Venus is the second planet from the Sun, orbiting at a distance of 0.723 AU once every 224 days.
It is our neighbour in the solar system, orbiting just inside the Earth's orbit. It is of a similar size to the Earth – around 95% of an Earth-width across – and has similar abundances of chemical elements such as carbon. For this reason, Venus is often considered to be the Earth's 'twin' planet.
Even though Venus orbits closer to the Sun than the Earth, its surface receives less heat from the Sun since it is permanently enshrouded by thick cloud layers. These reflect so much sunlight back into space that they more than compensate for the increased intensity of the Sun's light. They also make it virtually impossible to observe Venus's surface, meaning that the planet presents a bland and largely featureless disk.
Venus never ventures far from the Sun in the sky – at most, 46° – and so always appears low in the western sky in the evening, or low in the eastern sky in the morning sky.
Its appearances follow an 18-month cycle, appearing once in the evening sky and once in the morning sky within each cycle.
At each apparition, Venus is visible for a couple of months, attaining a maximum brightness of magnitude –4.2, surpassing any other star or planet by 2.5 magnitudes to become the brightest object in the sky after the Sun and Moon.
After passing behind the Sun, Venus first appears in the evening sky for a couple of months, lying to the east of the Sun. It then undergoes inferior solar conjunction, passing between the Earth and Sun about two months after reaching greatest prominence in the evening sky. Moving into the morning sky, it then reaches greatest prominence as the 'morning star' a further two months later.
The reverse journey, from the morning sky into the evening sky, takes much longer as Venus travels a long path around behind the Sun. The moment when Venus passes behind the Sun is typically around seven months after reaching greatest prominence in the morning sky. It does not return to the evening sky until 14 months after its disappearance from the morning sky.
Seen through binoculars or a telescope, Venus's disk is the largest of any of the planets on account of its closeness to the Earth, reaching a maximum size of 66" at inferior conjunction.
Nonetheless, it remains just too small to be discernible to the naked eye. Even through a telescope, it is not possible to resolve any detail on its surface without specialist equipment, owing to the thick featureless cloud layers which surround it.
However, as it orbits the Sun, Venus shows phases akin to those of the Moon, and these are quite apparent even through a pair of binoculars, especially if image stabilised.
Venus was the first planet ever to be visited by a space probe – after a failed Soviet attempt, the first spacecraft to successfully fly past Venus was the American Mariner 2 (1962).
It has since been visited by more than 20 spacecraft, most recently the American Magellan (1989) and European Venus Express (2005).
These have revealed that the planet's surface temperature to be 480°C – the highest average surface temperature of any body in the Solar System, and hot enough to melt lead and to visibly glow red hot. Its atmosphere has been found to be composed primarily of carbon dioxide, with traces of other noxious compounds such as sulphuric acid, which gives its clouds an acidic yellow hue.
These extreme climatic conditions are less a result of Venus's proximity to the Sun, and more a result of the greenhouse effect: Venus has the strongest known greenhouse effect of any body in the Solar System. Current thinking is that the Earth and Venus may have been very similar planets at the time of their formation, but that a point of divergence came early in their history, when oceans formed on the Earth, allowing carbon dioxide to dissolve in the water and become locked up in carbonaceous rocks such as limestone (calcium carbonate). On Venus, this process never happened, and most of its carbon remains in the form of carbon dioxide in its atmosphere.
Never attempt to view Venus through a telescope or binoculars if the Sun is still above the horizon. A momentary glance at the Sun through such an instrument is sufficient to cause permanent blindness.