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 let's focus on the north pole. It's lit in the northern hemisphere's summer, and unlit in winter, regardless of the time of day. Here's a representative diagram, with winter at the top and summer at the bottom.

The solar terminus completes this 180° flip in 182.63 days, meaning it 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.

So why don't we lose a day every year? Because "day" doesn't mean "rotation". A rotation is a full 360° spin on Earth's axis, while "day" is the time between sunrises. We usually think of those two terms synonymously, but some math shows the difference: the equator moves at 1,040.4 miles per hour, and therefore goes 24,969.6 miles in 24 hours, 68.2 miles further than Earth's actual circumference 24,901.5 miles. Earth rotates more than 360° per day.

Thus, one rotation doesn't take 24 hours, but 23 hours, 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.