond that the distances become so great that the number of zeroes involved in writing them becomes awkward indeed.
Instead, astronomers have developed a variety of shorthand systems for describing stellar distances and interstellar distances more quickly. Traditionally, these were based on angles and degrees of difference in the night sky. These ranged from parallax measurements within the solar system to the largest unit, the parsec, equal to one side of a right triangle drawn between certain stars in the sky.
Since the modern understanding of astronomy began to conceive of the universe as a massive volume of space in which Earth occupies no special position, rather than being largely limited to tracking pinpoints of light in the night sky, other units have become more common. Within our own solar system, astronomers will most currently refer to distances in terms of Astronomical Units, or AU. One AU is equivalent to the distance between the Earth and the Sun: that is, about 93 million miles. Within the inner solar system, the planets are close enough together that we can still describe distances in miles: for instance, Mars is 50 million miles farther from the Sun than us, and Venus is about 26 million miles in the other direction.
Beyond the Asteroid Belt, however, distances rapidly grow much greater. Jupiter lies at 5 AU, or five times as far from the Sun as the Earth. Saturn’s orbit is 9 AU from the Sun, Uranus’s is about 19 AU, and Neptune’s is 29 AU. Beyond that, the dwarf planets of the Kuiper Belt follow extremely elliptical orbits that can take them even greater distances from the Sun. For instance, Pluto’s orbit takes it from 29.5 AU at its closest, to 49 AU at its farthest point. Eris, the dwarf planet which is larger than Pluto and whose discovery prompted the Pluto planet debate in 2006, ranges from under 38 AU to over 97.5 AU.
Outside the solar system, even the Astronomical Unit starts to become awkwardly small, however. For this reason, astronomers most commonly refer to interstellar distances using another shorthand measurement: the light-year, or the distance that a beam of light travels in one year. One light-year is equal to about six trillion miles, or 63,000 AU. Alpha Centauri is 4.35 light-years away, but the Milky Way Galaxy is tens of thousands of light-years across, and the nearest major galaxy to our own, Andromeda, is about 2.2 million light-years away. Currently, the edge of the observable universe is about 13.7 billion light-years away.
