Weather Blog

So, when does the sun officially set?

So, when does the sun officially set?

With the equinox passing and sunrise/sunset times nearly equal, sunset topics get a lot of play, so I've got a few to answer some common questions.

First up: How come in Seattle, today (Sept. 25) is the day with the nearly equal 7:00 am sunrise and 7:01 pm sunset, when the equinox was three days ago (When the sunrise was 6:58 am and the sunset was 7:04 pm)?

It's a complex thing called the "Equation of Time". If you're daring, here's a link that explains it

OK, that out of the way, something a little more easily explained:

What time *is* the official sunset?

In Seattle proper, the official sunset time is a bit of a misnomer. The United States Naval Observatory calculates the official sunset as the time when the last tip of the top of the sun goes below the flat horizon.

(The sunrise is when the tip pokes above the horizon.)

Mountains or other topography are not taken into account. So for Seattle, with the Olympic Mountains blocking the sun as it nears the horizon to our west, the sun appears to set a few minutes earlier than listed, as opposed to a city that has a flat west horizon like, say, Ocean Shores.

But! Even at Ocean Shores, the sunset time isn't really the sunset, except we say it is. That's because no matter the horizon, the sun has actually set a few minutes before you see it set!

This is due to atmospheric interference. The sun's light rays get bent in the atmosphere as the sun goes down -- giving it the appearance that the sun is still slightly above the horizon, when it had already set a few minutes ago. However, the Naval observatory says that refraction is factored into the sunset (and sunrise) times, so the time listed is when you see it set, not when the sun actually sets.

Now to run this into a "Weekly Time Waster" entry (which I'll also post separately), here is the best site to find the sunrise/set for any place in the world:

http://aa.usno.navy.mil/data/

You can also find such items as when Civil Twilight begins and ends (i.e., when it gets officially "dark" after sunset) and the moon rise/set for a specific date anywhere.  The twilight info is good for wedding our outdoor planning events. Just note the times are based on clear weather. Cloudy weather makes it darker earlier.

And for those wondering when next summer starts -- find when every season begins through 2020. (That is UTC time -- subtract 7 hours for spring, summer and fall when we're on PDT, and 8 hours for winter when we're on PST)

There's plenty of other little goodies on there as well.