(1) Everyone, of course, has seen a rainbow. (2) Rainbows occur when the sun shines through a rainfall and a multi-colored arc appears not too far above the land. (3) There are, however, other visions in the sky that may not be as common but are certainly just as spectacular. (4) Among these spectacles in the sky (known as "optical phenomena") is a brilliant burst of greenish light that may last only a second. (5) It is known, appropriately enough, as a "green flash," and it happens precisely at the moment the sun sets on an ocean horizon. (6) Because it is rare and wonderful, people often gather in seaside locations at sunset to watch for it. (7) Another phenomenon that's sure to draw a crowd is the Northern Lights. (8) These ghostly pink and green and violet lights shimmer and pulsate so quickly that it is often difficult to photograph them. (9) They appear and disappear in sheets of color that can fill the entire sky. (10) And a moonbow, the nighttime version of the rainbow, can be seen on occasion when the moon is exceptionally bright and shines through a night rainfall. (11) While the moonbow's colors are subtler and blurrier than a rainbow's, the image of a curve of color across the moon is an extraordinary and memorable treat. The tone of the passage is a. prideful. b. encouraging. c. appreciative.
