In the construction of analytical theories of planetary motion, the effect is considered of errors in the adopted values of masses, orbital elements and ephemeris time upon the accuracy of computed co-ordinates of the inner planets in accordance with Newcomb's expansions and constants (with Ross's elements for Mars). These are accurate to five decimal places. The errors in the orbital elements are the principal source of inaccuracy in the positions of the planets. The precision of the ephemerides can be considerably improved by introducing appropriate corrections.