What is the difference between a solstice and an equinox?
Four days during our calendar year -- two solstices and two equinoxes -- mark the beginnings of the four seasons. At the extremes are the winter and summer solstices, with the vernal and autumnal equinoxes occupying the midpoints. The solstices and equinoxes also mark four important points in Earth's orbit around the Sun. It's Earth's position in its orbit -- and the orientation of its tilted axis at these points in its orbit -- that defines the seasons.
The solstices are six months apart and mark the days when the northern and southern hemispheres receive either maximum (summer) or minimum (winter) sunshine. In the north, the summer solstice, usually around June 21, is the longest day of the year; the winter solstice, six months later, is the shortest day of the year. The two equinoxes occur roughly midway between the solstices: the autumnal equinox in September and spring equinox in March. At these times, day and night are roughly of equal length.
The times of the solstices and equinoxes were important to ancient agrarian cultures, since these observations warned of changing weather conditions. For instance, the spring equinox signals the beginning of warmer weather.
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