Growing up, many of us learned about the solstices: those days marking the longest or shortest spans of daylight in a year. And if you’re anything like me, you probably assumed that the equinoxes – with a name suggesting equality – would naturally bring us daylight and night hours of equal length. So, imagine my surprise when I decided to check the sunrise and sunset times for my hometown of Huntsville, AL, on this first day of Autumn: the daylight hours were slightly longer than 12 hours. In fact, for Huntsville, the sunrise was at 6:34 a.m., and the sunset was at 6:41 p.m. While a 7-minute variation might seem minuscule, I’d expected a deviation of perhaps a minute, if any, due to rounding. So, what’s the underlying cause? Why isn’t there an exact balance between daylight and night on the equinox?
Science Simplified
On the equinox, many assume that daylight and night hours divide the day into two perfect halves, each lasting exactly 12 hours. Yet, intricate natural phenomena cause the daylight hours to extend slightly beyond this. Let’s explore why the first day of fall and spring might not provide a perfect equilibrium between daylight and night hours.
The Sun Isn’t Just a Dot in the Sky!
When we discuss the sunrise or sunset, it’s easy to conceptualize the Sun as a mere point in the sky. But it’s far more than that. The Sun is an immense sphere of incandescent gases, primarily hydrogen and helium. It’s not flat or two-dimensional; it has depth and volume and is about 864,400 miles in diameter.
What does this mean for us on Earth? As this massive ball of glowing gas rises or sets, it doesn’t do so all at once. Sunrise occurs when we see the topmost edge of the Sun cresting the horizon. As time progresses, more and more of the Sun becomes visible until it’s entirely above the horizon. The same phenomenon occurs in reverse during sunset. The Sun begins to dip below the horizon from its bottom edge, continuing until its last light – the topmost edge – finally disappears from view.
Due to the Sun’s size and spherical nature, this gradual appearance and disappearance extend the daylight duration by several minutes. It’s like watching a giant beach ball slowly rise or sink behind a wall, compared to a tiny marble doing the same. The larger object takes more time; in the Sun’s case, this translates to a slightly longer span of daylight.
Bend it Like … Light? The Atmosphere’s Role
Have you ever placed a straw in a glass of water and noticed how the straw appears bent or even broken at the water’s surface? This optical illusion is a result of a phenomenon called refraction. Refraction occurs because light travels at different speeds in different materials. For instance, light moves slower in water than in air. So, when light rays from the straw travel from the water (where they’re moving slower) into the air (where they speed up), they change direction, making the straw look bent.
Similarly, our atmosphere is made up of layers with varying densities. As sunlight passes into the denser parts of the Earth’s atmosphere from the vacuum of space, it slows down and bends. This bending allows us to witness the Sun when it’s actually about 0.5 degrees below the horizon, translating to seeing it roughly 2-4 minutes before its geometric rise at sunrise and the same duration after its geometric set at sunset. This subtle yet captivating dance of light in our atmosphere grants us an additional 4-8 minutes of daylight every day.
Sunset over the Gulf of Mexico in Destin, FL
In Conclusion
The balance of day and night during the equinoxes is more complex than many of us learned in school. Factors like the Sun’s apparent size and the bending of its light through our atmosphere bring about subtleties and nuances. These intricacies remind us that nature is ever fascinating, often defying our initial assumptions.
I encourage you to take a moment and look up the sunrise and sunset times for your hometown or region. Discover for yourself how these natural phenomena manifest in your part of the world. Every locale’s unique latitude and atmospheric conditions offer a slightly different story of daylight and night hours during the equinoxes.
So, the next time someone mentions the “first day of Autumn” or “first day of Spring,” you’ll be armed with the knowledge of why the day isn’t precisely 12 hours long. And in sharing this knowledge, you’ll spread the wonder of our incredible world.