On this page we present some of the technical details on the Venus transit.


With a mean distance from the Sun of about 68 million miles (Earth is about 94 million miles from the Sun), Venus is the second closest planet to the Sun and our nearest planetary neighbor. While the Earth orbits the Sun in (about) 365 days, Venus orbits the Sun in only 225 days. The picture below shows the positions of the planets with respect to the Sun.


Venus rotates in the opposite direction as compared to Earth (the stars rise in the west and set in the east)and a Venus day is longer than its year. It takes Venus 243.0187 Earth days to make one rotation and 224.701 Earth days to orbit the sun.


A transit of Venus across the disk of the Sun are among the rarest of planetary alignments and only six Venus transits have occurred since the invention of the telescope (1631, 1639, 1761, 1769, 1874 and 1882). A Venus transit occurs when Venus passes directly in front of the Sun. This is similar to a solar eclipse when the Moon passes in front of the Sun. However, unlike the Moon, which covers most of the Sun, Venus appears as a small dot crossing the face of the Sun. An example of what this will look like is shown in the image below (a computer generation).



Transits of Venus are only possible during early December and early June when Venus's orbital nodes pass across the Sun. Futhermore, the Venus transit presently upon us comes in a pair. The transit on June 8, 2004 will be followed by another transit on June 6, 2012. The last Venus transits occurred in 1882 and 1874. The next pair will occur in 2117 and 2125. Although the transits currently occur in pairs, this is not always the case, sometimes there is only one transit. Finally, only the inner planets (Venus and Mercury) can transit, because these are the only planets between the Earth and the Sun. However, if you were on Mars, you could see an Earth transit!

The chart below shows the relationship of the cycles:

The times in red reveal the primary cycle.
The times in blue reveal the dual sub-cycle occurring between pairs.
The times in yellow reveal the duration between the last transit of one pair and the first transit of the second pair.





SIDEBAR - 2003 Mercury transit:

Fortunately, there was a Mercury transit in 2003 and many images of it were acquired.

Mercury nears the limb of the Sun just prior to the end of the transit in this Royal Swedish Academy of Sciences image.
Mercury is a lonely dot against the huge solar disk in this SOHO image of the 2003 transit event.

For more information on the Mercury transit, click here .

To view a NASA SOHO video clip of the Mercury transit, click here .



The principal events during a transit are characterized by the contacts shown in the image below. The event begins when the planet's disk first blocks the Sun (contact point I). At contact point II the entire disk of the Venus is first seen. During the next 6 hours, Venus gradually traverses the solar disk at an angular rate of approximately 4 arc-min per hour. The planet reaches the opposite side of the Sun at contact point III. The transit ends at contact IV when the planet's disk no longer blocks any part of the Sun. Contact points I and II define the phase called ingress while contacts III and IV are known as egress (click on the picture for a larger version).


The global visibility of the 2004 Venus transit is shown in the Goddard Space Flight Center image below (which also shows expected cloud coverage on the day of the transit). The entire transit (all four contacts) is visible from Europe, Africa (except western parts), Middle East, and most of Asia (except eastern parts). The Sun sets while the transit is still in progress from Australia, Indonesia, Japan, Philippines, Korea, easternmost China and Southeast Asia. Similarly, the Sun rises with the transit already in progress for observers in western Africa, eastern North America, the Caribbean and most of South America. None of the transit will be visible from southern Chile or Argentina, western North America, Hawaii or New Zealand (click on the picture for a larger version).



The image below shows in detail the visibility of the Venus transit in the United States. Note that neither the 2004 nor 2012 transits will be fully visible in the United States (click on the picture for a larger version).




The Transit of Venus gives us the opportunity to see how transits make it possible to detect extrasolar planets, i.e. planets orbiting stars other than the Sun.

The most obvious (but naïve) method to detect a planet around a star would be to take a picture of the star. However, since the star is a billion times more brilliant than any planet, the light from the star would mask the planet.

Thus, one needs indirect approaches and the most successful up to now is the radial velocity search method. This method is based on the fact that as the planet orbits its host star, it also induces a tiny motion in the star. This periodic motion can be measured by recording the radial velocity of the star as a function of time. The radial velocity method has made possible the detection of 103 planetary systems containing 118 planets. Extrapolated to the entire galaxy, this reveals that the galaxy must contain at least 7 billion planets. However, the radial velocity method is not able to detect Earth-like planets as the amplitude exerted on the Sun by the Earth is too small to be detected.

A more promising method is therefore the transit method. During a transit a planet can slightly and temporarily darken a star around which it orbits. For a star like our Sun, a giant planet like Jupiter will create a drop in luminosity of 1% and for a planet of the size of the Earth a drop in luminosity of 0.01% results. Detection of these luminosity changes are not a problem with present-day technologies.

Finally, two giant exoplanets (HD 209458 b and OGLE-TR-56 B) have been shown to transit their host stars. The first one was even detected by an amateur astronomer.


Links to pages with technical information:

  • A Venus and Venus transit page by Nick Anthony Fiorenza .

  • A Venus transit page by Jurgen Giesen .

  • The US Naval Observatory's page on the Venus transit (timing information).

  • The Internet Projects's page that discusses the observing, photographing and evaluating the 2004 (for educators).

  • A paper by Udo Backhaus that discusses the observation of the Venus transit and its usefullness in determining the distance between Earth and Sun.

  • A DePaul University site that describes a "paper plate" method to understand the mechanics of the transit.