This easy Achievement can be unlocked once you have completed the Dawnguard DLC quest "Touching the Sky", where you receive Auriels Bow along with 12 Sunhallowed elven arrows and 12 normal elven arrows.
Once equipped you simply need to shoot at the sun, if the sun is not visible wait until noon the next day. If you sided with the Vampires then you can also unlock this by using the blood tainted Bloodcursed Elven Arrows.
http://game-play.tv/video/W7XN1K657HUU/Skyrim-Auriels-Bow