This June 8, 2004 photo shows the transit of Venus, which occurs when the planet Venus passes between the Earth and the Sun, is pictured in Hong Kong. (VINCENT YU/AP)

Look to the west Tuesday evening, June 5, Venus begins to cross the sun at 6:04 p.m. EDT Tuesday evening, as a notch in the sun. By 6:22 p.m., from our perspective, Venus becomes a black dot moving across the solar disc.

Link: As Venus crosses the sun, a rare treat for skywatchers

On this transit, it’s a 6-hour, 40-minute trek for our neighboring, interior planet – and because the sun sets – we only see a few hours of this cosmic memory.

Venus transits occur in pairs, eight years apart, alternating between 105.5 years and 121.5 years apart. Tuesday’s transit is paired with the crossing that last occured in June, 2004.

MUNICH, Germany: Picture taken by multiple exposure in Munich shows five different phases of the planet Venus crawling across the sun, 08 June 2004, during the "transit" of Venus last seen in 1882. (JOERG KOCH/AFP/GETTY IMAGES)

* 1631/1639 (each in December)

* 1761/1769 (June);

* 1874/1882 (December);

* none in the 20th century;

* 2004/2012 (June).

Looking ahead, the next pair won’t occur until Dec. 11, 2117 and Dec. 8, 2125.

For many cities in the eastern U.S., including Washington, the transit starts at 6:04 p.m. on Tuesday. The central and western time zones can see more of the transit, while Hawaii can see the whole event.

Link: Live webcasts from NASA

Observers on seven continents, and even a small part of Antarctica, will be able to see at least a portion of it.

Link: Start times in major U.S. cities

Astronomers once thought that by studying transits of Venus, they could ascertain the scale of the solar system, and note the sun’s distance from Earth. The method was less than perfect.

Video: How a common paper plate can illustrate the mechanics of the Transit of Venus

But for Tuesday’s transit, the Hubble Space Telescope will face Earth’s moon to note any change or flux of sunlight during the transit. Scientists will use the transit to calibrate the detection of exoplanets – planets around the rest of the universe – in hope of finding one that sustains life.

Video from Science@NASA explaining the transit event

A man watches the Venus transit through binoculars covered with a special protective foil in Munich on Tuesday, June 8, 2004. (JAN PITMAN/ASSOCIATED PRESS)

For this event, please be aware that looking directly at the sun, or through binoculars or a telescope aimed at the sun, will lead to blindness. Organized viewing events are best for seeing this through solar-filtered telescopes. Viewing the fully bright sun – through bare optics – will burn your eyes instantly!

Link:’s website on eye safety | NASA Eye Safety website

A little piece of history

Like a double helix, the transits of Venus and world history become intertwined.

Bishop explains that Johannes Kepler predicted the 1631 transit, but had died the year before. Edmund Halley (who died two decades before the 1761 transit) advocated for global expeditions to observe and analyze the transit. Dozens of expeditions formed.

For the 1761 transit, the The Royal Society of London – using the grants from King George II – dispatched Charles Mason and Jeremiah Dixon to modern day Indonesia to measure the transit. Before the men departed the English Channel, a French frigate engaged the English ship in battle, where eleven sailors were lost. The ship limped back to port and sought repairs at Plymouth, according to Simon Newcomb’s 1903 autobiography. (Newcomb was once director of the Nautical Almanac Office, U.S. Naval Observatory here.)

Mason and Dixon promptly resigned their commission, noting the danger. The Royal Society – who employed Mason and Dixon – strongly disagreed. In a threat to Mason and Dixon by The Royal Society, the employers intimated that resigning the commission, “would bring an indelible scandal upon their character, and probably end in their utter ruin,” Newcomb wrote.

With their frigate repaired, Mason and Dixon set sail, but only so far as Cape of Good Hope. There, they made transit observations … and their good reputations remained intact. Two years after the 1761 transit, the men would earn fame here (1763-67) by settling a border dispute between the colonies of Pennsylvania and Maryland.

Meanwhile, for the 1761 transit, the Royal Society of England also dispatched Rev. Nevil Maskelyne – who would later become the Astronomer Royal of England for a half-century – to the island of St. Helena, an isolated place in the South Atlantic, off the African coast. Clouds prevented Maskelyne from observing the transit, but posterity notes an itemized listing of “necessaries of an astronomer of the time,” reports Newcomb. The cost of Maskelyne’s liquor bill was almost three times that of his board at St. Helena.

For the next transit pair, 1874 and 1882, the United States had been through Civil War and stood at the centennial’s doorstep. With Europe’s transit expeditions being planned, the Naval Observatory’s Newcomb and other scientists, sought to plan global expeditions of their own.

It would be the first era that transits were photographed, but none of the 1874 photographic plates surivived. But, the expeditions needed funding. James A. Garfield, chairman of the House committee on appropriations, noted for his “rigidly economical [stance] in grants of money” was keenly aware of the transit’s importance. (Yes, he’s that Garfield, a member of the House of Representatives before he was president.)

In lieu of formal hearings, Garfield invited astronomer Newcomb and others to dinner at his home, to discuss the transit ventures. Newcomb noted, “As may readily be supposed, the transit of Venus did not occupy much time at the table.” Nevertheless, Garfield led Congress to grant $50,000 in 1872 for telescopes and then in 1874, $100,000 for initial expeditions.

(Eleven Naval Observatory photographic plates survived the 1882 transit: the images )

NASA Space Shuttle tributes: A young English naval lieutenant named James Cook led a 1769 transit expedition to Tahiti on the Endeavour, notes Bishop. (Yes, he’s that James Cook and he rose to captain.) Some of the best observations in the 1874 transit, says Newcomb, came from the astronomers aboard the British ship Challenger.

Making history in modern times: The Naval Observatory will attempt to observe the transit with their historic 5-inch Alvan Clark Transit of Venus telescope. Geoff Chester, public affairs officer at the Naval Observatory, explains that this instrument, Number 856, successfully observed the transit of 1874 from Vladivostok, Siberia and the 1882 transit from San Antonio, Texas. (The telescope was loaned to the Panama Canal Zone government in 1928, where it was used for almost five decades.)

Chester says: “It was employed to successfully observe the 2004 transit following restoration by the observatory’s instrument shop. If successful this year, we will have the only instrument known to have observed four of the seven transits that humans have recorded.”

Sources: Roy Bishop, Observer’s Handbook 2012, Royal Astronomical Society; “The Reminiscences of an Astronomer,” Simon Newcomb, 1903, Riverside Press; USNO press release, Geoff Chester; Science@NASA; Transit of Venus, NASA web pages, by Fred Espenak.

Additional links:

NASA Goddard Space Flight Center 2012 Transit of Venus website

NASA Solar Dynamics Laboratory Venus Transit website