Venus is hotter than Mercury because its thick atmosphere traps heat efficiently.

The longer version

A runaway greenhouse effect explains the heat. Venus has a dense atmosphere made almost entirely of carbon dioxide, a gas that lets sunlight in but blocks infrared radiation trying to escape. Heat therefore accumulates near the surface instead of radiating back into space. Surface pressure there is crushingly high as well.

Thick clouds of sulfuric acid add to the trapping and reflect much of the sunlight away, which is why the planet looks so bright from Earth. Mercury, despite being closer to the Sun, has essentially no air to hold warmth, so its heat escapes at night and its surface temperature drops sharply.

Where this entry sits

The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.