Saturn is about to reach its annual opposition this week, when it appears at its brightest and biggest all year and becomes the only planet visible all night.The short, exciting period for skywatchers has more to do with where Earth is in its orbit than with Saturn's journey around the sun. Opposition occurs when Earth moves directly between a planet and the sun, placing the two planets on the same side of the solar system. For observers on Earth, that means Saturn appears opposite the sun in the sky, just like a full moon (hence the name "opposition"). Saturn therefore rises in the east around sunset, 100% illuminated, before reaching its highest point around midnight and finally sinking to...