# Calculating Sunset
**URL:** https://boards.straightdope.com/t/calculating-sunset/718249
**Category:** Factual Questions
**Created:** [April 22, 2015, 2:34am UTC](https://boards.straightdope.com/t/calculating-sunset/718249 "2015-04-22T02:34:27Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@dolphinboy](https://boards.straightdope.com/u/dolphinboy)
#### Post date: [April 22, 2015, 2:34am UTC](https://boards.straightdope.com/t/calculating-sunset/718249/1 "2015-04-22T02:34:27Z")
There must be a simple answer for this, but how do they calculate sunset for any given zip code?
As an example, for my zip code sunset was 8:05 this evening, however I live in the Rocky Mountains so ‘perceived sunset’ will vary greatly based on whether you are at the bottom of a valley or at the top of a high mountain.
Does the sunset time on weather websites mean anything in the real world?
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### Author:  [@zombywoof](https://boards.straightdope.com/u/zombywoof)
#### Post date: [April 22, 2015, 2:44am UTC](https://boards.straightdope.com/t/calculating-sunset/718249/2 "2015-04-22T02:44:58Z")
They’re based on the [horizon](http://en.wikipedia.org/wiki/Horizon), and don’t take into account if your view of said horizon is blocked by mountains or buildings (I suppose “perceived sunset” happens quite early on a Manhattan avenue!) or anything else.
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### Author:  [@Reply](https://boards.straightdope.com/u/Reply)
#### Post date: [April 22, 2015, 2:51am UTC](https://boards.straightdope.com/t/calculating-sunset/718249/3 "2015-04-22T02:51:51Z")
If you want to calculate the actual annual sun coverage at your specific location, taking into account shading, you can use a Solar Pathfinder:
[http://www.solarpathfinder.com/](http://www.solarpathfinder.com/)
It’s basically a sun chart under a fish-eye dome, and you use it trace an outline of your surroundings to figure out whether that tree or that building will shade this spot after a certain time at a certain month.
There might be apps that do similar things, now.
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### Author:  [@dolphinboy](https://boards.straightdope.com/u/dolphinboy)
#### Post date: [April 22, 2015, 2:52am UTC](https://boards.straightdope.com/t/calculating-sunset/718249/4 "2015-04-22T02:52:57Z")
So is the horizon just a certain distance west of any given point on earth? How does that work on the north or south pole?
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### Author:  [@zombywoof](https://boards.straightdope.com/u/zombywoof)
#### Post date: [April 22, 2015, 3:05am UTC](https://boards.straightdope.com/t/calculating-sunset/718249/5 "2015-04-22T03:05:15Z")
I guess to be more clear, I should have said they are calculated to determine when the sun falls below the “true” horizon at a given earth location at a given date - see [here](http://en.wikipedia.org/wiki/Sunrise_equation).
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### Author:  [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)
#### Post date: [April 22, 2015, 3:27am UTC](https://boards.straightdope.com/t/calculating-sunset/718249/6 "2015-04-22T03:27:10Z")
> [@dolphinboy](#):
>
> So is the horizon just a certain distance west of any given point on earth? How does that work on the north or south pole?
Pretty much. The horizon is a certain distance in any direction from any given point on the Earth. The sun only sets exactly west twice a year because the Earth is tilted (which is what creates our seasons.) At any time other than the equinox the sun is setting some way north or south of local “west”
This may help to visualise it, particularly at the poles, though the animation is a bit floored in that it doesn’t show the daily rotation of the Earth.
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### Author:  [@Reply](https://boards.straightdope.com/u/Reply)
#### Post date: [April 22, 2015, 3:46am UTC](https://boards.straightdope.com/t/calculating-sunset/718249/7 "2015-04-22T03:46:15Z")
> [@dolphinboy](#):
>
> So is the horizon just a certain distance west of any given point on earth? How does that work on the north or south pole?
In this context the horizon is how far you can see, assuming sea level, before the curve of the Earth bends the ground away from your line of sight (ignoring atmospheric refraction and the like). From the height of an average adult male, it’s about 3 miles in every direction:
> **[Horizon](https://en.wikipedia.org/wiki/Horizon#/media/File:Horizons.svg)**
>
> The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whether it intersects the relevant body's surface or not.
> The true horizon is a theoretical line, which can only be observed to any degree of accuracy when it lies along a relatively smooth surface such as that of Earth's oceans. At many locations, this line is ...
Sunset, then, is when the Earth rotates out of direct view of the sun (when the sun “sets” below the visible horizon):
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But the specific time of sunset changes both depending on where you are and which day of the year it is. The Earth’s axis of rotation is tilted relative to the sun ([https://www.nc-climate.ncsu.edu/secc\_edu/images/Latitude3.jpg](https://www.nc-climate.ncsu.edu/secc_edu/images/Latitude3.jpg)) and this, combined with our orbit around the sun, determines the lengths of days (and also the seasons) at any given location and time.
Try going to:
[http://suncalc.net/](http://suncalc.net/) and dragging it around to get a feel.
If you go North enough, past the Arctic Circle the sun doesn’t set at all in the summer and doesn’t rise at all in the winter, and spring/fall have nearly as long/short days. There’s a similar Antarctic Circle in the south.
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### Author:  [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)
#### Post date: [April 22, 2015, 4:18am UTC](https://boards.straightdope.com/t/calculating-sunset/718249/8 "2015-04-22T04:18:25Z")
[http://astro.unl.edu/naap/motion1/animations/seasons\_ecliptic.html](http://astro.unl.edu/naap/motion1/animations/seasons_ecliptic.html)
Missing link from my post.