Elongation (astronomy) - Elongation of Superior Planets

Elongation of Superior Planets

Superior planets, dwarf planets and asteroids undergo a different cycle. After superior conjunction, such an object's elongation continues to increase until it approaches a maximum value larger than 90° (impossible with inferior planets) and typically very near 180°, which is known as opposition and corresponds to a heliocentric conjunction with Earth. In other words, as seen from an observer on the superior planet at opposition, the Earth appears at inferior conjunction with the Sun. Technically, the exact moment of opposition is slightly different from the moment of maximum elongation. Opposition is defined as the moment when the apparent ecliptic longitudes of the superior planet and the Sun differ by 180°, which ignores the fact that the planet is outside the plane of the Earth's orbit. For example, Pluto, whose orbit is highly inclined to the Earth's orbital plane, can have a maximum elongation significantly less than 180° at opposition.

All superior planets are most easily visible at their oppositions because they are near their closest approach to Earth and are also above the horizon all night. The variation in magnitude caused by changes in elongation are greater the closer the planet's orbit is to the Earth's. Mars' magnitude in particular changes with elongation: it can be as low as +1.8 when in conjunction near aphelion but at a rare favourable opposition it is as high as −2.9, or seventy-five times brighter than its minimum brightness. As one moves further out, the difference in magnitude caused by the difference in elongation gradually fall. The maximum and minimum brightness of Jupiter differ by only a factor of 3.3 times, whilst those of Uranus – which is the most distant Solar System body visible to the naked eye – differ by a factor of 1.7 times.

Since asteroids travel in an orbit not much larger than the Earth's, their magnitude can vary greatly depending on elongation. Although more than a dozen objects in the asteroid belt can be seen with 10x50 binoculars at an average opposition, only Ceres and Vesta are always above the binocular limit of +9.5 at small elongations.