Why Does The Latest Sunrise Of The Year Not Fall On The Winter Solstice?

If you are in the Northern Hemisphere, we made it! We have passed the longest night and shortest day. The winter solstice was this weekend, and depending on where you were above the equator, you got the smallest amount of sunlight for the year. It might be surprising to know, though, that while the days are getting longer, the sunrise is not getting earlier. At least, not quite yet.

The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.

The problem is that the Sun and our average clocks are not exactly in agreement, due to the many astronomical motions of the planet. And those motions are mostly regular, but not as regular as our precision timekeeping society requires.

Let’s start with the length of the day. A day, as we understand it, is 24 hours of mean solar time. "Mean" is the keyword there. The day used to be calculated between two consecutive noons, when the Sun passes over a specific meridian. While over the course of a year this averages to 24 hours plus or minus some milliseconds, the length of the day does not stay the same when taken day by day. The days are longer near the solstices and shorter at the equinoxes.

This causes a shift between the real noon of the Sun’s position in the sky and the time measured by clocks. And this is not all. Earth’s orbit is close to but not exactly a perfect circle. This means that the planet gets closer to the Sun and farther away. This variation in distance is about 5 million kilometers (3.1 million miles), with the farthest point (aphelion) happening in early July and the closest (perihelion) happening in early January. This fact might have people in the Northern Hemisphere wondering: how is it winter here, if we are nearer the Sun?

The climate throughout the year doesn’t depend on the distance from the Sun. The reason for the season(s) is the tilt of our planet. The Earth has a pretty spectacular tilt of 23.7 degrees, and it always points in the same direction (to Polaris in the Northern Hemisphere) as it goes around the Sun. This creates the seasons with the equinoxes and solstices – in winter, the Hemisphere is farther back, getting sunlight at a lower angle, and in summer it is the other way around.

The combination of the elliptical orbit, affecting the speed of the planet, and the axial tilt, causing the length of daylight to change unevenly, is the key. The orbital effects lag behind the tilt’s effect, so in the winter, the latest sunrise is a few weeks after the solstice; in the summer, it is the latest sunset that is offset.

So stay strong – we’ll be getting brighter mornings soon!