Why isn't the Winter Solstice the Earliest Sunset?

The earliest sunset of the year doesn’t happen on the Winter Solstice. This fact was a surprise to me when I first stumbled on it a few years ago. 

My intuition told me that because the Winter Solstice is the “darkest day of the year”, that would be the day of the earliest sunset. And from feedback I’ve gotten, most folks also think this.

But no: the earliest sunset in Montréal is on December 10th each year. Why?

I’ll admit: I’ve read about this for years and I’m still trying to wrap my head around it. I am going to do my best, but I am going to link to the resources that have helped me at the end and hopefully at least one of these will help you understand.

OK, let’s get into it.

What is the Winter Solstice?

Let’s get this out of the way: what is the Winter Solstice? It is the day with the least amount of total daylight.

Above is a sample of dates for Montréal in December. You can visit this page and look at the Daylength column for the full data.

On December 21, we see the number 8:42:14 (8 hours, 42 minutes, and 14 seconds of daylight). All other days of the year have a higher number than this. This is why we call this the “darkest day of the year”.

Everybody got that? Good.

The Dates and Times

Next, let’s look at the actual date and times of the sunset in Montréal. Look again at the table above.

If you visit this page, look at the Sunset column. Here’s a few to illustrate the pattern:

• Dec 1: 4:12 PM
• Dec 10: 4:10 PM
• Dec 21: 4:13 PM
• Dec 31: 4:20 PM

The takeaway: the date with the earliest sunset is December 10 at 4:10 PM.

On the Winter Solstice on December 21, it’s at 4:13 PM. The earliest sunset is not on the Winter Solstice.

Everybody got that? Good.

The Length of a Day Changes Throughout the Year

When trying to understand this entire concept, I stumbled over this particular part for a while. So hopefully I can help you avoid that.

We have multiple ways of measuring how long a day is.

1. Using a clock. The length of it doesn’t change. It’s exactly 24 hours every single day. 
2. Using the Sun. The length of it varies throughout the year. Not much: only by a few seconds. Some days it’s 24 hours and 20 seconds long. Some days it’s 23 hours, 59 minutes and 40 seconds long.

How do we measure how long a day is using the Sun? Pretty simple:

• When the Sun is exactly South in the sky, start a timer.
• The Earth will slowly rotate for about 24 hours, and the Sun will appear to move.
• Wait for the Sun to return again exactly South in the sky. Stop your timer.

You would think that this number would be the same every single day. But it’s not! How weird!

How much does it vary?

Time for a Graph

OK, it’s graph time. Sorry if that scares you, but I promise this will help!

Here’s a graph you’ll see over and over again if you research this topic. I’ve modified it slightly from Explaining Science.

This shows how many seconds plus or minus 24 hours we get throughout the year. Here’s a few examples (note that I’m eyeballing these values from the graph):

• Jan 1: +27 seconds
• Apr 1: -17 seconds
• Jun 1: +10 seconds
• Sep 15: -22 seconds
• Dec 1: +20 seconds

So on January 1, the day lasts: 24 hours and 27 seconds.

And on April 1, the day lasts: 23 hours, 59 minutes, and 43 seconds. 

And so on.

Honestly, this kinda blew me away. The length of a day changes?! Yes… if you were to measure the time just using the Sun.

And this is the crux of why the earliest sunset is on December 10 instead of December 21. There is a discrepancy between how we humans measure time on our watches (24 hours) and the Solar Time (which varies).

We don't need to know why the solar day changes to continue, but if you're interested, I highly recommend this blog post.

When do sunsets start getting later?

What I found clicked for me when researching this was flipping the question on it's head:

Instead of thinking When is the earliest sunset and why is it not the Winter Solstice?, I found it easier to think When do sunsets start getting later?

For me, this next graph made the concept finally sink in. I've recreated what ExplainingScience.org has done in their blog post, but adapted it for Montreal.

Each orange bar shows the date that the Sun rises and sets. For example, December 1st, the Sun rises at 7:13 AM and sets at 4:12 PM.

It also shows the amount of daylength in hours, minutes, and seconds.

Remember how at this time of year, the Solar Day lasts about 24 hours and 30 seconds long? This means that, on our standard 24 hours watches and clocks, the sunrise will be "pushed" about 30 seconds later the following day.

The pink arrows illustrate this push.

What this also means is that the sunset on the following day will be "pushed" later, too.

The Push vs The Shrink

Look closely at the daylength of December 9, 10, and 11. The days keep shrinking and shrinking, and you would expect then, the sunset to get earlier and earlier.

But on December 11, the "push" has a greater effect than the "shrink". On December 11, the sunset happens about 1 second later than December 10.

Conclusion: This Isn't Easy

If you found yourself really scratching your head here, that's totally OK! As an amateur myself, this took a lot of blog post reading, video watching, and walks outside to have it wrap around my head.

If I had to summarize this entire concept, it would be this:

• the length of a day with our human-made clocks is 24 hours long
• the length of a day using the Sun varies through the year
• the length of a day with the Sun varies a few seconds above or under 24 hours
• because of this discrepancy in time keeping, the Sunrise can be "pushed" ahead of what our clocks say it should be
• this "push" causes our Sunsets to start later in Montréal on December 11

I'm purposely omitting some stuff here, as I found it's what I needed to grasp the concept. I hope that it works for you, too.

Further Reading

I have to say a huge thanks to ExplainingScience.org for their videos and blog posts. If you want to fully understand the concept, start by watching their brief video:

Then, read their blog posts:

• How the length of a day changes over the year
• The Shortest Day

You'll hear a term called "The Equation of Time" as you read about this topic. There's even a graph that repeatedly comes up. I had a draft of this post with that graph, but found that for myself, it was too much information and my brain was full even as I wrote this.

I omitted it. I found the "push" concept was all that was needed for somebody totally fresh to this early sunset idea to get the basics.