In a previous article I wrote that I am using my Raspberry PI as status screen showing the weather among things, but I wanted to make the widget that shows the current weather a bit more interesting. Instead of having the background black (for nights) and white (for days) I want to have a better approximation of the lightness of the sky. In order to be able to do this, I need to know: Where the Sun in the sky is? (In Britain, of course that would be: Where in the sky is the Sun behind the clouds.)

With PHP's date_sun_info() function you can easily calculate when the Sun rises and sets, but it's not useful to determine how far above or under the horizon the Sun is. For that, I needed to implement a little bit more maths. I found an excellent tutorial online that explains the formulas that are used to calculate the position of the Sun. The trigonometry and maths go beyond me at the moment though!

I've implemented some of those functions in a simple library, called "astro". You can find it on GitHub at https://github.com/derickr/astro. Right now, it doesn't implement a lot more than just the position of the Sun, but I am intending to implement the rest of the algorithms too.

Of course, just a C-library of some maths isn't very useful if your language of choice is PHP, so I also implemented a tiny PHP extension wrapping the astro library. It's called solarsystem and available on GitHub as well. There is only an earth_sunpos() function so far, but again, I am intending to extend on that.

In order to make use of it, you'll have to run:

git clone git://github.com/derickr/php-solarsystem.git
cd php-solarsystem
git submodule init
git submodule update
phpize
./configure
make
make install

Then you can either add extension=solarsystem.so to php.ini, or run scripts with php -dextension=solarsystem.so yourscript.php. In the tests/ directory of the Git checkout you can find a script called sun-position.php. If we examine that, we will see that for four cities (Johannesburg, London, Longyearbyen and Oslo) we calculate the position of the Sun for every 15 minutes during January 14th, 2013. The main function there is earth_sunpos() which takes a Unix timestamp, as well as the latitude and the longitude of the location for which we want to calculate the Sun's position.

The script produces CSV, that I redirect into a file:

php -dextension=solarsystem.so sun-position.php > sunpos.csv

I then opened this file in LibreOffice and made a pretty graph out of it:

For Longyearbyen (yellow line) it shows that the Sun never rises as it always stays below the horizon. It also shows that the highest point over the horizon is different for London and Oslo—mostly because they are 10° apart horizontally. For London, you can also see that sunrise happens around 08:00 and sunset around 16:20.

The position over the horizon, combined with the weather forecast allows me to calculate the likely lightness of the sky. But that will have to wait to a future blog post.