Position and orbit

Venus orbits the Sun in a nearly circular orbit, as shown by its small orbital eccentricity. Its mean orbital radius is just over 108 million kilometres, and it passes within 40 million kilometres of Earth, closer than any other planet.

Figure 5.3 Orbital path of Venus (distance circles are in astronomical units, AU).

Observation of Venus is easy because of its close proximity to Earth, and it is the brightest object in the sky apart from the Sun and Moon. Venus can be seen either in the eastern sky before sunrise or in the western sky after sunset. It is so bright it can often be seen during daylight and is often mistaken for a UFO. Cloud cover on Venus is 100 per cent, compared to an average of 40 per cent on Earth; so optical telescopes on Earth cannot be used to see the surface of Venus.

Venus orbits the Sun in about 225 Earth days. Because it is closer to the Sun than Earth, Venus is seen to go through phases just like our Moon. Venus's size appears to vary according to its phases because of its changing distance from Earth. Venus is in its full phases when furthest from Earth, and when close to Earth it is seen as a thin crescent phase. It is possible to view the phases through binoculars or a small telescope.

As Venus and Earth travel around the Sun, Venus can be seen near the opposite side of the Sun about every 584 days. When Venus is moving towards Earth, the planet can be seen in the early evening sky (west). When Venus is moving away from Earth, the planet can be seen in the early morning sky (east).

Figure 5.4 The 2004 transit of Venus across the southern part of the Sun. (Photo: J. Wilkinson)

At rare intervals, observers from Earth can see Venus transit, or pass in front of, the Sun. The last transit occurred on 8 June 2004, and the next is due on 6 June 2012. A transit should be viewed by projecting the Sun's image onto a white screen from a telescope. The planet can be seen as a black dot slowly moving across the Sun's image (Figure 5.4). Care should always be taken when viewing the Sun - never look directly at the Sun through a telescope.

Venus and Earth have almost the same mass, diameter and average density.

The strength of gravity on Venus is slightly less than that on Earth. A 75 kg person on Earth weighs 735 N, but on Venus they would only weigh 607 N. Venus is the third densest planet in the solar system. This high density is because Venus has a large rocky core made of mostly nickel and iron. The core is about 3340 km in radius and is surrounded by a molten silicate mantle about 2680 km thick. There is also a thin outer layer or crust about 50 km thick that is similar to the crust on Earth.

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