Understanding Solstices and Equinoxes

Use the Sky as Your Seasonal Guide

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

Imagine you had no watch or mobile phone or clock or calendar where you lived. How would you tell time? Know what time of year it is? It could be tough, unless you had a way to simply look around you and tell time by the objects you could see.

That's the way that prehistoric people lived. They used the sky as a timekeeper and calendar. In some places, such as Stonehenge (in England), they built monuments to track the motions they saw in the sky.

 

The rhythms of the Sun's apparent motions determine how life on Earth behaves. We say "apparent" because it isn't really the Sun that's moving. It appears to because Earth is turning on its axis, like a merry-go-round. As we spin around, we see the Sun appear to rise and set. 

The Sun appears to rise in the east and set in the west, as do the Moon, planets, and stars. The period from one sunrise to the next is just over 24 hours. The Moon shows us changes in its appearance (called phases) according to a cycle of roughly 28 days, which is the basis of our month. 

How Are Solstices and Equinoxes Determined? 

If you watch the sunrise and sunset each day (and remember never to look directly AT our hot, bright Sun), you'll see 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".

Essentially, early observers noticed 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 and the start of summer and the longest day of the year for people south of the equator. 

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, you'll see a very distinct shift of the positions north or south. 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!

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Petersen, Carolyn Collins. "Understanding Solstices and Equinoxes." ThoughtCo, Sep. 18, 2017, thoughtco.com/what-are-solstices-and-equinoxes-3073393. Petersen, Carolyn Collins. (2017, September 18). Understanding Solstices and Equinoxes. Retrieved from https://www.thoughtco.com/what-are-solstices-and-equinoxes-3073393 Petersen, Carolyn Collins. "Understanding Solstices and Equinoxes." ThoughtCo. https://www.thoughtco.com/what-are-solstices-and-equinoxes-3073393 (accessed January 16, 2018).