Weather Blog

Shortest day… or not?

Shortest day… or not?

At 9:47 am today we waved goodbye to autumn and said "hello" to winter. You'll hear meteorologists like myself tell you on TV that today is "the shortest 'day' of the year," offering us the least amount of sunlight of any day on the calendar.

Sounds simple, right? Actually it's not.

Let me explain.

My intuition told me that if today is the "shortest day," then December 21 would have the latest sunrise and the earliest sunset. Not so easy.

Take a look at this link: www.timeanddate.com

You'll see that the earliest sunset has already happened (4:18 PM from December 5-16), but the latest sunrise won't occur until next week (7:58 AM on December 29). Turns out that the earliest sunset near the Equator is actually in November!

Why is this?

It's actually a very complicated explanation (if you want the full explanation, check out this site: http://www.larry.denenberg.com/earliest-sunset.html).

Here's a brief overview: Our "day" lasts 24 hours, but a "solar day" - the time between the sun's peak in the sky from one day to the next (called the "solar noon") - is not constant.

A solar day averages out to 12 hours throughout the entire year, but it's a little longer than 12 hours in the winter and a little shorter than 12 hours in the summer.

Think of it this way: Since the sun angle is lower in the fall/winter, it takes a bit longer to cross the same point (in this case, Seattle) than it does in the summer (with a higher sun angle). It's only a few minutes, but it's enough to mess with our sunrise/sunset times.

Another reason why the times are off is because we are not equidistant from the sun all year long. We orbit the sun in an elliptical, not circular, pattern. That also changes the length of the solar day throughout the year.

So what does this mean? It means that today - December 21 - is actually the shortest day (2 seconds less sunlight than yesterday -- tomorrow will have 3 seconds more sunlight!), but if you're an early riser and judge the shortest day by the "latest sunrise" you'll have to wait until next week before you can look forward to spring!

Happy winter!