What is the difference between solstice and equinox?
The words solstice and equinox refer to four specific days in the year—one in each of the four seasons. A solstice occurs in summer and winter; an equinox, in spring and fall.
The word solstice comes from the Latin words sol (sun) and stitium (standstill). At the solstice, the sun appears to "stand still". What does this mean?
During the winter and spring months in the northern hemisphere, the sun rises at a more northerly point on the horizon each day. When the sun reaches its extreme northerly point, it rises above the line of latitude known as the Tropic of Cancer (about 23.5° north of the equator). In the northern hemisphere, that day marks the summer solstice, the longest day of the year and the beginning of summer.
For a few days before and after the solstice, the change in position of the sunrise is so slight that the sun seems to come up at the same point on the horizon each day. Thus, it appears to stop or "stand still" for a few days before it changes direction and begins to rise farther and farther to the south each day. When the sunrise reaches its extreme southerly point, it is directly over the Tropic of Capricorn (about 23.5° south of the equator). In the northern hemisphere, that day marks the winter solstice, the shortest day of the year and the first day of winter.
In the northern hemisphere, the summer solstice happens on or around June 21, and the winter solstice happens on or around December 22. (Of course, because the seasons are reversed below the equator, the southern hemisphere enjoys its summer solstice around December 22 and experiences its winter solstice around June 21.)
The word equinox comes from the Latin words aequus (equal) and nox (night). At the equinox, the night is almost equal in length to the day. Why does this happen?
After the winter solstice, as the sun rises farther and farther to the north, the days in the northern hemisphere grow longer, while the nights grow shorter. Midway through this process, at the spring equinox, the daytime and nighttime hours are almost equal. After the summer solstice, the longest day of the year, the process is reversed. The days grow shorter and the nights longer, until they are once again almost equal, at the fall equinox.
The spring equinox (called the vernal equinox) happens on or around March 21, and the fall (or autumnal) equinox happens on or around September 23.
If you plan to read more on this topic, the following terms may help you: