Why Are Orbital Elements Not Preferred for Ephemerides Calculation?

[pc2-brazil]

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.

 

[lippyka]

Ephemerides are typically calculated by integrating a set of equations describing the motion of the body in question. This requires knowledge of the position and velocity of the body at some point in time (the epoch) and the equations of motion. The initial conditions needed to integrate the equations of motion can be derived from the orbital elements, which describe the shape and orientation of the orbit, its size and its orientation in space. However, the orbital elements change with time due to perturbations from other bodies, so they may not provide accurate positions over longer periods of time. Therefore, it is preferable to use direct numerical integration methods, such as numerical solutions of the equations of motion, rather than relying on orbital elements to calculate ephemerides.