Astronomical Unit (au)
One astronomical unit (1 au) is the "average" straight-line distance between the center-of-the-Earth and the center-of-the-Sun, and a convenient Solar System distance scale. Numerically, 1 au is a distance of approximately 150 billion meters (93 million miles) -- with an exact nominal value of 149,597,870,700 meters as defined by the International Astronomical Union's Resolution B2 (2012). The actual Sun-Earth distance varies between 0.9833 au and 1.0167 au throughout the year due to Earth's oval orbit. The Sun is exactly 1 au distant only twice/year about April 4 and October 5 (a few weeks after the spring and autumn equinoxes). Read more