The Sun, the controlling body of the Solar System, is the only star close enough to be studied in detail. It is 270 000 times closer than the nearest stars beyond the Solar System, those of the α Centauri group. Data are given in Table 2.1.

DISTANCE

The first known estimate of the distance of the Sun was made by the Greek philosopher Anaxagoras (500–428 BC). He assumed the Earth to be flat, and gave the Sun's distance as 6500 km (using modern units), with a diameter of over 50 km. A much better estimate was made by Aristarchus of Samos, around 270 BC. His value, derived from observations of the angle between the Sun and the exact half Moon, was approximately 4 800 000 km; his method was perfectly sound in theory, but the necessary measurements could not be made with sufficient accuracy. (Aristarchus also held the belief that the Sun, not the Earth, is the centre of the planetary system.) Ptolemy (c. AD 150) increased the distance to 8 000 000 km, but in his book published in AD 1543 Copernicus reverted to only 3 200 000 km. Kepler, in 1618 gave a value of 22 500 000 km.

The first reasonably accurate estimate of the Earth–Sun distance (the astronomical unit) was made in 1672 by Giovanni Cassini, from observations of the parallax of Mars. Some later determinations are given in Table 2.2.