I noticed that our sunrise today was at 6:37am, while sunset is 6:35pm — just under 12 hours. We also just experienced the fall equinox, and I got to wondering about the relationship.
It turns out they are strongly related, but not quite the same. The US Naval Observatory has a nice explanation. In short: the sun is very big, and the definitions of sunrise, sunset, and equinox refer to different parts of the sun.
- Sunrise is when the upper edge of the sun is visible. The majority of the sun is still below the horizon.
- Sunset is when only the upper edge of the sun is still visible; the majority of the sun has already gone below the horizon.
- Twilight is when the sun is below the horizon, but atmospheric refraction still gives us some light. (Bonus light!)
The equinox is about the geometric center of the sun crossing the celestial equator. That center is visible for exactly 12 hours. But sunrise occurs before the center is visible, and sunset occurs after it has ceased to be visible.
The time when there is 12 hours of daylight is known as equilux. (Etymologically, “equinox” is equal dark and “equilux” is equal light, but their meanings have seemingly evolved: “equinox” is about solar means, while “equilux” is about actual apparent daylight.)