Seasons begin and end on different dates in different countries. In the United States, people say the seasons begin at the solstices and equinoxes. The summer solstice is the longest day of the year, and the winter solstice is the shortest. The equinox is the time when the day and the night are the same number of hours, assuming the sun were a point of light at its center. Since civil dawn occurs when the edge of the sun first appears over the horizon and civil dusk occurs when the edge drops over the horizon, the civil length of day is 12 hours long several days prior to the equinoxes. In the USA, summer begins at summer solstice, winter at winter solstice, spring at the spring ("vernal") equinox and autumn at the autumnal equinox.