Answer:
June 21 is on or about the summer equinox when the sun is directly overhead at the equator - both hemispheres receive equal sunlight.
Dec 22 is on or about the winter solstice - since the axis of the earth is tilted about 23 deg from the plane (perpendicular to) of the earth's orbit, the southern hemisphere receives more direct  sunlight because of the orientation of the earth's axis and the plane of the earth's orbit
