If you're still giddy about the annular solar eclipse we had a few weeks ago, you'll be happy to note that an even rarer astronomical event is coming up Tuesday, and it's a sight that no one alive today will ever see again (unless they solve that whole aging problem, or someone will make it to about 120 years old.)
Tuesday evening, those across North America will be treated to the "Transit of Venus" -- or when the planet Venus will cross directly in front of the sun.
Unlike the moon, whose size relative to the sun can cover up the whole thing, Venus in relative size is about 1/32nd the size of the sun. So it'll be equivalent to a small dot moving across the sun. So small that trying that pinhole viewer that you might have put together for the annular eclipse might not have enough resolution to pick up Venus moving across the sun. (And yes, same eclipse safety "Don't look at the sun!" viewing rules apply to looking for Venus.)
According to NASA, the transits of Venus are quite rare, coming in pairs 8 years apart, but then having to wait over 100 years for the next couplet of sightings.
The last sightings were on June 8, 2004, and Tuesday's will be the last time it'll happen again until December 2117.
(As ScienceDaily points out, Venus actually crosses between the sun and Earth about every 584 days, but due to the angled orbits of Earth and Venus, most of those transitions have Venus pass above or below the sun, relative to us.)
Unlike a solar eclipse, whose peak is done in a few minutes and the entire show is maybe a couple hours, the Venus transit will go for almost 7 hours.
For Portland, the show starts at 3:06 p.m. PDT when the dark spot of Venus will appear at the top center of the sun. The planet will then take a curved path to the southeast until it exits the sun on its right side -- technically at 9:48 p.m. However, sunset is in Portland is at 8:56 p.m. so we won't get the entire show, but most of it.
Even with proper viewing glasses, it'll be hard to notice with the naked eye.