With clear skies, the transit of Venus should be visible from Chicago on Tuesday, starting just after 5 p.m.
"We see it as a small dot gliding slowly across the face of the sun later on today. This is going to pass between the earth and the sun, peaking at 5:04 p.m. our time," Tracy Butler said. But astrologists warn those who want to see the planet move across the sun should not look directly into it with the naked eye.
To view the transit of Venus safely, the Chicago's Adler Planetarium suggests three options for safely watching the transit:
The Adler Planetarium will host a viewing event for the transit of Venus, which starts at 4 p.m. CDT and runs until 9 p.m. The actual transit will be viewable between 5:04 p.m. and about 7:30 p.m., when the sun sinks below the city's skyline. It is open to the public and is free for Illinois residents. Find out about the Adler event at http://www.adlerplanetarium.org/experience/events/transitofvenus
Next transit of Venus viewable in 2117
It's your last chance to catch one of the rarest cosmic spectacles -- Venus slowly crossing the face of the sun. Weather permitting, the transit of Venus will be visible from much of Earth -- Tuesday from the Western Hemisphere and Wednesday from the Eastern Hemisphere. This sight won't come again until 105 years from now -- in 2117.
The nearly 7-hour show can be seen in its entirety from the western Pacific, eastern Asia and eastern Australia. Other parts of the globe will catch portions of the transit.
Here's a sampling of local viewing times: 12:10 p.m. Honolulu, 3:06 p.m. Los Angeles, 5:06 p.m. Mexico City, 6:04 p.m. New York -- all on Tuesday -- and 5:37 a.m. London, 6:10 a.m. Beijing, 6:38 a.m. Cairo, 7:10 a.m. Tokyo, 8:16 a.m. Sydney, 10:15 a.m. Auckland on Wednesday.
As in a solar eclipse, do not look directly at the sun. There are ways to watch the Venus transit without blinding yourself.
If you still have your pair of eclipse glasses from the May 20 "ring of fire" solar eclipse, now is a good time to reuse it. You can also find the special viewing glasses at your local museum -- if they're not already sold out. Another option is to buy welder's glasses from a home improvement store, but make sure it's number 14 or darker.
NASA plans a live webcast from Mauna Kea, Hawaii. Slooh.com and the Exploratorium in San Francisco are among others that will the sky show broadcast online.
The transit of Venus -- a planetary spectacle that won't occur again until 2117 -- won't be enough to significantly block the sun's light, but it will give Earth's closest star a moving beauty mark. Venus is currently Earth's closest neighboring planet, although Mars occasionally has that distinction.
"In terms of rarity, to be here at a time when it's happening, you almost have to look at it," said Geoff Chester of the U.S. Naval Observatory, who saw the last transit in 2004. "It ain't going to happen again in my lifetime."
The transit is happening during a 6-hour, 40-minute span starting just after 6 p.m. EDT in the United States. What you can see and for how long depends on what the sun's doing in your region during that exact window, and the weather. Those in most areas of North and Central America will see the start of the transit until the sun sets, while those in Western Asia, the eastern half of Africa and most of Europe will catch the transit's end once the sun comes up.
Hawaii, Alaska, eastern Australian and eastern Asia including Japan, North and South Korea and eastern China will get the whole show since the entire transit will happen during daylight in those regions.
Don't stare directly at the sun without eclipse glasses, a properly filtered telescope or a strong welding visor. Permanent eye damage could result.
Astronomers across the globe are using the rarity of the moment to spark scientific curiosity among the public, and to document the transit with the latest technology available.
Sul Ah Chim, a researcher at the Korea Astronomy and Space Science Institute in the central South Korean city of Daejon, said he hoped the event will help people see life from a larger perspective and "not get caught up in their small, everyday problems."
"Most people consider 105 years to be a very long time," Sul said. "But when you think about it from the context of the universe, 105 years is a very short period of time, and the Earth is only a small, pale blue spot."
In Hawaii, university astronomers planned viewings at Waikiki Beach, Pearl Harbor and Ko Olina. At Waikiki, officials planned to show webcasts of the transit as seen from telescopes from volcanoes Mauna Kea on the Big Island and Haleakala on Maui.
NASA planned a watch party at its Goddard Visitor Center in Maryland with solar telescopes, images from its Solar Dynamics Observatory Mission and expert commentary and presentations. The observatory will produce "Hubble-quality" images, according to NASA's website.
Groups of scientists from the University of North Texas planned to watch from points in Alaska and Hawaii to recreate the 1769 expedition of British Capt. James Cook, who used the transit to calculate Earth's distance from the sun. The scientists will use atomic clocks, GPS and high-end telescopes to take measurements, and will use high-end video gear to capture time-lapse video.
Experts from Hong Kong's Space Museum and local astronomical groups were organizing a viewing Wednesday outside the museum's building on the Kowloon waterfront overlooking the southern Chinese city's famed Victoria Harbor. The transit begins there around 6 a.m. local time.
The transit also coincides with a national holiday in South Korea. Choi Hyungbin, head of the Daejon Observatory, said he was expecting more visitors than might otherwise come out to watch the transit. Local media urged curious residents to visit observatories for the event, reiterating the danger of looking directly at the sun.
This will be the seventh transit visible since German astronomer Johannes Kepler first predicted the phenomenon in the 17th century. Because of the shape and speed of Venus' orbit around the sun and its relationship to Earth's annual trip, transits occur in pairs separated by more than a century.
It's nowhere near as dramatic and awe-inspiring as a total solar eclipse, which sweeps a shadow across the Earth, but there will be six more of those this decade.
The Associated Press contributed to this report. All rights reserved.