Your Guide to Solstices and Equinoxes

Stonehenge, near Salisbury, Great Britain
David Nunuk / Getty Images

Solstices and equinoxes are interesting terms that show up each year on our calendars. They are related to astronomy and the motions of our planet. Most people think of them as the "start" of a season. That's true as far as a date on a calendar is concerned, but they don't necessarily predict climate or weather.

These terms are related to the rhythms of the Sun's apparent motions technically determine the exact day and time of solstice and equinox.

The word "apparent" is important because it isn't really the Sun that's moving. It only appears to move because Earth is turning on its axis, like a merry-go-round. As the planet spins around, people see the Sun appear to rise in the east and set in the west.  The Moon, planets, and stars all appear to do the same thing, for the same reason. 

How Are Solstices and Equinoxes Determined? 

Watch the sunrise and sunset each day (and remember never to look directly AT our hot, bright Sun), and notice its rise and set points change throughout the year. Notice also that the Sun's position in the sky at noon is farther north at some times of year and more southerly at other times. The sunrise, sunset, and zenith points slide slowly to the north from December 21-22 to June 20-21 each year. Then, they appear to pause before starting the slow daily slide toward the south, from June 20-21st (the northernmost point) to December 21-22 (the southernmost point).

Those "stopping points" are called the solstices (from the Latin sol, which means "sun", and sistere, which means "stand still"). These terms stem back to a time when early observers had no knowledge of Earth's motions in space but did notice that the Sun appeared to stand still at its northernmost and southernmost points, before resuming its apparent motion south and north (respectively).

Solstices

Summer solstice is the longest day of the year for each hemisphere. For northern hemisphere observers, the June solstice (the 20th or 21st), marks the beginning of summer. In the southern hemisphere, that's the shortest day of the year and marks the beginning of winter.

Six months later, on December 21st or 22nd, winter begins with the shortest day of the year for northern hemisphere people. It's the start of summer and the longest day of the year for people south of the equator. This is why such solstices are now called the December and June solstices, rather than "winter" or "summer" solstices. It recognizes that the seasons for each hemisphere correspond to north or south location. 

Equinoxes

Equinoxes are also connected to this slow change of apparent solar position. The term "equinox" comes from two Latin words aequus (equal) and nox (night). The Sun rises and sets exactly due east and due west on the equinoxes, and day and night are of equal length. In the northern hemisphere, the March equinox marks the first day of spring, while it's the first day of autumn in the southern hemisphere. The September equinox is the first day of fall in the north and the first day of spring in the south.

 

So, the solstices and equinoxes are important calendar points that come to us from the apparent position of the Sun in our sky. They are also intimately connected to the seasons but are not the sole reason why we have seasons. The reasons for the seasons are linked to Earth's tilt and its position as it orbits the Sun. 

Take a moment each day to observe the sky; notice sunrise or sunset and mark where those occur along your horizon. After a few weeks, it's very easy to note a  very distinct shift of the positions north or south. Check out the appearance points of sunrise and sunset against the printed calendar and see how close they come to matching. It's a great long-term science activity for anyone to do, and has been the subject of more than a few science fair projects! 

While solstices and equinoxes hark back to a time in human history when sky observers had no way of knowing about our planet's motions in space, they still mark important dates that give people clues about the change of seasons.

Today, ancient astronomical markers such as Stonehenge remind us that people have been looking to the sky, and measuring its motions, since the dawn of human history.