Today marks the beginning of summer, the longest day of the year, and a time of celebration at Stonehenge and everywhere else across the northern hemisphere. What exactly is the summer solstice, and what makes it worthy of feast and fun? In short, the solstices (one occurring on or around June 21 and one occurring on or around December 21 each year, marking the beginning of summer or winter, depending on your hemisphere) occur because the Earth is tilted on its axis; it is on the solstices that the sun is tilted most toward (the summer solstice, with longer, warmer days) or away from (the winter solstice,  with cooler, shorter days) the sun. Because of the Earth's tilt and the