The forecast for tomorrow shows the rain moving out by mid-day, giving stargazers a good shot at seeing Saturday evening's unusually fine lunar occultation. In case you missed today's story in The Sun's Science & Health section (call 410 539-1280 to subscribe), here's the scoop:

Just after sunset tomorrow, the slim crescent moon, on its west-to-east orbit around the Earth, will pass in front of a beautiful star cluster called The Pleiades (or the Seven Sisters).  As it plows through the cluster, as seen from our perspective, the moon's unlit dark edge will eclipse the bright stars one by one, causing them to wink out abruptly. An hour or so after they vanish, each star will reappear on the sunlit side of the moon, although the glare will make that part of the event hard to see.

These occultations of the Pleiades occur in clusters separated by 18 years. Some are better than others, and this is one will be the best for our part of the world since 1969, astronomers say. Scientists have long used lunar occultations to map the mountains and valleys on the moon's edge as viewed from Earth, and to track its orbit. Some still do. (Click that link, or see story.)

It's best observed with binoculars, or perhaps a small telescope. And the darker the sky the better. I put my 10x50 binocs on the Pleiades two nights ago, at about 7:30 p.m., in a test run. I was in Cockeysville, surrounded by porch lights, street lights and the urban glare to my south. But skies were very clear, and while it was hard to find the cluster with the naked eye, they popped out nicely in the binoculars. Of course, tomorrow evening they'll be easy to find because the moon will be in the middle of them.

What's interesting about these things is how vivid the motion of the moon in its orbit becomes when you can see it passing in front of "fixed" background stars. It travels a distance equal to its own diameter in about an hour.

And the fact that the stars wink out instantly, instead of fading out, gave early astronomers their best clue that the moon had no atmosphere. If it did, these stars would fade to black the way the sun does here as it drops below the horizon. It also tells us how far away the stars really are. If they were close, they would have enough diameter to cause their light to be eclipsed more gradually. The fact that they vanish all at once tells us their light comes to us from a distant pinpoint in the sky.

Just find a spot with a good view toward the west (where the sun went down).  Best time to look will be from dusk - whenever the stars become visible - until 9 or even later if you have an big appetite for this sort of thing.