Ok, I’m sure this has been troubling you, as it has me.

I had an alarm set on my phone to mark the winter solstice (3.03pm Sun 21st Dec). Ok, it’s a little geeky, but it meant that when the moment came I could rejoice in the idea that we were heading back to the summer.

“But how quickly?” I wondered. Over the next few days I watched the time of sunrise on my weather app, waiting to see a change. I was expecting it to be just a minute earlier every few days . For the first few days it stayed resolutely at 08.06 am (for London), and then it went up to 08.07. It is still 08.07.

WAIT! WHAT? That’s the wrong direction! Sunrise should be getting earlier!

Perhaps, I thought, the daylight hours are getting longer overall because sunset is getting later by larger increments than sunrise? And, maybe the counter-intuitive advance of sunrise is caused by an effect such as precession or nutation?

(If you have ever taken a sextant sight on the star Polaris, you’ll know all about precession and nutation. The Earth wobbles like a spinning top on its axis. The large circle is Precession and the mini wobbles, as it describes the precession circle, are Nutation. Navigators have to apply a combined correction for precession & nutation to get their latitude from Polaris. BTW it’s a lovely & simple sight to take.)

Well, good guess, but it turns out I was wrong. The reason sunrise times continue to increase after the winter solstice before decreasing in early January is due to a phenomenon called the ‘Equation of Time‘. While this is influenced by the Earth’s orbit, it is not primarily caused by precession or nutation.

The shift occurs because our clocks and the Sun do not keep the same time. Two main factors cause this discrepancy:

Elliptical Orbit: The Earth’s orbit is not a perfect circle. In early January, the Earth is at perihelion (its closest point to the Sun) and travels at its fastest orbital speed. Because it is moving faster, the Earth must rotate slightly more than a full 360 degrees for the Sun to return to the same position in the sky (solar noon). This makes the “natural” solar day about 30 seconds longer than the 24 hours on our clocks.

Axial Tilt: The 23.5° tilt of the Earth’s axis also affects the Sun’s apparent motion. Around the solstices, the Sun’s movement is primarily eastward, which further contributes to lengthening the solar day beyond 24 hours.

Between late December and early January, these two effects combine to make the Sun “run late” relative to our clocks. After the Solstice, even though the days are technically lengthening (the time between sunrise and sunset is increasing), the entire daylight window is shifting later by roughly 30 seconds each day due to the longer solar day.

For the first few days of January, the shift of the solar day “later” is greater than the gain in daylight from the returning Sun. This causes sunrises to continue occurring later by the clock until around January 5th (at London’s latitude), when the gain in daylight finally overcomes the clock shift.

There you are! I know it’s been burning you up. Now you can relax!