Illustration of the apsidal motion. At time t₀ the apsis (semi-major axis of the orbit) is oriented at an angle of ω₀ (right). At time t it moved to the direction characterized by (left). Traditionally, the argument of periastron is measured from East to the direction of the observer, however, the radial velocity (and the TTV) equations are invariant if we measure it from West. This causes no difficulties in the measurements of , and the East–West discrepancy can be resolved via time-series done via interferometry, astrometry or direct imaging.