April 24, 2009: If you're reading this at the end of the day on Sunday, April 26th—stop! You're supposed to be outside looking at the sunset.

On Sunday evening, the crescent Moon, Mercury and the Pleiades star cluster will gather for a three-way conjunction in the western sky. It's a must-see event.

The show begins before the sky fades to black. The Moon pops out of the twilight first, an exquisitely slender 5% crescent surrounded by cobalt blue. The horns of the crescent cradle a softly-glowing image of the full Moon. That is Earthshine—dark lunar terrain illuminated by sunlight reflected from Earth. If the show ended then and there, you'd be satisfied.

Right: A crescent Moon with Earthshine over Manassas, Virginia, on Dec. 30, 2008. Photo credit: Karen Schmeets. [Larger image]

But there's more.

Shortly after the Moon appears, Mercury materializes just below it. The innermost planet has emerged from the glare of the sun for its best apparition of the year in late April—perfect timing for a sunset encounter with the Moon. To the naked eye, Mercury looks like a pink 1st-magnitude star. The planet itself is not pink; it only looks that way because it has to shine through dusty lower layers of Earth’s atmosphere. A backyard telescope pointed at Mercury reveals a tiny fat crescent. The innermost planet has phases like the Moon!

Next, do nothing. Spend some quiet moments absorbing the view. As the twilight deepens, your eyes will dark-adapt and—voilà! There are the Pleiades.

Also known as the Seven Sisters, the Pleiades are a cluster of young stars about a hundred light years from Earth. They form a miniature Little Dipper located, on this particular evening, halfway between Mercury and the Moon. The brightest stars of the cluster are only 2nd magnitude, not terrifically bright. Nevertheless, the Pleiades are compelling in disproportion to their luminosity. Every ancient culture--Greek, Maya, Aztec, Aborigine, Maori and others—put the cluster in its myths and legends. On April 26th you may discover why, even if you cannot articulate your findings.

The Pleiades, Moon and Mercury are all visible to the naked eye even from light-polluted cities. Nevertheless, if you have binoculars, use them. A quick scan of the threesome reveals a rugged moonscape in startling detail, the rich pink hue of Mercury, and many more than seven sisters (there are hundreds of stars in the cluster).

Still reading? Stop! Twilight awaits.