Timelapse footage of the transit of Venus across the Sun, on 8th June 2004. Transits of Venus occur when the Sun, Venus and the Earth are aligned in space, and Venus appears to cross the face of the Sun. The sequence begins with the first contact, where Venus is seen just touching the limb of the Sun. After it moves fully onto the face, it appears to be connected to the dark edge by the 'black drop effect'. This is an optical illusion caused by turbulence in Earth's atmosphere. The view then pulls back to show the relative apparent sizes of Venus and the Sun. Historically the transits were used to attempt to determine the distance from the Earth to the Sun. The Sun is some 149 million kilometres (km) from Earth, with Venus around 41 million km from Earth at this time. Venus has a diameter of 12,100km, compared to the Sun's 1,392,000km. Transits of Venus occur in a 243-year cycle, with pairs eight years apart separated by 121.5 then 105.5 years. The 2004 transit was the first since 1882. The next transit will be on 5-6th June 2012, with the following one on 10-11th December 2117.