The Moon Covers the Pleiades

The slender crescent Moon plays a game of astronomical tag with a cluster of bright young stars, the Pleiades, on the evening of March, 20. In fact, the Moon appears to “catch” the Pleiades, hiding the stars from our view, before it sets.

Of course, the Moon only appears close to the Pleiades. The cluster of several hundred stars is really about 400 light-years from Earth, some 10 billion times more distant than our Moon. Still, the view should be impressive, particularly through a good pair of binoculars.

The Pleiades is one of the loveliest naked-eye objects in the night sky. Six tightly grouped stars in the shape of a small dipper are normally visible to the naked eye and dozens more can be seen with binoculars. The brightest star in the Pleiades is Alcyone, a blue-white giant several hundred times more luminous than our Sun.

Look for the Moon and the Pleiades fairly high in the west as twilight deepens on March 20. Depending on what part of North America you live in, the Moon already may appear in front of the cluster. As the evening progresses the Moon keeps moving relative to the cluster and many of the cluster’s stars will be occulted, or hidden from view, by the Moon. Because the Moon has no atmosphere and the stars appear as pinpoints of light, the individual Pleiades disappear instantly when the dark limb of the Moon passes in front of them.

The Pleiades cluster, named for the mythological Seven Sisters of the Pleiades (the daughters of Atlas), contains several hundred stars. The Pleiades became known as the Sailors’ Stars during classical times when the cluster ascended into the eastern sky at the beginning of the Mediterranean Sea’s calm-weather season.

moon pleiades

The Moon Passes the Pleiades Star Cluster. Image credit: The University of Manchester / Derekscope, with the Moon from a 2009 conjunction.