ABSTRACT
In this chapter, the authors discuss the problem of referring the theoretical orbit to the observations. First, they examines how the apparent positions can be determined at every instant from the knowledge of the orbital elements; then, the inverse and more difficult problem of how the orbital elements can be derived from the observed positions is briefly examined. The maximum elongations of Mercury and of Venus are 23° and 46°, respectively (some authors call elongation the geocentric angle Sun–planet, while strictly speaking the name refers to the distance along the ecliptic, namely, the difference in ecliptic longitudes). The inverse problem, of deriving the six orbital elements of a Solar System object from the observations, is more complex than the direct one. The study of binary stars has made great progress by virtue of modern optical techniques, such as adaptive optics and multiple telescope array interferometry.
