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The Transit of Venus, June 8, 2004
A very rare event is due to happen this spring! For the first time in 122 years, Venus will pass directly between the Earth and Sun. For several hours, the planet will appear silhouetted against our daytime star.
A rare event...
Venus is the second planet from the Sun. Its orbit is therefore smaller than Earth's: While our planet circles the Sun in one year, Venus completes its orbit in only 225 days.
Every 584 days, Venus catches up to our planet and passes between the Earth and Sun: This is known as inferior conjunction. Normally though, Venus’ orbit, which is tilted 3.4° with respect to that of Earth, carries it either above or below the Sun’s disk. But on rare occasions, when the inferior conjunction occurs in early June or early December, Venus’ alignment carries it directly in front of the Sun. This is called a transit.
Transits of Venus usually occur in pairs, eight years apart: Each pair is separated from the next by either 105 ½ or 121 ½ years, in alternating sequence. The last transit of Venus was in 1882, so no living person has seen one!
...but very noteworthy
It was Johannes Kepler who computed the first prediction of a transit of Venus. The transit of 1631 was not visible from Europe and likely went unobserved. Curiously, Kepler's calculations were still rough and failed to predict a transit in 1639. Acting on his intuition, however, Englishman Jeremiah Horrocks tackled the task of recomputing the event: Completed with less than a month to spare, his calculations showed that Venus would indeed transit the Sun on December 4, 1639. In those days, the delay was too short to spread the word to other astronomers in Europe: Horrocks and his friend William Crabtree were, therefore, the only witnesses of the first transit of Venus ever observed.
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1631 December 7
1639 December 4
1761 June 6
1769 June 3
1874 December 9
1882 December 6
2004 June 8
2012 June 6
2117 December 11
2125 December 8
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In 1677, astronomer Edmund Halley — famous for proving that some comets are periodic, and accurately predicting the return of one of them — showed that transits of Venus offered a unique opportunity to calculate the absolute distance between the Earth and the Sun. To accomplish this feat, one would need to combine accurate timings of the ingress and egress of Venus in front of the Sun, observed from locations as widely separated as possible. Once the Earth-Sun distance was established, it would then be possible to determine the distance of all the planets in the solar system — and even the closest stars! At that time, transits were thus considered extremely valuable, scientifically speaking.
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Closer to the Sun, Mercury also undergoes transits, and more frequently than Venus—as often as 13 times per century! However, Mercury is much smaller than Venus and farther away from us. The planet's tiny disk is therefore unsuited to reliably measure the Earth-Sun distance...
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During the 18th and 19th centuries, a number of expeditions were mounted to observe the transits of Venus (in 1761, 1769, 1874, and 1882) from the four corners of the globe. The goal was to accurately time the moments when the small, round silhouette of Venus entered and left the face of the Sun. For a variety of reasons, however, measuring the contact times with the required accuracy turned out to be a near impossible task. Overcoming these disappointments, 18th and 19th century astronomers were nevertheless able to establish the distance scale of our solar system.
Today, the distance to the planets is measured with great precision thanks to radar. The transit of Venus is now of interest only because it is rare, and because of the history it evokes.
Transit 2004
On June 8, 2004, Venus will cross the Sun’s disk in roughly 6 hours and 12 minutes. This transit will be visible in its entirety throughout Europe, and most of Africa and Asia. In the eastern Americas, the transit will already be underway at sunrise.
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World visibility map of the 2004 June 8 transit of Venus.
Courtesy of Fred Espenak, NASA/GSFC.
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In Montreal, when the Sun rises at 5:06 EDT, Venus will be more than two-thirds of the way through its passage across the solar disk. But the final portion of the transit will be the most interesting. From 7:05:18 to 7:25:18, the planet will cross the edge of the Sun — a total of 20 minutes. These times are referred to as contacts 3 and 4. During this time, the Sun will be 20 degrees above the horizon. In Québec City, the Sun rises at 4:51 EDT; Contacts 3 and 4 occur at 7:05:12 and 7:25:11 EDT respectively. Elsewhere in Quebec and Canada, the exact time of egress will vary by several seconds, depending on the observer’s location. This table provides local circumstances of the transit for a number of cities across Canada.
Observation
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CAUTION!
Great care must always be taken when observing the Sun: Intense solar radiation can cause instantaneous burns to the retinas and permanent eye damage. When observing the Sun directly, either with the naked eye, or a telescope, one must always use specially designed solar filters.
Safe observing methods
In case of doubt, why not show up at one of the public observing sessions organized by amateur astronomers!
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During its transit, Venus will appear like a perfectly round, black dot measuring about 1/32nd of the Sun’s diameter. Using special protective filters, those with good eyesight should be able to see the silhouette of Venus with the naked eye.
In order to fully appreciate the event, a telescope or binoculars are suggested, but they must be equipped with a special solar filter.
The Sun’s image can also be projected on a screen with a pair of binoculars or small telescope. In this case, a filter should not be used. However, the setup must be monitored at all times to ensure that no one accidentally looks through the optics.
Under all circumstances, please follow these tips and techniques for a safe and enjoyable obervation of the transit. And there's always the possibility of joining one of the public observing sessions in your area!
If you miss this transit of Venus, the next one will occur on June 6, 2012. After that, you’ll have to wait until December 11… 2117!
More information...
The following websites are great starting points for more information on the transits of Venus:
Sites in English
Sites in French/Other languages
See the Transit of Venus on the Web
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