The summer solstice is Sunday, marking the longest day of the year in the Northern Hemisphere and the start of astronomical summer. It’s the opposite in the Southern Hemisphere, where it is the shortest day of the year and winter will start. The Earth’s axis is tilted at an angle as it travels around the sun. The solstices mark the times when the Earth is tipped most extremely either toward or away from the sun. This means the hemispheres are getting very different amounts of sunlight, and days and nights are at their most unequal.