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In the 18th century, astronomers used the solar transit of planet Venus to calculate the distance from the Earth to the sun. After the 1882 Venus transit, John Philip Sousa scored a march in its honor. Since it was first predicted in the 17th century, we've only had a chance to view the phenomenon six times.
And Tuesday's our day. Starting just after 6 p.m. ET and lasting about six hours, the planet will glide slowly across the face of the sun. And with the right equipment, skywatchers in the continental United States can view the phenomenon occurring. Venus will appear as a small black dot silhouetted against the sun before it dips below the horizon.
Edward Murphy, associate professor of astronomy at the University of Virginia, explains more about the transit in the video above. (Note: The hula hoop represents the Earth's orbit; the tilted platter represents the 225-day orbit of Venus.)
To prevent eye damage, you'll need the right equipment - eclipse glasses, a properly filtered telescope or a strong welding visor - to safely view the transit. You can view our previous report on safe viewing here.
This extraordinarily rare event occurs in pairs. Each eight-year pair occurs every 105.5 or 121.5 years. This represents the second member of the pair - the last transit occurred in June 2004. And if you miss this one, it won't occur again until December 2117.
The farther west you live, the more you'll be able to see. For most of the country, the sun will set before Venus has completed the journey. Only in Hawaii and Alaska will the full transit be visible.
To see the transit timetable from your location, click here
For more, visit transitofvenus.org or NASA's Transit of Venus page, where there will be a livestream of the event.
