The author's excellent 'powers of observation' [=ability to notice and pay close attention to things] are evident in the book's detailed descriptions.
The plants seem to have recovered from the disease, but they are still 'under observation'. [=someone is watching them carefully]
They're keeping him at the hospital 'for observation' [=so that doctors can watch him carefully] for another few days.
(..) the very center of the universe... ...as a portal to attack us. We have sent a squadron of Lanterns to the sun to begin monitoring. They'll be setting up observation platforms to conserve energy. Meanwhile, all important documents and artifacts will be moved from Oa... for safekeeping. (..)2011 Green Lantern: Emerald Knights