JUNE 5/6, 2012

On June 5/6 2012 Venus crossed the Sun's disk, producing an extremely rare Venus Transit. Venus could be seen in silhouet as a large black spot.

Astronet and 'Planetary Paul' were organising a live webcast of the Venus transit from The Netherlands. Unfortunately, Planetary Paul was clouded out completely. But luckily there were many other stations around the world and from outer space that provided webcasts and live streamings of the event.

Images from outer space taken by NASA's Solar Dynamics Observatory (SDO).


Excellent images were brought by Slooh Space Camera

The Transit of Venus was succesfully webcasted by

NASA's did live streaming from the top of Mauna Kea, Hawaii.

Unfortunately Planetary Paul, Haarlem, The Netherlands. was completey clouded out.


The Venus transit could also be followed on the H-alpha images that are continuously broadcasted by the NSO/GONG H Alpha Network Monitor of solar telscopes around the Earth.

Astronomy Picture of the Day, in cooperation with NASA/SDO provided very detailed solar images.

YouTube: The Venus Transit was observed and photographed from the International Space Station (ISS)

Don Pettit, flight engineer on board of the International Space Station (ISS), will photograph the Venus Transit from one of the windows of the cupola of ISS. Pettit's pictures will be posted at this Photostream.

Find local activities worldwide.


Transits of Venus across the disk of the Sun are among the rarest of planetary alignments. Only seven such events have occurred since the invention of the telescope (1631, 1639, 1761, 1769, 1874, 1882 and 2004). Transits of Venus are only possible during early December and early June when Venus's orbital nodes pass across the Sun. If Venus reaches inferior conjunction at this time, a transit will occur. Transits show a clear pattern of recurrence at intervals of 8, 121.5, 8 and 105.5 years. This transit is the last one of the part of the pair 2004 June 8 and 2012 June 5/6. The next pair of Venus transits occur over a century from now on 2117 Dec 11 and 2125 Dec 08.

Animation of the Venus Transit, by Marc van der Sluys of Dutch website

The principal events occurring during a transit are characterized by contacts. The event begins with contact I which is the instant when the planet's disk is externally tangent with the Sun. The entire disk of the Venus is first seen at contact II when the planet is internally tangent with the Sun. During the next several hours, Venus gradually traverses the solar disk at a relative angular rate of approximately 4 arc-min/hr. At contact III, the planet reaches the opposite limb and is once again internally tangent with the Sun. The transit ends at contact IV when the planet's limb is externally tangent to the Sun. Contacts I and II define the phase called ingress while contacts III and IV are known as egress. Greatest transit is the instant of minimum angular separation between Venus and the Sun as seen from Earth's geocenter. Position angles for Venus at each contact are measured counterclockwise from the north point on the Sun's disk.

The Venus Transit. Courtesy: Fred Espenak

As can also be seen in above's diagram, the geocentric phases are:

EventUniversal TimePosition Angle
Contact I22:09:3841o
Contact II22:27:3438o
Contact III04:31:39293o
Contact IV04:49:35290o

These geocentric times are for an observer at Earth's center. The actual contact times for any given observer may differ by up to ±7 minutes. This is due to effects of parallax, since Venus's 58 arc-second diameter disk may be shifted up to 30 arc-seconds from its geocentric coordinates depending on the observer's exact position on Earth.

Here is a list of predicted contact times and corresponding altitudes for a number of cities around the world. Here are lists of similar predictions for locations throughout Canada and the USA.

The entire transit (all four contacts) is visible from northwestern North America, Hawaii, the western Pacific, northern Asia, Japan, Korea, eastern China, Philippines, eastern Australia, and New Zealand. The Sun sets while the transit is still in progress from most of North America, the Caribbean, and northwest South America. Similarly, the transit is already in progress at sunrise for observers in central Asia, the Middle East, Europe, and eastern Africa,. No portion of the transit will be visible from Portugal or southern Spain, western Africa, and the southeastern 2/3 of South America. (Note that due to the International Date Line the Western Hemisphere will see the transit on June 5.)

