Looking at something can often mean little more than "allowing your gaze to rest on something, with no clear purpose or special concentration". As @StoneyB comments, to look on is normally only used to mean either to stand by (idly) and watch something, or to have an opinion about something (to look on X as Y). Neither of which apply here. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.