You would really need a battery too, to keep the interest coming 24/7, light and dark!
It must be do-able though I suspect the pay-back on the solar gear would take years as the Pi is so cheap to run.
A quick search and there are several solutions you can have the unit power down on low power.
http://www.instructables.com/id/Solar-Powered-Raspberry-Pi/The charger is @ $25 battery $15. you would build it for $50. This is more than the energy to runit for several years.
But the cool factor is worth way more