Today, March 19, 2020, is the March equinox. Calendrically speaking, it’s the first day of spring in the northern hemisphere and the first day of autumn in the southern hemisphere. So, happy new season, wherever you are hunkering down during the world-wide pandemic of COVID-19. If you can, step outside for some fresh air and wave to your neighbors (distantly) and enjoy the new season!
The Equinox, Solstice, and our Calendar
Solstices and equinoxes are calendar dates actually related to astronomy and the motions of our planet. Most people think of them as the “start” of a season. They aren’t weather predictions, but are related to positions of the Sun as seen from Earth’s surface. It appears to move, but it’s actually a result of Earth spinning on its axis.
Think of a merry-go-round. If you’re on it and look out at people standing around watching it spin they appear to move. But, they’re really standing still. It’s YOU (and the merry-go-round) that is moving.
It’s the same with Earth. As the planet spins around on its axis, we see the Sun appear to rise in the east and set in the west. Same thing with the Moon and stars. They look like they’re moving across the sky. That apparent motion is at the heart of why we calculate equinox and solstice dates. (Solstices happen in December and June.)
Understanding the Solstice and Equinox
Watch the sunrise and sunset each day (and remember: don’t directly at the Sun). If you do this for a while, you’ll notice that its rise and set points change throughout the year. The Sun also seems to move farther north or farther south through the year. Its sunrise, sunset, and zenith points slide slowly to the north from December 21-22 to June 20-21 each year. After that, they almost stop (or stand still) before stopping. Then, the Sun appears to pause for a day or two before starting the slow daily slide toward the south. That southward trip goes from June 20-21st (the northernmost point) to December 21-22 (the southernmost point).
Those “stopping points” are called the “solstices” (from the Latin word sol, which means “sun”, and sistere, which means “stand still”). These terms are a heritage from our ancient ancestors. They had an incomplete understanding 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).
Solstice Facts
The 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.
Then, six months go by. On December 21st or 22nd, winter begins with the shortest day of the year for the northern hemisphere people. The lucky folks south of the equator get to enjoy the start of summer the same day, with the longest day of their year.
Equinoxes
The word “equinox” also has Latin roots. It comes from aequus (equal) and nox (night). Early observers noticed that the Sun rises and sets exactly due east and due west on the equinoxes. It also appears to cross the celestial equator, a projection of Earth’s equator out to the sky. In addition, day and night are approximately the same lengths.
In the northern hemisphere, the March equinox denotes spring and 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.
Observing the Solstices and Equinoxes
While you can’t “see” the equinox or solstice actually happen, you can chart the motions of the Sun to see how it was that the ancients first came up with these ideas. Take a moment each day to observe the sky. Make a note of where sunset or sunrise takes place along the horizon. Check out your chart against the calendar to see how close they line up.
This is actually a great long-term science activity and I know that it’s a perennial favorite for science fairs. A couple of years ago, I actually went out each night at sunset and got a picture of the sunset using my smartphone. That’s certainly an easy way to do it, and lets you document how the Sun appears to wander north and south along the western horizon.
