exupero's blog

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 a full exploration of what's going on and what it means deserves its own post.

From the winter solstice and the summer solstice, Earth travels from one side of the sun to the other, causing the opposite side of the globe to be illuminated. That can be counterintuitive, given that the planet spins every day, so it helps to think in terms of the poles, which point the same celestial direction regardless of which side of the sun we're on. Knowing that the north pole is lit in the northern hemisphere's summer and unlit in winter helps develop some intuition for the movement of the solar terminus:


Since the solar terminus completes a 180° flip in 182.63 days, it creeps around the globe at just under 1° per day. Every night we have a little extra darkness to travel through. Sunrise is always getting later.

So why don't we lose a day every year? Because Earth rotates more than 360° in 24 hours. With a velocity of 1,040.4 miles per hour, a point on Earth's equator travels 24,969.6 miles in 24 hours, which is 68.2 miles further than Earth's actual circumference 24,901.5 miles. And what's 1/365.26 of Earth's circumference? 68.2 miles.

Thus, our measure of 24 hours is determined by the time between sunrises, not between times when Earth is oriented the same way against the stars. Twenty-four hours is the length of a day, not a rotation, which only takes 23 hours, 56 minutes. So while a year is 365.26 days long, it's 366.26 rotations long, that one extra rotation consumed by the year-long pirouette of the solar terminus.