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The superior planets, orbiting outside the Earth's orbit, do not exhibit a full range of phases since their maximum phase angles are smaller than 90°. Mars often appears significantly gibbous, it has a maximum phase angle of 45°. Jupiter has a maximum phase angle of 11.1° and Saturn of 6°, [1] so their phases are almost always full.
For some objects, such as the Moon (see lunar phases), Venus and Mercury the phase angle (as seen from the Earth) covers the full 0–180° range. The superior planets cover shorter ranges. For example, for Mars the maximum phase angle is about 45°. For Jupiter, the maximum is 11.1° and for Saturn 6°. [1]
This diagram shows various possible elongations (ε), each of which is the angular distance between a planet and the Sun from Earth's perspective. In astronomy, a planet's elongation is the angular separation between the Sun and the planet, with Earth as the reference point. [1] The greatest elongation is the maximum angular separation.
Diagram showing the eastern and western quadratures of a superior planet like Mars. In spherical astronomy, quadrature is the configuration of a celestial object in which its elongation is a right angle (90 degrees), i.e., the direction of the object as viewed from Earth is perpendicular to the position of the Sun relative to Earth.
The phase curves of many asteroids [32] have also been observed and they too may exhibit opposition surges. Asteroids can be physically classified in this way. [33] The effects of rotation can be very large and have to be factored in before the phase curve is computed. An example of such a study is reported by R. Baker and colleagues. [34]
Seen from a superior planet, an inferior planet on the opposite side of the Sun is in superior conjunction with the Sun. An inferior conjunction occurs when the two planets align on the same side of the Sun. At inferior conjunction, the superior planet is "in opposition" to the Sun as seen from the inferior planet (see the diagram).
In the reference frame of the Earth, where the terms were originally used, the inferior planets are Mercury and Venus, while the superior planets are Mars, Jupiter, Saturn, Uranus and Neptune. Dwarf planets like Ceres or Pluto and most asteroids are 'superior' in the sense that they almost all orbit outside the orbit of Earth.
For example, if a TNO is incorrectly assumed to have a mass of 3.59 × 10 20 kg based on a radius of 350 km with a density of 2 g/cm 3 but is later discovered to have a radius of only 175 km with a density of 0.5 g/cm 3, its true mass would be only 1.12 × 10 19 kg.