The Earth's tilt is to thank for this demarcation of seasons: Summer solstice occurs when the North Pole is tilted directly toward the sun. (The Southern Hemisphere experiences its summer solstice in December, when the South Pole is directly tilted toward the sun.)Signifying growth and life, the event has a long history of being recognized as a critical event in the calendar, and many cultures have developed celebrations and rituals to coincide with it.—Katie Langin