pyephem calculation of Mars opposition via earth_distance - pyephem

Question

I looped through all days in 2014 looking for the date with the smallest Earth-Mars distance. I expected this to occur on Mars opposition on Apr 8, 2014 but April 14 was the date with the lowest distance. Why?
date time mars.earth_distance (AU)
2014/4/1 00:00:00 0.636615
2014/4/2 00:00:00 0.633898
2014/4/3 00:00:00 0.631388
2014/4/4 00:00:00 0.629085
2014/4/5 00:00:00 0.626992
2014/4/6 00:00:00 0.625109
2014/4/7 00:00:00 0.623436
2014/4/8 00:00:00 0.621973
2014/4/9 00:00:00 0.620720
2014/4/10 00:00:00 0.619676
2014/4/11 00:00:00 0.618842
2014/4/12 00:00:00 0.618216
2014/4/13 00:00:00 0.617798
2014/4/14 00:00:00 0.617585
2014/4/15 00:00:00 0.617578
2014/4/16 00:00:00 0.617774
2014/4/17 00:00:00 0.618171
2014/4/18 00:00:00 0.618768
2014/4/19 00:00:00 0.619563
2014/4/20 00:00:00 0.620554
Similarly, the largest apparent size occurs on 12/15 and brightest magnitude occurs on Apr 12 rather than at opposition on Apr 8.

Answer 1

The moment of “opposition” is not the moment that a planet is closest or brightest, but is (if I recall) the moment when the elliptical longitude of the planet is 180° — exactly halfway across the sky, which is as far as possible — away from that of the Sun from the point of view of Earth.
If the planets traveled in perfect circles centered on the sun and at a uniform velocity, then the moment of opposition would also be the moment when the planet is closest and brightest. But since the planets are in elliptical orbits and move at a non-uniform rate, they can pass opposition but still be moving toward each other — or already be moving away from each other.
Magnitude is affected by both the phase of the planet and its distance, so we would expect the maximum magnitude to fall between the moment when the phase is greatest (at the moment of opposition) and the moment when the distance is smallest — and that is in fact what your numbers seem to indicate.