On or about Monday, June 20, 2016, our planet will experience its summer solstice. The word "solstice" means "sun-stopping," because after this date, the place where the sun rises and sets reverses course. For example, from the winter solstice on, the sunrise and sunset points move further and further north until the summer solstice, when it reaches its northernmost point. That's why in the northern hemisphere, it's the longest day of the year, and in the southern hemisphere, it's the shortest. After the summer solstice, the sunrise and sunset points gradually move more and more south until we reach the winter solstice, where the sun rises and sets at its southernmost point.