Optical Illusions and the Equinox

As this post goes out, it’s the fall equinox in the Northern Hemisphere (spring in the Southern Hemisphere), arriving at 8:05 pm EDT on September 22 (1205 GMT on September 23). “Equinox” comes from Latin and means “equal night.” However, even today, daylight and darkness are not equal in length. Even more surprising, sunset and sunrise are optical illusions.

Sunrise is the moment the first speck of the Sun becomes visible above the horizon—not the whole Sun or even half of it. Likewise, sunset is when the last speck of the Sun disappears below the horizon, not just part of it. However, when we see the Sun rising, it is still below the horizon, and when we see it set, it is already below the horizon. This discrepancy happens because, when the Sun is near the horizon, Earth’s atmosphere bends the light, creating an optical illusion. As the Sun appears closer to the horizon, refraction increases, making the bottom of the sphere look slightly flattened. The good thing is that sunlight refraction in our atmosphere gives us a little more light at both sunset and sunrise.

Even after the Sun disappears below the horizon, the atmosphere gives us what we call “bright” twilight, so we can enjoy a little more time outdoors until the top edge of the Sun is six degrees below the horizon. Even then, we experience “astronomical” twilight until the Sun’s rim is 18 degrees below the horizon. At that time, the sky in dark-sky areas becomes completely dark. Unfortunately, for those of us who like to observe the night sky, artificial light has given us light pollution in all but a few rural areas.

The bottom line is that when the equinox occurs, we still have more light than total darkness. And we can’t forget that “God made the two great lights—the greater light to rule the day and the lesser light to rule the night” (Genesis 1:16). We refer to the Sun rising and setting, even though we know the Sun remains still and Earth rotates. The Bible repeatedly uses the same language as we are earthly observers. In fact, the creation story in Genesis is written from the perspective of an observer on Earth, because that made it easy for even ancient people to understand.

Psalms 19 begins with, “The heavens declare the glory of God; the skies proclaim the work of his hands.” Poetically, the psalmist David wrote that the Sun is “like a bridegroom coming out of his chamber” and “a champion rejoicing to run his course. It rises at one end of the heavens and makes its circuit to the other…” We still speak of the Sun rising, moving across the sky, and setting, even though we could say that is an optical illusion. As the equinox is upon us, we can thank God that we have predictable days and seasons on this planet in a finely tuned system, not an optical illusion.

— Roland Earnst © 2026

Reference: space.com HERE and HERE