What is the second brightest natural object in Earth's night sky?

Venus, after the Moon - bright enough to cast shadows on a dark night and to be visible in daylight if you know where to look. It is the most common cause of UFO reports, and there are documented cases of aircraft being scrambled towards it.

It is bright because it is close and because it is entirely wrapped in cloud, reflecting about 75 per cent of the sunlight that hits it - the highest albedo of any planet. Mars, which is closer at times, is a dull red rock by comparison.

It also goes through phases like the Moon, which is what broke the old cosmology. Galileo observed in 1610 that Venus shows a full set of phases and changes size dramatically - it is small and full when far away on the other side of the Sun, and large and crescent when close. That is impossible if Venus orbits the Earth and exactly what happens if it orbits the Sun, and it is the single cleanest piece of observational evidence against the Ptolemaic system.