The subject of IAU Colloquium 165 and the year 1996, which is the 150th anniversary of the discovery of the planet Neptune, give the opportunity to recall facts which have led to the discovery of three new major planets in the Solar System.

Five planets plus the Earth, the Sun and the Moon were the only permanent objects known in the Solar System from Antiquity up to the 17th century when Galileo (1564–1642) discovered four new bodies around Jupiter.

The question of the dimensions of the Solar System and the distances of the stars soon became one of the main problems. From the parallax of Mars J.-D. Cassini (1625–1712) deduced the diameter of the Earth’s orbit and the astronomers attempted to determine the stellar parallax at six-month intervals at the Paris and Greenwich Observatories, leading Bradley (1693–1762) to the discoveries of aberration in 1726, and nutation in 1745.