Venus is the second-closest planet to the Sun, orbiting it every 224.7 Earth days. After Earth's Moon, it is the brightest object in the night sky, reaching an apparent magnitude of 4.6. As an inferior planet, from Earth it never appears to venture far from the Sun, and its elongation reaches a maximum of 47.8°. Venus reaches its maximum brightness shortly before sunrise or shortly after sunset, and is often referred to as the Morning Star or Evening Star. A terrestrial planet, it is sometimes called Earth's "sister planet", as the two are similar in size and bulk composition. The planet is covered with an opaque layer of highly reflective clouds and its surface cannot be seen from space in visible light. Venus was a subject of great speculation until some of its secrets were revealed by planetary science in the 20th century. Venus has the densest atmosphere of the terrestrial planets, consisting mostly of carbon dioxide, and the atmospheric pressure at the planet's surface is 90 times that of the Earth. Venus' surface has been mapped in detail only in the last 20 years. It shows evidence of extensive volcanism, and some of its volcanoes may still be active today. In contrast to the constant crystal movement seen on Earth, Venus is thought to undergo periodic episodes of plate tectonics, in which the crust is sub ducted rapidly within a few million years separated by stable periods of a few hundred million years. The planet is named after Venus, the Roman goddess of love, and most of its surface features are named after famous and mythological women. The adjective Venusian is commonly used for Venus, though the Latin adjective is the less commonly-used.