The just-past-full Moon has an almost-full companion tonight: the planet Saturn. They’re low in the southeast at nightfall, with Saturn close to the right or lower right of the Moon. The solar system’s second-largest planet looks like a bright star, so it’s hard to miss.
The Moon is full when it lines up opposite the Sun in our sky. At that angle, the Sun illuminates the entire lunar hemisphere that faces Earth — creating a full Moon.
Since the Moon is on the opposite side of Earth from the Sun, that configuration is known as opposition. And the planets experience opposition, too. In fact, Saturn will be at opposition next week. So like the full Moon, it’ll rise around sunset, remain in the sky all night, and shine at its brightest.
Unlike the Moon, though, Saturn doesn’t go through a cycle of phases. It always stays outside Earth’s orbit around the Sun, so we never see a “new” Saturn, or a crescent one. Instead, it’s always almost fully illuminated as seen from Earth, so when it’s not full, it’s almost full.
Saturn shines brightest at opposition for a couple of reasons. For one thing, it’s closest to Earth for the year at opposition — about 800 million miles. And for another, it’s a more effective mirror at opposition, so more of the sunlight that strikes its cloudtops is reflected in our direction.
So keep an eye on Saturn over the next few nights, as it puts in its best appearance of the year. We’ll have more about Saturn next week.