On Tuesday evening, April 8, you can experience one of the most beautiful events the sky can deliver. Head outside no later than an hour or so after sunset (around 8:30 P.M. local daylight time) and look to the west. Your eyes should land immediately on the slender crescent Moon, oriented with its cusps standing nearly straight up from the horizon. Point your binoculars at the Moon to reveal a stunning sight: the bright Pleiades star cluster (M45) sparkling like a clutch of tiny diamonds accenting the primary jewel.
When you first gaze at the Moon, you may see only its brightly lit crescent. Look a little closer and you’ll see an ashen light filling out the “dark” part of the Moon’s disk. This light comes from sunlight reflecting off Earth’s dayside up to the Moon and back to us. Literally, the Moon is bathed in earthshine. If it appears particularly bright, you should see some dark lunar “seas” and bright craters. These will stand out more if you observe through a small telescope.
This conjunction will show you how fast the Moon moves against the starry background. Around 8:30 P.M. from central North America, Earth's satellite sits to the Pleiades’ right. An hour later, the crescent Moon covers some stars at the cluster’s upper right. And an hour after that, the Moon lies directly above the Pleiades.
Although the Moon and Pleiades appear close to one another in our sky, a lot of space lied between the two. The Moon is only a stone’s throw from Earth: about 225,000 miles, or 1.2 light-seconds. The Pleiades, on the other hand, lies more than 2 million billion miles from Earth, or some 440 light-years.
That distance has had its ups and downs. Until 1997, most astronomers thought the Pleiades were about 440 light-years away. That year, the European Space Agency (ESA) claimed it was some 10-percent closer, or a little under 400 light-years. This estimate came from ESA’s Hipparcos satellite, which was supposed to deliver the most accurate stellar distances yet.
Fortunately, the Hubble Space Telescope came to the rescue. Using the orbiting telescope’s ultra-precise Fine Guidance Sensors, Hubble scientists pinned down the Pleiades’ distance at 400 light-years. Early data analysis on Hipparcos observations contained a small but noticeable error when lots of bright stars contaminated the field. As you gaze at the Moon and the Pleiades this Tuesday, think about how far in the background the cluster’s stars really are.