Planet Venus

Not to be confused with Venus, the Roman goddess of love.

Venus is the second planet from the Sun and the brightest planet in the night sky. It is also the hottest planet in the Solar System, with a mean surface temperature of 735 K (462 °C; 863 °F), even though Mercury is closer to the Sun. The Romans named it after Venus, their goddess of love and beauty, whom the Greeks knew as Aphrodite.

The Planet

Venus orbits the Sun in about 225 Earth days, a little less than a year on Earth. It is only slightly smaller than the Earth, with a diameter of about 12,100 km, roughly 95 percent of Earth’s. It spins very slowly and in the opposite direction to most planets, so that the Sun rises in the west there. One turn on its axis takes about 243 Earth days, longer than its year.

The atmosphere of Venus is made up mostly of carbon dioxide, with some nitrogen and clouds of sulphuric acid that hide the surface from view. The pressure of the air at the surface is about ninety times that on Earth, and the trapped heat makes the surface hotter than a household oven. Venus has no moons.

Phases and Brightness

Venus lies inside the Earth’s orbit, so it never appears far from the Sun in the sky, at most about 47 degrees away. It shows phases like the Moon, from a thin crescent to a full disc, and it is brightest as a crescent when it is closest to Earth. It is the brightest object in the sky after the Sun and the Moon. Venus has been known since prehistoric times.

Morning Star and Evening Star

For part of its cycle Venus rises before the Sun and is seen in the east before dawn; for another part it follows the Sun and shines in the west after sunset. Because of this, Venus was long thought to be two separate stars. The Greeks called the morning star Eosphorus or Phosphorus (“bringer of light”) and the evening star Hesperus. Hesiod makes Eosphorus the star-child of Eos, the goddess of dawn (Hesiod, Theogony 381). In the Iliad, Hesperus is “the fairest star that stands in the heavens” (Homer, Iliad 22.317–318), and the Morning Star rises to herald the light of dawn (Homer, Iliad 23.226).

Pliny reports that Pythagoras of Samos was the first to discover the nature of the star, that the Lucifer of the morning and the Vesper of the evening were one planet (Pliny the Elder, Natural History 2.36–37). Hyginus writes that the fourth star is Venus, called Lucifer, which many accounts also call Hesperus. Some said it was the son of Aurora and Cephalus, who rivalled Venus in beauty (Hyginus, Astronomica 2.42).