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Astronomical Unit (au)

One astronomical unit (1 au) is essentially the straight-line distance between the center-of-the-Earth and the center-of-the-Sun. 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). By specifying exactly how many meters are in an astronomical unit, the au is a convenient Solar System distance unit.

The mean distance from the ☉ Sun to the planet ☿ Mercury is ~0.4 au and ♇ Pluto is ~40 au. The furthest human-made spacecrafts, NASA's Voyager I and II (launched in 1977) are ~146 au and ~121 au, respectively, distant from the Sun. Both Voyagers have exited the Solar System and are in the interstellar space beyond the Sun's heliosphere (its influence boundary).

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).

Light, which travels at a speed of 63,240 au/year, takes ~499 seconds (8 minutes 19 seconds) of time to travel the 1 au distance from the Sun to the Earth.

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