May 18, 2004. Copyright, 2004, Graphic News. All rights reserved

Goddess of love flirts with the Sun

LONDON, May 18, Graphic News: Given a cloudless sky on Tuesday, June 8, observers throughout Europe, the Middle East, and most of Asia and Africa will be able to witness one of the rarest events seen from Earth -- the tiny black silhouette of the planet Venus moving across the disc of the Sun.

So-called ÒtransitsÓ occur when the complex orbits of the Earth and Venus around the Sun result in all three bodies being lined up briefly in space, causing Venus to pass directly between the Earth and the Sun.

Observers lucky enough to view the entire transit will see our nearest planetary neighbour crossing the southern hemisphere of the Sun from left to right. The planet, entering the disc of the Sun at the 8 oÕclock position, will take six hours to cross the bright face before exiting at the 5 oÕclock position.

Transits are so rare that only five have been observed since the invention of the telescope in 1608, and they have led to some of the most important discoveries in astronomy. The last such occurrence, in 1882, inspired an international effort to use the event to answer one of the most pressing scientific questions of the day: what is the exact distance between the Sun and Earth?

In 1619, German astronomer Johannes Kepler published his Third Law of Planetary Motion, which enabled the scale of the solar system to be calculated from just two values: the time it took a planet to orbit the Sun and the average distance between the Earth and the Sun -- a value known as the Òastronomical unitÓ.

In the 1960s scientists bounced radar off the Sun and Venus to establish an average Sun-to-Earth distance of 92,955,859 miles (149,597,954km).

Kepler suggested that observers placed at widely different points on Earth could calculate the distance to Venus from the different angles from which they viewed the transit. From this value they could indirectly calculate the distance between the Earth and Sun, and hence the exact distances between the planets.

The transits occur in a predictable pattern of two occurring in an eight-year period, followed by one 105 years later and another eight years after that. After an additional 122 years, the pattern repeats.

Although Kepler had forecast VenusÕ crossing of the Sun in 1631, the first observation of a transit occured in 1639. Since then transits have occurred in 1761, 1769, 1874 and 1882. If you miss JuneÕs transit, there will be another opportunity in 2012. After that, more than a century will pass before the next transits, in 2117 and 2125.

In 1769, the British explorer Captain James Cook took his ship, the Endeavour, on its first voyage to the South Pacific to observe the transit from Tahiti.

The 1874 and 1882 transits became the 19th century equivalent of the space race, with rival expeditions from the United States, Britain, Germany, France, Italy, the Netherlands and Russia.

The June 8 transit will allow astronomers to test new techniques and instruments aboard three Sun-watching satellites that can be used to detect planets in other solar systems.

Of the 120-plus extra-solar planets that have been discovered to date, most were revealed because their massive gravity affected the motion of their stars.

The new techniques will allow the detection of far planets by measuring the fluctuations that they cause in light from the stars they circle. In 2007, NASA plans to launch the Kepler spacecraft to monitor Sun-like stars in hope of detecting Earth-size planets through small decreases in star brightness.

Note: As with a solar eclipse, scientists warn people not to look directly at the Sun during the transit. This can result in permanent blindness. Eclipse viewing glasses should be used or the image of the Sun can be projected on to a screen using a small telescope.
/ENDS

Sources: Royal Astronomical Society, NASA