Astronomical Unit
An astronomical unit (AU) is the average distance from Earth to the Sun, about 150 million kilometers, used to measure distances in the solar system.
Formula
1\ \text{AU} \approx 1.496 \times 10^{8}\ \text{km}
Definition
An astronomical unit, or AU, is the distance from Earth to the Sun, about $150$ million kilometers, used to compare distances inside the solar system; Earth is $1$ AU from the Sun. It is the average distance, about $1.496 \times 10^8\ \text{km}$, since Earth's slightly elliptical orbit ranges from about $147$ million km in early January to $152$ million km in early July, and Kepler's third law takes its simplest form when distances are in AU and periods in years. Since 2012 the IAU has defined the AU as exactly $149{,}597{,}870{,}700\ \text{m}$, replacing the earlier definition tied to Earth's orbit. It anchors the parsec, the distance at which $1$ AU subtends one arcsecond, and was historically measured by planetary parallax and transits of Venus and today by radar ranging to planets.
Example
Mars orbits at about $1.5$ AU and Jupiter at about $5$ AU, so saying Neptune is $30$ AU away is simpler than saying $4.5$ billion kilometers. Light takes about $8.3$ minutes to cross $1$ AU and about $5.5$ hours to reach Pluto at an average of $39$ AU. Radar pulses to Venus at closest approach return after about $4.6$ minutes; multiplying by $c$ and halving gives about $0.28$ AU, consistent with $1\ \text{AU} - 0.72\ \text{AU}$.
Key Insight
Earth is closest to the Sun in early January, during Northern Hemisphere winter, which shows that distance is not what causes seasons; the $23.5^\circ$ tilt of Earth's axis is. The AU is convenient but small: there are about $63{,}000$ AU in a light-year, so the nearest star at $4.2$ light-years is roughly $270{,}000$ AU away, about $9{,}000$ times farther than Neptune.