The Astronomical Unit

Abstract

The mean distance between the Earth and the Sun is designated as the astronomical unit (AU).¹ This is perhaps the most basic unit of astronomy. Astronomers have been concerned about its determination for over two thousand years. By the early seventeenth century, the sizes of the orbits of the known planets relative to the Earth’s orbit were obtainable with reasonably good precision. To translate these distances into miles, meters, or kilometers, however, the distance from the Earth to the Sun (or, more easily, the distance from the Earth to another planet) had to be determined in those units. Since the distance in AU was already known, the AU would then be calibrated. The transits of Venus were observed in previous centuries for just such a purpose²; in the twentieth century, to obtain a higher precision, the minor planet Eros and others were observed.³ In the latter half of the twentieth century, the Doppler ranging technique, which employs the principles discussed in this chapter, was first applied to the planets.⁴