ABSTRACT
This chapter discusses the problem of referring the theoretical orbit to the observations. It examines how the apparent positions can be determined at every instant from the knowledge of the orbital elements; then, the inverse and much more difficult problem of how the orbital elements can be derived from the observed positions will be briefly examined. The inverse problem of deriving the six orbital elements of a Solar System object from the observations, is much more complex than the direct one, so that only some basic notions can be dealt. The intersection of the planet's orbital plane with the ecliptic is a line passing through the Sun called the line of the nodes. Other interesting properties of the orbits of the planets are their small eccentricities and modest inclination to the ecliptic, the largest values being those of Mercury and of Pluto, and the direct sense of their motion.
