A rare positioning of planets Venus (L) and Jupiter (R) and the crescent moon of the Earth provides a "smiley" effect that captivated Asia on Monday night.
People in most of Hawaii, Alaska, Australia, the western Pacific, and eastern Asia will be able to see the planet Venus cross in front of the Sun, which will last 6 hours 40 minutes on 5th June.
Most people in North America will be able to observe the first several hours before the Sun sets. Most people in Europe will see the final stages of the transit after sunrise.
This will be only the eighth such transit of Venus since the invention of the telescope, and the last one until December 10-11, 2117.
“This is it, folks,” Robert Naeye, Editor in Chief of Sky and Telescope magazine, said.
“Unless modern medicine comes up with a miracle to extend human lifespans, this transit of Venus will be your final opportunity to watch our sister planet cross the Sun’s fiery disk as seen from Earth,” he said.
In the U.S., the transit will begin at approximately 6:04 p.m. EDT, 5:04 CDT, 4:05 MDT, and 3:06 PDT. The transit commences at the moment of first contact: when Venus’s silhouette touches the outer edge of the Sun.
For the next 18 minutes or so, Venus will slowly cross the solar limb, until second contact, when Venus is entirely in front of the Sun.
“This 18-minute period of ingress will be the most dramatic time to view the transit, especially in a telescope,” Sky and Telescope Associate Editor Tony Flanders, said.
During ingress, observers using telescopes might be able to see a faint ring or arc of brightness (the “aureole”) around Venus’s edge due to sunlight refracting through the planet’s upper atmosphere.
Around second contact, telescopic observers should look for a thin bridge of darkness connecting Venus to the Sun. This so-called “black-drop effect” is due to telescope optics, the distorting effects of Earth’s atmosphere, and the fact that the Sun’s brightness decreases near its outer edge.
For the next several hours, Venus will slowly creep across the solar disk, and the view will not change much from one minute to the next. But telescopic observers should pay careful attention if Venus comes near a sunspot.
“Since we see the night hemisphere of Venus, the planet will be pitch black. Surrounding sunspots should appear brighter, and this should provide an interesting contrast effect,” Naeye said.
For observers in parts of the world that can see the end of the transit, the egress will essentially be a mirror image of ingress. Just before third contact, telescopic observers should look for the black-drop effect. And observers should look for the aureole during the entire 18 minutes of egress.
“During the transit, Venus will present a disk about one arcminute across. That’s large enough that people with normal vision should be able to see the silhouette without binoculars or a telescope, just using a safe solar filter,” Sky and Telescope Senior Editor Alan MacRobert said.
“However, optical aid will greatly improve the view, allowing viewers to see exquisite detail,” he said.
If Venus and Earth orbited the Sun in the same plane, we’d enjoy a transit of Venus every 584 days. But Venus’s orbital plane is tipped 3.4 degrees relative to Earth’s orbital plane.
The planes intersect at two points called nodes. For a transit to occur, Venus has to be at inferior conjunction at the same time it’s at a node.
One of these nodes occurs in early June and the other in early December, meaning these are the only times of year when transits of Venus can occur. In early June, Venus appears to be diving “downward” (or south), so astronomers call this the descending node.
In early December, Venus is moving “upward” (or north) in its orbit, so this is an ascending node.
Since the 1600s and lasting until the early 3000s, transits of Venus occur in a bizarre 243-year cycle in which transits come in pairs.
The two transits in a pair occur at the same node and almost exactly eight years apart. The upcoming transit is the second of a pair; the last one was in June 2004.
But more than a century will elapse until the next pair of transits. It will take 105.5 years between the June 2012 transit of Venus and the December 2117 transit (which occurs at Venus’s ascending node).
That transit will be followed by another one on December 8, 2125. But because Earth’s orbit is slightly elongated, it will take 121.5 years for the next pair of descending-node transits in June 2247 and 2255.