The moon hides Spica

¡SkyCaramba! Weekly astronomy blog for the week ending March 30, 2013

On March 28, the moon will hide the bright star Spica, if you are watching from southeast Asia, Malaysia, Indonesia, or some islands northeast of Australia. This is just one in a series of lunar occultations of Spica that started last year and will continue until later this year. Some of those occultations were or will be daytime events. Other than knowing those ones are happening, there isn’t much to get excited about. But the one happening as March ends is almost entirely a nighttime event.

In some locations, the local date may actually be March 29. To minimize confusion, let’s refer to UTC. In UTC, the moon will be right on top of Spica at 14:48 on the 28th as seen from northeast of Australia in the Coral Sea. The event will be earlier west of there and later east of there. Wherever you are, the event will last a little less than an hour and a half.

Watching the moon pass right in front of a star or planet can be exciting to astronomers of any experience level. For beginners, it gives a sense of how fast the moon is moving. Somewhat more experienced astronomers can enjoy the challenge of photographing the object disappearing behind the moon or grazing the lunar limb. And very experienced scientists can monitor how a star or planet dims as it disappears and how it brightens as it reappears to find clues about its atmosphere or other objects orbiting it. They may also learn more about the moon’s topography. It’s hard to believe, but as well mapped as the moon is, there’s still more to learn about its surface.

Some double stars have been discovered by astronomers monitoring lunar occultations. When a very bright star vanished, a very dim star very close to it showed up temporarily not obscured by the brighter star’s glare.

The moon isn’t the only celestial object that can hide other objects. Planets and asteroids sometimes pass in front of other stars. And rarely, a planet passes in front of another planet. These occultations can also spur discoveries. When a star dimmed and brightened several times before Uranus passed in front of it in 1977, astronomers knew Uranus had rings. The rings had never been seen through telescopes. Pluto has passed in front of stars allowing astronomers to study its atmosphere.

If you don’t happen to be in the zone where the moon occults Spica on the 28th, maybe you will be in one of the places where an occultation is visible later this year. And if you aren’t even that well situated, take comfort in the fact that you’re not alone. You can still enjoy the view of the moon passing very close to a bright star and observe how fast the moon moves by it.