Good morning, A method of calculating positions of planets and other bodies of the solar system is to compute them from the orbital elements given for some epoch, using methods described, for example, in the site below. http://ssd.jpl.nasa.gov/?bodies#elem However, this site states that, in general, orbital elements should not be used to calculate ephemerides. My doubt is: Why? How are ephemerides calculated? Aren't orbital elements used at least at some point? Thank you in advance.