Simple. Just think of it this way. The north pole and the south pole always have opposite seasons and daylight exposure. When it's the longest sunlit day of the year at the north pole, it's automatically the shortest and darkest day of the year at the south pole. So, the longest sunlit day of the year at any point north of the equator is always June 21st, and the shortest is always 6 months later on December 21st. If I'm standing at the equator on December 21st then and want more sunlight, I would have to go south, not north. So the farther south you go, the more sunlight you would have. 40 degrees south is the farthest south of your three points and puts you almost half way to the south pole.
