solstice

A solstice is the moment when the Sun reaches its most southern or northern position in the Earth sky, which is on the days when the hemisphere receives the most or least sunlight, i.e., the days of the year with the longest and shortest daylight. The summer solstice occurs on the longest day in the summer, and the winter solstice, the shortest day in winter. As summer and winter are opposed in the two hemispheres, summer solstice in the northern hemisphere is the same moment as winter solstice in the southern, and vice versa. These days occur because given the Earth's axial tilt, along with the fact that this axis-direction remains (very-nearly) the same over the course of the Earth's orbit. The hemispheres are angled so the Sun shines more on one than the other over half Earth's orbit (and year), then more on the other hemisphere over the other half. The solstices are the points of time within the Earth's orbit when one of the two hemispheres (and its pole) is most-tilted toward the Sun.