Rarest of All Eclipses

On June 8, Venus will appear to cross in front of the sun as viewed from Earth.

1882 Sunplate (false color) from last Venus transit Image Credit: JHA XXIX, 1998

Only the two inner planets, Mercury and Venus, can show this phenomenon when they move in between the Earth and the Sun. In contrast to Mercury, where transits happen at a rate of 13 to 14 per century, transits of Venus are very rare. Venusian transits currently happen in pairs, 8 years apart which repeat only after more than a century. From Earth, the event is seen only at intervals of 8, 121.5, 8, and 105.5 years.

The last "Venus transit" occurred in 1882. The next two Venus transits are on June 6, 2012, and Dec. 11, 2117.

No one alive has viewed this rarest of all eclipses. On Earth, one can rarely see Venus when it passes in front of the sun because its orbit is inclined by 3.4 degrees to the plane of Earth’s orbit [banner image]. Therefore, the sun, Venus and Earth rarely line up. Only six such events have occurred since the invention of the telescope; the most recent in 1882.

The Venus transit will be visible from approximately 75 percent of the Earth . Precise transit times for cities worldwide are available. The entire continent of Europe, along with most of Asia and much of Africa, will be in the prime viewing area to see the entire transit from start to finish. Western Africa and the easternmost parts of North and South America will be able to see the transit at sunrise. Australia will see it at sunset. Western North Americans will not see the eclipse at all.

"People using a filter approved for safe solar viewing can expect to see a small black dot, about 1/30 the size of the solar disk, very slowly moving across the sun," said Fred Espenak, an eclipse expert at NASA’s Goddard Space Flight Center, Greenbelt, Md.

Previous transit pairs were in (1761,1769) and (1874,1882) – it was to observe the 1769 transit from Tahiti that Captain Cook had astronomers on board his ship. As early as 1609 Kepler could determine very accurately a model of the Solar system by using his laws, but only after (at least) one distance was accurately measured, could all the others be deduced with equal precision. One of these direct methods of determining the distance between the Earth and Sun is to observe the transit of Venus simultaneously from different places on the globe.

Transits are used in modern extrasolar planet discovery. A dip in brightness is observed as a prospective planet transits in front of its parent star Credit: ESO

This astronomical event gave astronomers at the turn of the last century an idea of the scale of our solar system.

In 1882, no one could have imagined that one day such distances in the solar system would be refined to within centimeters–as they are presently known– by bouncing radar beams off Venus or laser light from a mirror put on the Moon by astronauts.

As the transit begins, the leading edge of Venus first touches the bottom left side of the sun’s disk. Astronomers call this Contact I. About 20 minutes later, Venus’ opposite edge touches the same point on the edge, or limb, of the sun as Contact I. This creates a "neck" where the appearance of the planet appears a little distorted –the so-called "black drop effect." When the "black drop effect" occurs, University of Arizona associate astronomer Glenn Schneider said, Venus no longer resembles a disk. "It kind of looks like a tear drop," he said. Schneider, who is known to chase anywhere in the world to view a solar eclipse, is partly responsible for solving the "black drop effect" problem that comes with the Venus transit.

As the transit ends, Venus’ leading edge touches the bottom right side of the sun’s limb, again creating a dark neck along the edge, during Contact III. Venus’ opposite edge touches the sun’s limb during Contact IV, as Venus completes its journey across the sun.

Astronomers once attributed the black drop effect to Venus’ thick atmosphere. But then they discovered a black drop effect during Mercury’s transits, which can be seen from Earth 13 or 14 times a century. Because Mercury has almost no atmosphere, astronomers realized the idea that the transiting planet’s atmosphere creates the black drop effect is wrong.

Schneider, along with Jay Pasachoff of Williams College and Leon Golub of the Harvard-Smithsonian Center for Astrophysics, saw Mercury’s faint black drop effect in 1999 data from NASA’s Transition Region And Coronal Explorer (TRACE) spacecraft observations of a Mercury transit.

Schneider and his colleagues determined that there were two factors causing the black drop effect. The first was due to the point spread function, which occurs when the optics in smaller telescopes used on Earth cause the object being viewed to appear blurry. The second factor they attributed to limb darkening, when the brightness of the sun decreases towards the edge of it disk. The black drop effect hindered earlier astronomical attempts at making exact measurements, even though they were only about one minute off the time it took Venus to cross the face of the sun.

Astronomers have since discovered the distances between the sun and all the planets of the solar system. However, Schneider said, observing the transit of Venus still plays an important role in astronomy today. Those who search for extrasolar planets can use the same observational methods to find terrestrial planets in other star systems where hot, gaseous, Jupiter-like planets have been found, he said.

NASA’s Kepler mission is scheduled for launch in October 2007. It will allow astronomers to find planets, perhaps the size of Earth, orbiting other stars by looking for tiny dips in the brightness of a star when a planet crosses in front of it. Periodic brightness dips will signal the presence of a planet in orbit around the star, even if the planet is not directly visible.

Observing Guide to the 2004 Transit

The main event may be safely observed over the Internet with images from solar observatories and satellites. For Internet viewing options, a live webcast is available from Athens, Greece , made in partnership with the Exploratorium in San Francisco, Calif.

Compositional map from Magellan radar mission to Venus
Image Credit: NASA/Magellan

If you are able to receive NASA TV , you may watch the webcast live from Greece between 1:00 and 1:45 AM EDT on the initial passage of Venus onto the disk of the Sun, and between 7:00 and 7:30 AM EDT on the emergence of Venus from the disk of the Sun. From 1:45 to 7:00 AM there will be 5 minute updated solar images but no narrated program.

Each observatory listed below will make 12 observations of Venus against the sun from sunrise to third and fourth contact.

Participating Observatories

* South Florida Science Museum
* Burke-Gaffney Observatory
* Crosby Ramsey Memorial Observatory
* Dyer Observatory
* Fernbank Science Center
* Coca Cola Science Center
* Muncie Indiana Astronomy Club
* Observatorio Astronomico de Minas, Uruguay
* St. Paul’s School
* University of North Dakota, Delhi India
* University of Florida
* Howard University Astronomical Observatory