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Astronomical unit (AU, or au), a unit of length effectively equal to the average, or mean, distance between Earth and the Sun, defined as 149,597,870.7 km (92,955,807.3 miles). Alternately, it can be considered the length of the semimajor axis—i.e., the length of half of the maximum diameter—of Earth’s elliptical orbit around the Sun. The astronomical unit provides a convenient way to express and relate distances of objects in the solar system and to carry out various astronomical calculations. For example, stating that the planet Jupiter is 5.2 AU (5.2 Earth distances) from the Sun and that Pluto is nearly 40 AU gives ready comparisons of the distances of all three bodies.

In principle, the easiest way to determine the value of the astronomical unit would have been to measure the Earth-Sun distance directly by means of the parallax method. In this approach, two observers stationed at the ends of a long, accurately known baseline—ideally, a baseline as long as Earth’s diameter—would simultaneously record the position of the Sun against the essentially motionless background of the distant stars. Comparison of the observations would reveal an apparent shift, or angular (parallax) displacement, of the Sun against the remote stars. A simple trigonometric relationship incorporating this angular value and the baseline length then could be used to find the Earth-Sun distance. In practice, however, the method cannot be applied, because the Sun’s intense glare blots out the background stars needed for the parallax measurement.

By the 17th century astronomers understood the geometry of the solar system and the motion of the planets well enough to develop a proportional model of objects in orbit around the Sun, a model that was independent of a particular scale. To establish the scale for all orbits and to determine the astronomical unit, all that was needed was an accurate measurement of the distance between any two objects at a given instant. In 1672 the Italian-born French astronomer Gian Domenico Cassinimade a reasonably close estimate of the astronomical unit based on a determination of the parallax displacement of the planet Mars—and thus its distance to Earth. Later efforts made use of widely separated observations of the transit of Venus across the Sun’s disk to measure the distance between Venus and Earth.