The Sky at Night – Venus, Earths Twin


The Sky at Night – Venus, Earths Twin – How can two such similar planets have become so different? One is the crucible of life, the other an inferno with a surface scorched by raining acid, yet both began as almost identical bodies. With Venus prominent in the sky in May, the team explores our nearest neighbour, discovering how it formed and how ESA’s Venus Express spacecraft has revealed the secrets of its atmosphere.


Dr Lucie Green explores what happened to leave Venus with searing temperatures and acid rain, and talks to Climate expert Dr Hugo Lambert about whether it could happened to Earth.


The Sky at Night – Venus, Earths Twin


Venus is the second planet from the Sun. It is named after the Roman goddess of love and beauty. As the brightest natural object in Earth’s night sky after the Moon, Venus can cast shadows and can be visible to the naked eye in broad daylight. Venus’s orbit is smaller than that of Earth, but its maximal elongation is 47°; thus, it can be seen not only near the Sun in the morning or evening, but also a couple of hours before or after sunrise or sunset, depending on the observer’s latitude and on the positions of Venus and the Sun. Most of the time, it can be seen either in the morning or in the evening. At some times, it may even be seen a while in a completely dark sky.

Venus orbits the Sun every 224.7 Earth days. It has a synodic day length of 117 Earth days and a sidereal rotation period of 243 Earth days. Consequently, it takes longer to rotate about its axis than any other planet in the Solar System, and does so in the opposite direction to all but Uranus. This means that the Sun rises from its western horizon and sets in its east. Venus does not have any moons, a distinction it shares only with Mercury among the planets in the Solar System.

Venus is a terrestrial planet and is sometimes called Earth’s “sister planet” because of their similar size, mass, proximity to the Sun, and bulk composition. It is radically different from Earth in other respects. It has the densest atmosphere of the four terrestrial planets, consisting of more than 96% carbon dioxide. The atmospheric pressure at the planet’s surface is about 92 times the sea level pressure of Earth, or roughly the pressure at 900 m (3,000 ft) underwater on Earth.

Even though Mercury is closer to the Sun, Venus has the hottest surface of any planet in the Solar System, with a mean temperature of 737 K (464 °C; 867 °F). Venus is shrouded by an opaque layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in light. It may have had water oceans in the past, but these would have vaporized as the temperature rose under a runaway greenhouse effect. The water has probably photodissociated, and the free hydrogen has been swept into interplanetary space by the solar wind because of the lack of a planetary magnetic field.

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