Venus: Earth's hellish twin

The numbers

Note rows six and seven together: Venus takes 243 days to rotate once and 225 days to orbit the Sun. Its day is longer than its year, which is a genuinely strange arrangement.

The surface pressure is comparable to being about 900 metres underwater on Earth. Early landers survived on the surface for only minutes to a couple of hours before being destroyed.

The runaway greenhouse

Venus is closer to the Sun than Earth, but distance alone does not explain a 465°C surface. It receives roughly twice Earth's sunlight and its thick clouds reflect most of it away — Venus actually absorbs less solar energy than Earth does.

The temperature comes almost entirely from the greenhouse effect. Carbon dioxide traps outgoing infrared radiation, and Venus has an enormous amount of it.

The likely history is a feedback loop. Early Venus may have had oceans. Being nearer the Sun, more water evaporated; water vapour is itself a strong greenhouse gas, which raised temperatures further, which evaporated more water. Eventually the oceans were lost entirely, water vapour reached the upper atmosphere where ultraviolet light broke it apart, and the hydrogen escaped to space.

Without liquid water, the process that on Earth locks carbon dioxide into carbonate rocks stopped. So the carbon dioxide stayed in the atmosphere, and the greenhouse became permanent.

Why it spins backwards

Venus rotates retrograde — opposite to almost every other body in the solar system. From its surface, the Sun would rise in the west.

Two explanations are debated. One is a giant impact early in its history that flipped or reversed the rotation. The other, currently favoured by many, involves atmospheric tides: Venus's enormously thick atmosphere is dragged by solar heating, and over billions of years the torque combined with solid-body tides could have slowed the rotation, stopped it, and reversed it.

The second explanation is more satisfying because it requires no special event, only time and a very thick atmosphere.

Observing it

Venus is the brightest object in the sky after the Sun and Moon, reaching about magnitude −4.5 — bright enough to cast faint shadows from a dark site, and to be seen in daylight if you know exactly where to look.

It shows phases like the Moon, which is straightforward in a small telescope. This mattered historically: Galileo's observation of Venus's full range of phases was incompatible with the geocentric model, in which Venus could never appear gibbous. It was among the strongest early observational arguments for heliocentrism.

Venus never strays far from the Sun in our sky, so it appears as the 'morning star' or 'evening star' and is never visible in the middle of the night. Its brightest appearances come as a crescent, when it is closer to us and more of it is lit from behind.

The astrological Venus

Venus signifies love, beauty, value and attraction, and rules Taurus and Libra. As with Mars, that meaning descends from the Roman deity the planet was named for, which was itself attached to it because Venus is the most beautiful object in the sky.

So the chain runs: brightest planet → named for the goddess of beauty → signifies beauty. It is a coherent human association and it says nothing about sulphuric acid clouds at 92 bar.

Which is not a criticism of either description. It is a reminder that they are answering different questions, and that keeping them apart is the whole discipline.