Here formula_45 is the effective inclination of Saturn's rings (their tilt relative to the observer), which as seen from Earth varies between 0° and 27° over the course of one Saturn orbit, and formula_46 is a small correction term depending on Uranus' sub-Earth and sub-solar latitudes. formula_47 is the Common Era year. Neptune's absolute magnitude is changing slowly due to seasonal effects as the planet moves along its 165-year orbit around the Sun, and the approximation above is only valid after the year 2000. For some circumstances, like formula_48 for Venus, no observations are available, and the phase curve is unknown in those cases.