Astronimical unit
Length of the semimajor axis of Earth's orbit around the
Sun, 92,955,808 mi (149,597,870 km), often defined simply as the average
distance from Earth to the Sun.
Direct measurement through the parallax method cannot be
used for accurate determinations, because the Sun's glare blots out the light
of the background stars necessary to make the measurement. The most precise
values have been obtained by measuring the distance from Earth to other objects
orbiting the Sun. This indirect method requires an accurate proportional
mathematical model of the solar system; once the distance to one planet or
other object is determined, then the distance to the Sun can be calculated.
