The Shortest Day, but Mornings Continue to Get Darker

Sunrise and Sunset on the Solstice

This year, in nearly all time zones, the December solstice falls on December 21.

In the Northern Hemisphere, it’s the winter solstice and the shortest day of the year—from now on, the amount of daylight increases.

Meanwhile, in the Southern Hemisphere, it’s the summer solstice and the longest day of the year. After this, the days become shorter.

We might expect that the shortest day of the year would coincide with the latest sunrise and the earliest sunset.

Likewise, we might suppose that the longest day of the year would give us the earliest sunrise and the latest sunset.

Surprisingly, however, we’d be wrong.

Northern Hemisphere: Mornings Keep Getting Darker

Our Sun Calculator gives the earliest and latest sunrises and sunsets for your city.

For cities in the Northern Hemisphere, the latest sunrise happens after the December solstice, whereas the earliest sunset happens before.

How many days before or after the solstice? It depends on your latitude, which is your distance from the equator measured in degrees. Here are some examples.

To put it another way: although December 21 is the shortest day, the mornings will continue to get a bit darker. On a brighter note, the evenings are already getting a bit lighter.

Southern Hemisphere: Evenings Keep Getting Lighter

Similarly, for cities in the Southern Hemisphere, the latest sunset comes after the December solstice; the earliest sunrise comes before.

This means that, although December 21 is the longest day, the evenings will continue getting a bit lighter. On the other hand, the mornings are already becoming a bit darker.

The Sun Gets Out of Sync with Our Clocks

Why do the latest and earliest sunrises and sunsets not coincide with the solstices? It’s because the length of a day is not always 24 hours.

Let’s clarify that: according to our clocks and watches, every day is exactly 24 hours long. But according to the Sun, it’s not.

The length of a solar day can be up to 22 seconds shorter or 29 seconds longer than 24 hours.

What is a solar day? It’s the amount of time from one solar noon to the next. Solar noon is the moment the Sun reaches its highest point in the sky on a particular day.

The consequence of all this is that, over the course of a year, our clocks and watches go in and out of sync with the Sun—leading to shifts in the times of sunrise and sunset.

Why Solar Time Seesaws

Of course, another question is: why does the length of a solar day fluctuate?

There are two reasons. First, Earth’s orbit around the Sun is elliptical—in other words, it’s not a perfect circle. Secondly, Earth’s spin axis is tilted—at the December solstice, the South Pole is pointed toward the Sun.

The difference between solar time and clock time is called the equation of time.