For centuries, humans have used different markers to note the changing seasons. The harvest moon has popularly been used to announce the fall, just as Groundhog Day tells us whether or not spring is on schedule.
But, seasonal change owes much more to the equinox and the solstice than to groundhogs.
What is an equinox?
First, because we’re a dictionary and all, we’ll look at the word equinox. It actually comes from Latin and means “equality of night and day.” The equinox occurs twice a year, marking the onset of both spring and autumn. During the equinox, the sun crosses the plane of Earth’s equator, making nighttime and daytime (roughly) equal length all over the world. Contrary to popular belief, the equinox doesn’t last for a full 24 hours; rather, it occurs at two specific moments in time when the sun is exactly above the celestial equator.
The spring equinox, or vernal equinox or March equinox, occurs around March 21st, when the sun moves northward across the celestial equator. The autumnal equinox, or September equinox, occurs around September 22nd or 23rd, when the sun crosses the celestial equator going south.
Fun fact: Using “March equinox” and “September equinox” avoids the Northern Hemisphere bias that spring occurs in March and autumn in September. (Since, it turns out, fall happens in March on the other side of the world. Who knew?)
What is the solstice?
The equinox is often confused with the solstice. The word solstice derives from the Latin solstitium, which literally means “the standing still of the sun.” The solstice occurs twice a year–to denote the beginnings of winter and summer for the Northern Hemisphere.
During the summer solstice, which occurs around June 21st each year, the sun is at its greatest distance from Earth’s celestial equator, giving us the longest day of the year. The winter solstice occurs around December 22nd, marking Earth’s closest proximity to the sun, and giving us the shortest day of the year.
A quick recap
So, a quick broad-stroke recap: When the equinox occurs, as the equi– prefix might suggest, day and night are of equal length, and it denotes the onset of spring and autumn.
The solstice, meanwhile, marks the beginnings of summer and winter, and it’s either the longest day of the year (on summer solstice) or the shortest (winter solstice).
Then again, if you still want to identify the beginning of spring using a groundhog’s shadow, be our guest.