«The Venus transit really brings home to us, in a unique way, how ordinary our sun is. It's just another star,» said scientist and author Mark Anderson.
«There's something very humbling out of this experience. We are another planet in orbit around another star in another galaxy in another corner of the universe. It really brings it home to us,» Anderson said.
Tuesday's transit, which bookends a 2004-2012 pair, begins at 6.09 p.m. EDT (2209 GMT) and lasts for six hours and 40 minutes. Times can vary by seven minutes depending on the location of the observer.
Skywatchers on seven continents, including Antarctica, will be able to see all or part of the Venus transit, which should only be observed with telescopes outfitted with solar filters to protect the eyes.
In Greece, the phenomenon is expected to be visible at 1.09 a.m. early Wednesday morning.
The Internet will be a hub of activity, with live video and pictures from an armada of space- and ground-based observatories.
Even astronauts aboard the International Space Station are joining in the event.
«I've been planning this for a while,» space station flight engineer Don Pettit said in a NASA interview. «I knew the transit of Venus would occur during my rotation, so I brought a solar filter with me.»
It's not all about pretty pictures. Several science experiments are planned, including studies that should help in the search for habitable planets beyond Earth.