Transit of Venus (2004)
On 8 June 2004 Venus passed in front of the Sun for the first time in 122 years. A small black dot crept across the solar disk for more than six hours.
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Available views: Orbit ✓ · Sky — (not visible in the sky)
The story
On 8 June 2004 a transit of Venus took place: Venus passed directly in front of the Sun. When Venus lines up exactly between the Sun and Earth, it appears as a small, round black dot on the Sun's disk. The dot touched the edge of the Sun at 05:13 UT and left the far side at 11:26 UT, a crossing of more than six hours.
The whole event was visible from Europe, most of Asia and almost all of Africa. In easternmost Asia, such as Korea and Japan, it began in the afternoon, but the Sun set before the transit finished, so most of it, up until sunset, could be seen there. In eastern North America only the end was visible, just after sunrise.
The previous transit of Venus had taken place on 6 December 1882. So in 2004 nobody alive had ever seen one. Many schools and science centres held projection events, in which a telescope throws an image of the Sun onto a sheet of paper, and many people saw the black dot moving across the Sun for the first time.
Transits of Venus are very rare. The orbits of Venus and Earth are tilted slightly against each other, so when Venus passes between the Sun and Earth it usually slips above or below the Sun. As a result, transits come in pairs eight years apart and then stop for more than a century. The partner of 2004 was 2012. Move the orbit view to 8 June 2004 and the Sun, Venus and Earth are reproduced standing in a line.
Viewing guide — where, when, how
Move the orbit view to 8 June 2004, follow Earth and keep the Sun in view, and Venus sits exactly between the two. Slow the playback to reproduce the six hours during which Venus moves a little along its orbit and passes in front of the Sun.
Select Venus in the sky view to see whether it is in the dawn or evening sky from your location, and how much of it is lit. During a transit Venus sits in the same place as the Sun, so moving the time to 8 June 2004 draws Venus on top of the solar disk.
The next transit of Venus is on 11 December 2117. Before that, Mercury will pass in front of the Sun on 13 November 2032. Mercury looks much smaller than Venus, so a telescope fitted with a proper solar filter is needed to see it. The Sun should never be viewed with the naked eye or a telescope without a filter.
See this for yourself — replay the moment in the orbit view
Why this eclipse is special
Transits of Venus have a long history as a way of measuring the distance to the Sun. In 1761 and 1769 astronomers from many countries travelled around the world to time the moments when Venus entered and left the Sun's disk. Seen from places far apart, Venus seems to follow slightly different paths across the Sun. This difference is called parallax, and it can be used to work out the distance from Earth to the Sun. Captain Cook's voyage to Tahiti in 1769 was made for this observation.
In 2004 many people repeated the old method for themselves. The VT-2004 project, led by the European Southern Observatory, drew 2,763 participants around the world, including nearly 1,000 school classes. Timings made from their own locations were gathered together, and they gave an Earth–Sun distance of about 149.61 million km, only 0.007 percent from the accepted value.
The same idea is used today to find planets around other stars. While Venus is in front of the Sun, sunlight drops by about 0.1 percent. When a planet passes in front of a distant star, the starlight also dims very slightly, and the Kepler space telescope found many exoplanets by searching for such tiny dips.
Background and earlier eclipses
The first person to observe a transit of Venus was the Englishman Jeremiah Horrocks, in 1639. From then until 2004 there were only six transits in all: 1639, 1761, 1769, 1874, 1882 and 2004. In 1874 several countries sent expeditions to places such as Japan, India and Egypt, and attempts were made to record the transit with photographs.
Early observers were troubled by the 'black drop effect'. Just as Venus touched the edge of the Sun, Venus and the edge seemed to stretch and stick together like a drop of water, which made exact timing difficult. Scientists think it comes from a combination of the Sun's edge looking darker than its centre and the limits of the telescope in forming a sharp image. The effect was seen from space during the 1999 and 2003 transits of Mercury, a planet with almost no atmosphere, which supports this view. In 2004 many observers reported the effect as much weaker than in earlier centuries.
The distance to the Sun is no longer measured with transits. In the 1960s radar measurements, which timed radio signals bounced off Venus, pinned the distance down to the kilometre, and in 2012 the International Astronomical Union fixed the astronomical unit (the average Earth–Sun distance) at 149,597,870.7 km. So the 2004 measurement was not meant to find a new value. It was an educational experiment in which people tried for themselves the method that eighteenth-century astronomers used to size up the solar system for the first time.
The partner of 2004, the transit of 6 June 2012, was the last transit of Venus of the twenty-first century, and it could be seen from start to finish in East Asia, eastern Australia and the western Pacific. The next pair falls on 11 December 2117 and 8 December 2125. After each pair comes a wait of 105.5 or 121.5 years, which is why the transit after 1882 was in 2004.
FAQ
How long did it take?
A little over six hours, from 05:13 to 11:26 UT.
Where could it be seen?
The whole transit, from about 05:13 to 11:26 UT, was visible from Europe, most of Asia and almost all of Africa. Easternmost Asia and Australia saw the start before sunset, and eastern North America and much of South America saw the end after sunrise.
Why did it take 122 years?
Transits come in pairs eight years apart, followed by gaps of 105.5 or 121.5 years, so the transit after 1882 was in 2004.
How accurate were the public measurements?
The VT-2004 project's result was about 149.61 million km, only 0.007 percent from the accepted value.
When is the next transit of Venus?
On 11 December 2117. The partner of 2004 was 2012.
Related
- Transit of Venus — last this century
- Transit of Mercury (2032)
- Mariner 2 — first planetary probe
- Venus — object index
- The Sun — object index
Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.