Rotation of the solar terminus

Earth's solar terminus rotates as Earth revolves around the sun, completely spinning once over the course of a year. This affects sunrise and sunset times, but this post dives more into the details of what's going on.

From the winter solstice to the summer solstice, Earth travels from one side of the sun to the other, causing the opposite side of the globe to be illuminated. It's a bit hard to think about, given that the planet spins every day, so consider only the north pole: it's lit in the northern hemisphere's summer, and unlit in winter, regardless of the time of day. Here's an animated diagram, with the north pole's winter at the top and summer at the bottom.

Notice how the solar terminus flips 180° in half a year. That's 182.63 days, meaning the solar terminus creeps around the globe at just under 1° per day. Every night we have a little more darkness to travel through; sunrise is always getting later.

We usually think of a day as being a rotation on the Earth's axis, but a day is not a 360° spin on Earth's axis, rather it's the time between sunrises, which is slightly longer than it takes to make a full rotation. Some math shows the difference: Earth's equatorial rotation velocity is 1,040.4 miles per hour, and therefore the equator travels 24,969.6 miles in 24 hours, 68.2 miles further than Earth's actual circumference 24,901.5 miles. Earth rotates almost 361° per day.

Thus, one rotation takes less than 24 hours, just 23 hours and 56 minutes—4 minutes short of a full day. So while a year is 365.26 days long, it's 366.26 rotations long. The year-long pirouette of the solar terminus subtracts one rotation, giving us a year of 365.26 days.