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Solstice vs. equinox

Question:

What is the difference between solstice and equinox?

Answer:

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.

Solstice

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.)

Equinox

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.

Vocabulary

If you plan to read more on this topic, the following terms may help you:

(solar) altitude:
the height of the sun relative to the horizon
autumnal equinox:
a brief period when day and night are basically equal in duration,on or around September 23
axial tilt:
the number of degrees the Earth leans toward or away from the sun, measured from its axis; the Earth has an axial tilt of about 23.5° at all times
axis:
an imaginary line that goes through the centre of the Earth, from the North Pole to the South Pole
equator:
an imaginary line that can be drawn around the Earth at its middle
horizon:
a point where the Earth and sky seem to meet
line of latitude (or parallel of latitude):
an imaginary line that can be drawn around the Earth, parallel to the equator (that is, the same distance from the equator at every point) The latitude of the equator is 0° and the latitude of the North and South poles is 90°.
line of longitude (or meridian of longitude):
an imaginary line that can be drawn around the Earth, starting at the North Pole and going to the South Pole
northern hemisphere:
the half of the Earth that is north of the equator
orbit:
the path followed by a planet (or star) around another planet (or star)
Polaris:
the North Star
southern hemisphere:
the half of the Earth that is south of the equator
summer solstice:
a brief period when the sun does not appear to move, on or around June 21
Tropic of Cancer:
the latitude line at 23.5° N (north)
Tropic of Capricorn:
the latitude line at 23.5° S (south)
vernal equinox:
a brief period when day and night are basically equal in duration, on or around March 21
winter solstice:
a brief period when the sun does not appear to move, on or around December 22
zenith:
the point in the sky directly overhead; the highest point above the horizon