venus

Transits of Venus are possible because the planet's orbit around the Sun is inside that of Earth. Venus Transits occur in pairs, with the second coming eight years after the first. But the pair events are separated by either 105.5 or 121.5 years.

Planet Venus

Venus is the only planet in the Solar System to turn in a clockwise direction on its axis, and it does this very slowly, taking 243 Earth days to do so. The "day" on Venus is even longer than a year on Venus, which is 225 Earth days long. Venus is covered in poisonous clouds containing sulphuric acid, and an atmosphere containing dangerous carbon monoxide.

In 1990-1994 the special imaging equipment on the Magellan space craft managed to look through the clouds to reveal a volcanic surface of the planet. Temperatures on Venus reach up to 480°C because heat from the Sun cannot escape through the thick atmosphere, which acts like a greenhouse covering the entire planet.

Sun-Earth Distance

Jeremiah Horrocks used his observations to estimate the distance between the Earth and the Sun - his calculation of 59,000,000 miles was far from the true number but much closer than any previous numbers. Still in the 17th century Rev. James Gregory and Sir Edmund Halley suggested that a more accurate measurement of the Earth-Sun distance could be made using the Transit of Venus from different places. In 1761 and 1769 the distance was computed to 93,000,000 miles. The photographs taken in 1882 were used in the following years to difficult but more detailed calculations of the Sun-Earth distance to 92,702,000 miles (Simon Newcomb), resp. 92,797,000 (William Harkness). The Transit of 2004 can be used for getting more precise number.

Long time ago in Egypt scientists tried to work out distances in space by measuring shadows in different places. Similar methods are used today. During the Transit Venus enters the Sun's disc in various times according to the position of the observer. Comparison of these times makes it possible to calculate the distance from the Earth to the Sun.