This script switches the wallpaper at sunrise and sunset.

Install:

0. get two wallpaper, one for days and another for nights

1. install the prequisite perl module as root:

cpan -i Astro::Sunrise

2. download the script and save it to anywhere. (e.g.: ~/.gnome2/nautilus-scripts)

3. set script attribute to executable (chmod +x filename)

4. edit the script,
set the pathes of the two wallpapers here:

$dayimg = '~/wallpapers/day.jpg';
$nightimg = '~/wallpapers/night.jpg';

and set your location's coordinates here:

$longitude = 19.1919;
$latitude = 51.5151;

5. set crontab to run the script at every 10 minutes:

crontab -e

then put the following line to crontab (change the path to your script location):

*/10 * * * * ~/.gnome2/nautilus-scripts/setbg.pl

6. add the script to the list of autorun applications.

System->Preferences->More Preferences->Sessions


Note: the script inspired by the wallpapers on the screenshot which are artworks of Asaki Hoshino and can be found here : http://hybridworks.deviantart.com/gallery/