Venus Due for a Date With the Sun in an Eclipse Last Seen 122 Years Ago

In a rare planetary alignment that has not occurred in 122 years, the planet Venus will appear to move across the face of the sun on Tuesday -- an astronomical event known as the transit of Venus.

Planet-watching parties are being organized across Europe, tourists are flocking to prime viewing sites such as Egypt and Sicily and amateur astronomers throughout the world are preparing to watch the event from their own backyards.

Major telescopes on Earth and in orbit will document the planet’s six-hour passage in front of the sun. A similar transit that will be visible from most of the Southern Hemisphere will occur in 2012, but for North Americans, the next visible transit will take place in 2117.

The event will not yield any new or unexpected scientific information and will not expand our understanding of the universe, said Roger W. Sinnott of Sky & Telescope magazine. Nonetheless, huge numbers of people want to see something that no one alive today has seen.

The transit provides a powerful link to our scientific past, a time when astronomers and adventurers working with crude tools traveled to the far reaches of Earth to view the transits in an effort to solve one of the earliest and most important astronomical puzzles -- the distance from Earth to the sun.

This year’s transit will be visible only east of the Mississippi River, and then only the tail end of it. People in Europe, Asia and most of Africa will be able to see it in its entirety.

The event is like an eclipse of the sun by the moon: In each case, a celestial body comes directly between the sun and the Earth, blocking the passage of light.

Although Venus’ diameter is 3 1/2 times that of the moon, it is so much farther away from us that it blocks only about 3% of the sun’s surface. That’s enough to be seen without a telescope. As with an eclipse, looking at it directly would damage the eyes.

Thousands of transits passed unobserved until the 17th century German astronomer Johannes Kepler developed tables of planetary motion that predicted a transit of Venus on Dec. 6, 1631. Kepler died in 1630, however, and did not see the transit.

The transit of Venus that year was visible only in the Southern Hemisphere and there is no record of anyone having seen it.

English astronomer Jeremiah Horrocks refined Kepler’s tables and realized that another transit of Venus would occur Dec. 4, 1639. He alerted fellow astronomer William Crabtree and they became the only people known to have observed a transit until the next one occurred in 1761.

The delay between transits arises because the orbits of Earth and Venus do not lie in the same plane, but are offset by 3.4 degrees. On most occasions when the two planets are on the same side of the sun, Venus is above or below the line between the sun and Earth. Only four times every 243 years are they properly aligned, and that occurs in a predictable pattern.

This month’s transit can be considered the first of the cycle. In eight years, there will be a second transit, also in June, this one observable primarily in the Southern Hemisphere. The eight-year gap occurs because 13 Venus years almost equal eight Earth years, so the two will be in nearly the same positions relative to the sun.

It will then be 105.5 years until another transit occurs, on Dec. 11, 2117. Another transit will occur eight years later, again in December, and then it will be 121.5 years until the cycle begins again.

A century after solitary sightings by Horrocks and Crabtree, the next two transits were seen around the world. Several nations mounted scientific expeditions to observe them because British astronomer Edmond Halley had shown that observations of the transit from two widely separated points on the Earth’s surface would allow calculation of the distance from Earth to the sun, a quantity known as one astronomical unit.

From widely separated points on Earth, Venus would appear to take slightly different elapsed times to cross the sun. Accurate calculation of that time difference would allow estimation of the distance from Earth to the sun. With that value in hand, it would be simple to calculate the distance to other objects in the solar system.

Among those observing the 1769 transit was British explorer Capt. James Cook, whose expeditions to the South Pacific were planned, largely, to monitor the event from an area in Tahiti still known as Point Venus. Those observations led to the discovery that Venus has an atmosphere.

Surveyors Charles Mason and Jeremiah Dixon were commissioned to view the transits by the Royal Society of London. Their trip to Sumatra for the 1761 transit was delayed when their ship was badly damaged by a French frigate during the Seven Years’ War. They set out again and their measurements of the transit, observed from South Africa, were sufficiently impressive that they were commissioned to survey the border between Maryland and Pennsylvania, establishing the Mason-Dixon Line.

But Cook and others reported a problem with their measurements. They could not precisely determine the exact time that Venus appeared to contact the sun or completed its crossing because, as the two objects neared each other, the black disc of Venus appeared to stretch out to meet the sun, a phenomenon known as the “black-drop effect.” Researchers are still not sure what causes the phenomenon, although slight faults in telescope lenses are part of the problem.

Because of the uncertainties, astronomers calculated that the distance to the sun was somewhere between 93 million and 97 million miles.

Because of that wide range, a greater effort was expended for the 1874 transit. Congress appropriated the equivalent of more than $2 million to send out eight missions, three to the Northern Hemisphere and five to the southern. Russia dispatched 26 missions, Britain 12, France and Germany six each, Italy three and Holland one.

“In the 19th century, it was really analogous to the space race,” said NASA historian Steven J. Dick.

A similar effort was undertaken for the 1882 transit, which so captured the public imagination that John Philip Sousa wrote a new tune, the “Transit of Venus March.”

Using data from the 19th century transits, astronomer William Harkness of the U.S. Naval Observatory calculated the average Earth-sun distance to be 92,797,000 miles plus or minus 59,700 miles. More recent data based on radar measurements and observations from spacecraft set the distance at 92,955,589 miles.