The visibility of the Venus Transit. Courtesy: Fred Espenak/NASA


The apparent semi-diameters of Venus and the Sun are 29 arc-seconds and 945 arc-seconds respectively. This 1:32.6 diameter ratio results in an effective 0.001 magnitude drop in the Sun's integrated magnitude due to the transit. This simply means that the Sun will be as dangerous for our eyesight at the time of the Venus Transit as it is on any normal day, when there is no planet in front of the solar disc.


The use of #14 shade welding glass or eclipse shades will permit a large number of people who do not have specialized equipment to observe this event. However, as the planet approaches the limb of the Sun, subtleties like the black drop effect may not be discernible. Pinhole projectors are a safe, indirect viewing technique for observing an image of the Sun. While popular for viewing solar eclipses, pinhole projectors suffer from the same shortcomings as unmagnified views when Venus approaches the edges of the Sun. Small features like the black drop effect and the halo around Venus while it straddles the solar edge may not be discernible.

You may project a magnified view of the Sun through a telescope onto a surface, but the technique often has its own limitations. For example, large reflector telescopes can generate too much heat by concentrating a lot of the Sun's energy on the secondary mirror and eyepiece. Likewise, Schmidt-Cassegrain telescopes can experience too much heat build-up as the light bounces internally. Also, magnified projections usually have an exposed focal point beyond the eyepiece where bystanders could inadvertently burn themselves. Constant attention is required.

Projecting a magnified view of the sun through a telescope on a surface. Illustration and photograph: European Southern Observatory.

The transit of Venus is perhaps best viewed directly when magnified, which demands an appropriate solar filter over the large end of the telescope. Do not use small filters that fit over the eyepiece, for the concentrated sunlight can shatter them. The sun's energy must be attenuated before it enters the telescope. A filtered, magnified view will show the planet Venus, the "black drop" effect, and sunspots.

No matter what technique you use for viewing the sun, do not stare continuously at the sun! Always give your eyes a break.


In earlier times, astronomers used transits of Mercury and Venus to get information about the dimensions of the solar system: the size of the Sun, the distance of Venus, and the distance between us and the Sun, which is called the astronomical unit (AU). To fix that important quantity, astronomers used the method of triangulation.

Aside from its rarity, the original scientific interest in observing a transit of Venus was that it could be used to determine the distance from the Earth to the Sun, and from this the size of the Solar System, by employing the parallax method and Kepler's third law. The technique involved making precise observations of the slight difference in the time of either the start or the end of the transit from widely separated points on the Earth's surface. The distance between the points on the Earth was then used as a baseline to calculate the distance to Venus and the Sun via triangulation. Illustration: Vermeer/Wikipedia.

The first astronomer to recognize the importance of observing transits of Mercury and Venus was Edmund Halley (1656-1742). It appeared to the observer, using a telescope as the lens of a camera obscura (never look directly at the Sun through a telescope!) as a black dot crossing the surface of the Sun. The first astronomers to use telescopes to observe the transit of Venus were Jeremiah Horrocks (1618-41) and William Crabtree (1610-44) in 1639.

Astronomers travelled to remote parts of the world to observe the transits of Venus in 1761 and 1769. To observe the transit of 1769, Captain Cook sailed from England to Tahiti. He discovered Hawaii and a few other places as bonuses along the way; it is not often that the side benefits of astronomical research are so apparent.

How accurate were the measurements of Cook? The "dusky shade round the body of the Planet" as he descrtibed the appearance of Venus was a problem. Intense sunlight filtering through Venus' atmosphere fuzzed the edge of the disk and decreased the precision with which Cook could time the transit. For this reason, his measurements disagreed with those of ship's astronomer Charles Green, who observed the transit beside Cook, by as much as 42 seconds.

Cook and Green also observed the "black drop effect." When Venus is near the limb of the sun--the critical moment for transit timing--the black of space beyond the Sun's limb seems to reach in and touch the planet. You can recreate the black drop effect with your thumb and index finger: Hold the two in front of one eye and narrow the distance between them. Just before they touch, a shadowy bridge will spring across the gap. According to John Westfall, writing for Sky & Telescope magazine in June 2004, "this is simply the result of how two fuzzy bright-to-dark gradients add together." The black drop effect, like the fuzziness of Venus' atmosphere, made it hard to say just when the transit began or ended.

