When the moon is exactly between the earth and the sun, we get eclipses. Full eclipses occur when the moon is near enough to the earth to block the entire disk of the sun, and the earth, moon, and sun are exactly in line (syzygy). Given that the moon and sun are rather small in the sky (half a degree) and that the moon's orbit precesses and is inclined by 5.5 degrees to the earth-sun (ecliptic) orbit, syzygy lineups are rare. Given that the moon is much smaller than the earth, its shadow passes over only a narrow strip, moving west to east.

If the moon is at apogee, the far extreme of its orbit, it does not block the entire sun, but leaves a ring of bright sunlight around the moon, an "annulus". The ring will be perfect near Crescent City, and the eclipse will block 94% of the sun's disk. Further north near Portland, the sun will peek over the top of the moon, and we will see a crescent, horns down, blocking 88% of the sun at Portland's latitude.

The eclipse becomes visible at sunrise over coastal China on Monday May 21, passes over Japan, the international dateline at local noon, south of Alaska on May 20 and then to Oregon near sunset. It would be way cool if someone would use something like Google Hangout to aggregate webcam feeds from around the Pacific Rim.

This may be the only chance in your lifetime to observe an annular eclipse without traveling halfway around the world, or into space. Eclipses are spooky and cool. Don't miss this one!