Elongation (astronomy)

Since an inferior planet is well within the area of Earth's orbit around the Sun, observation of its elongation should not pose that much a challenge (compared to deep-sky objects, for example).

When a planet is at its greatest elongation, it appears farthest from the Sun as viewed from Earth, so its apparition is also best at that point.

Thus Venus's synodic period, which gives the time between every two eastern greatest elongations, is 584 days; this also applies to the western counterparts.

When a planet is closer to the Sun it moves faster than when it is further away, so exact determination of the date and time of greatest elongation requires a much more complicated analysis of orbital mechanics.

For example, Pluto, whose orbit is highly inclined to the essentially matching plane of the planets, has maximum elongation much less than 180° at opposition.

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, which translates to seventy-five times brighter than its minimum brightness.

Since asteroids travel in an orbit not much larger than the Earth's, their magnitude can vary greatly depending on elongation.