Black drop effect

This was a problem for observers elsewhere, too, not only Cook in Tahiti. In fact, when all was said and done, observations of Venus' 1769 transit from 76 points around the globe, including Cook's, were not precise enough to set the scale of the solar system. Astronomers didn't manage that until the 19th century when they used photography to record the next pair of transits. And even then an incertainty in the measurements remained.

In recent years, radio signals emitted by spacecraft as they pass behind Venus have enabled us to obtain very accurate planetary positions and masses, as well as the distance between the Earth and the Sun. Because of these results from space exploration, observing this and future planetary transits will be of less scientific importance, but they will of course continue to be great public and educational interest.


Join the project of Prof. Dr. Udo Backhaus of Universität Duisburg in Essen! He will calculate the distance from the Earth to the Sun using your contact timings . He will also use photographic images taken from different places on Earth to do the calculations.

Dutch physicist Steven van Roode developed a special and free Phone App to report contact timings for a similar calculation.

Transit of Venus 2012 of the Bradford Robotic Telescope Project has an automatic algorithm for reporting contact timings. The results can be checked online. welcomes your photographs! Send your best pictures (3 at most, maximum size 1200 x 900 pixels) to:

What else is there to be seen? Look before and during ingress and during and after egress for a spectacular 'light ring' around the black silhoutte of Venus. This is caused by sunlight that is refracted by Venus' thick atmosphere. It was after observing this light ring in 1761 that Russian astronomer Mikhail Lomonósov realized that Venus is surrounded by an atmosphere.

The light ring in 1874

During the recent Venus Transit of 2004, June 8, the light ring around Venus could be clearly seen.


The 2012 transit will give scientists a number of research opportunities. Astronomers can measure of dips in a star's brightness caused by a known planet transiting a known star (the Sun). This will help astronomers when searching for exoplanets. Unlike the 2004 Venus transit, the 2012 transit occurs during an active phase of the 11-year activity cycle of the Sun, and is likely to provide practice in detecting a planet's signal around a "spotty" variable star.

The atmosphere of Venus can be observed simultaneously from Earth-based telescopes and from the Venus Express spacecraft. This will give a better opportunity to understand the intermediate level of Venus's atmosphere than is possible from either viewpoint alone, and will provide new information about the climatology of the planet.

The atmosphere of Venus can be studied using a spectrograph. The results of analysis of the well-understood atmosphere of Venus will be compared with studies of exoplanets with atmospheres that are unknown.

The Hubble Space Telescope cannot look at the Sun directly. But Hubble will be aimed at lunar crater Tycho and study the reflected sunlight to detect spectral fingerprints of Venus' atmosphere. This may provide another technique to study exoplanets.

Carl Koppeschaar


General information:

History of Venus Transits:

The Black Drop Effect

Topocentric contact times and corresponding altitudes of the Sun:

Past and future Venus Transits:


Time synchronization:

Solar Parallax Calculation:



As a result of posting this webpage on Wikipedia's Venus Transit web pages, my IP address was blocked by a certain "Melos" (Wikipedia administrator). This webpage and its Dutch language mirror took me many unpaid hours to compose. As anyone can see it contains no advertisments or hidden scripts whatsoever. I am an astronomer and a science journalist. I only want to inform the public, as I did many times before with live webcasts of solar and lunar eclipses, Mercury transits and the 2004 Venus transit.

Wikipedia should first investigate before they block a serious contribution! German poet, philosopher, historian and playwright Friedrich Schiller (Johann Christoph Friedrich von Schiller, 1759 - 1805) knew it:

"Mit der Dummheit kämpfen Götter selbst vergebens." (Talbot in Die Jungfrau von Orléans, III, 6)

In plain English:

"Against stupidity, the gods themselves contend in vain."

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