Venus crosses the Sun

This image of the transit was captured by placing a digital camera to the eyepiece of a scope. Credit: Dan Falk.

WAUKESHA, WISCONSIN: On June 8, 2004, two of the sky’s most brilliant objects will meet as Venus transits the face of the Sun. Weather permitting, the best spots to observe the transit from will be Europe, Africa, and Asia – from where the entire 6-hour event will be visible. The beginning of the transit will be seen from Japan, the Philippines, and Australia, but the Sun will set before the event ends. Observers in eastern North America, the Caribbean, and most of South America will see only the end of the event after the Sun rises.
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Understanding transits
A transit occurs when one celestial body crosses in front of a bigger one. Planetary transits result when either Mercury or Venus passes across the face of the Sun as seen from Earth. This can happen only when one of these planets lies between Earth and the Sun (a configuration called inferior conjunction). However, for the same reason we don’t see solar eclipses every two weeks, transits do not occur at each inferior conjunction. Because the orbits of both Mercury and Venus tilt with respect to the ecliptic, the plane of Earth’s orbit in space, the planets typically pass either above or below the Sun at inferior conjunction. (Mercury’s orbit tilts 7° to the ecliptic, and Venus’s orbit tilts 3.4°.) If inferior conjunction happens within a day or so of the planet crossing the ecliptic, a transit results. Transits of Mercury are uncommon – on average, thirteen occur each century. Transits of Venus are even more rare.

Transits of Venus occur in pairs eight years apart, but a recurring pattern of either 105.5 or 121.5 years between the last transit of one pair and the first transit of the next pair is the general rule. All transits of Venus fall within several days of either June 8 or December 9. Venus crosses the plane of Earth’s orbit twice during each of its orbits around the Sun. The two points of intersection are known as nodes. If Venus passes through inferior conjunction when its longitude is near 77° (around June 8) or 257° (around December 9), a transit occurs.
Transit 2004
Unless human life spans get a big boost in the next century, those of you reading this will have at most two opportunities to view a Venus transit: on June 8, 2004, and June 6, 2012. Following these events, another transit of Venus will not occur until December 11, 2117.

Location will matter most for viewing the 2004 transit. In the contiguous United States (observers in northern Alaska will be able to view the entire transit), no one will see Venus transit the Sun for more than two-and-a-half hours. Observers with a clear sky in eastern New York and New England will see at least 120 minutes of the transit. That number drops to 60 minutes for observers on a line curving from north central Minnesota to northeastern Florida. None of the transit will be visible from the Mountain states or the Pacific Coast. Even from the favored Northeast, the Sun will have climbed only about 24° above the horizon by the transit’s end. By contrast, observers in Europe, Asia, and Africa can follow the entire transit, which will last longer than 6 hours.

During the transit, four events – called “contacts” – define the beginning and end.

First contact

• occurs when the dark edge of Venus’s disk first touches the bright solar background.

Second contact marks the instant when the Sun first surrounds the entire disk of Venus completely.

Third contact occurs when the edge of Venus again touches the Sun’s edge.

Fourth contact is defined as the instant when the disk of Venus can no longer be seen silhouetted against the Sun.

North Americans will see only third and fourth contacts.

Venus’s disk will have a diameter 3.125 percent of the Sun’s, so astute observers should be able to detect the transit with their properly protected naked eyes. Because the Sun is involved, make sure to safeguard your eyes when viewing the transit (basic observing equipment includes a #14 welders’ filter or a solar filter for a telescope).