The Venus Transit 2004 European Science & Technology Week 2004Visit the websites of the VT-2004 organisers

The Venus Transit 2004

... Frequently Asked Questions

This page will provide answers to some of the most common questions asked in connection with this programme.

If you have other questions in connection with this programme, you may also contact the organisers of the ESO/EAAE Venus Transit 2004 project via email at

Q: Why is this event so important?

A : A Venus Transit observed with sufficient accuracy from the Earth allows the distance between the Earth and the Sun to be determined. This measure is crucial for astronomers to know because all other distance determinations in the universe ultimately depend on this value. Today astronomers use other, more precise methods for the determination of this "Astronomical Unit", so the present one is above all of historical and didactic interest.

Q: When was a Venus Transit first observed by astronomers?

A : The first Transit of Venus was observed in 1639.

Q: When was a Venus Transit first observed by astronomers in order to determine the distance to the Sun?

A : The first such attempt to determine the Astronomical Unit was undertaken in 1761, however with little success due to the observers' lack of experience.

Q: Is it also possible to observe a transit of planet Mercury?

A : Yes. As seen from Earth, the planet Mercury also passes in front of the Sun. Because Mercury circles the Sun every 88 days (compared with 225 days for Venus) and is also much closer to the Sun, transits of Mercury are much more common than those of Venus. On average, they occur approximately once every 7 years.

Q: Why is a Venus Transit such a rare event?

A : Venus Transits occur approximately 4 times in 243 years, more precisely in pairs of transit events separated by about 8 years and these pairs are separated by about 120 years. The reason for these long intervals lies in the fact that the orbits of Venus and the Earth do not lie in the same plane and a transit can only occur if both planets and the Sun are situated exactly on one line (the "line of nodes"). It takes Venus about 225 days and the Earth about 365 days to circle the Sun. This means that when Venus returns to one of the nodes, Earth is not yet there, and vice versa. In addition, a couple of smaller, complicated effects lead to the observed long and slightly irregular intervals. Following the event in June 2004, the next Venus Transit will happen on June 6, 2012, and thereafter in December 2117 and December 2125.

Q: Why should I participate in this experiment?

A : Many observations from all over Eurasia/Africa/Australia are needed in order to get a reasonably accurate result for the Astronomical Unit. And for every participant, this is a marvellous opportunity to take part in a first-ever "public" re-enactment of a famous historic observation.