Astro Bob: Watch the waxing moon sideswipe Dschubba
On Sunday evening, June 12, the gibbous moon will almost touch the star Dschubba in the Scorpion. For some observers, it will disappear altogether.
DULUTH — Your first question is probably "how do I pronounce that star's name?" It's JOOB-ah, and means the "forehead of the scorpion" from the Arabic jabhat. It's also known as Delta Scorpii and shines slightly brighter than magnitude 2.0, on par with the brightest of the Big Dipper stars.
Normally, it's easy to see even in light-polluted areas, but on Sunday night you'll need help from binoculars (any size will do) because the bright, waxing gibbous moon will practically sit on top of it. For observers in upper Midwest, the moon and Dschubba will be closest during early evening twilight. Start looking about 30-45 minutes after sunset. Just aim at the moon and look for a tiny sparkle of light below its bottom edge.
The farther north and east you live in the U.S., the closer star and moon will appear. From my hometown of Duluth, Minn. Delta shines just 30 arc-seconds — 1/60 of the moon's apparent diameter — to its lower right when closest at 9:22 p.m. CDT. That's only about 20 minutes after sunset, so we may have to wait a little while longer for the sky to darken. Farther south and west across the country their separation widens.
In a broad zone that includes Grand Marais, Minnesota, much of Michigan and all the northeastern states and northeastern Canada, the moon will cover or "occult" the star. Here's a list of cities with times of the star's disappearance and reappearance at the opposite side of the moon.
You'll need a small telescope to see this hide-and-seek show best. The moon travels along its orbit at around 2,500 miles an hour (1.1 km/sec) and hides and then "releases" many faint stars along its path every month. The bright ones like Delta get our attention.
Since all stars are basically points of light because of their enormous distances, and the moon lacks an appreciable atmosphere, they blink out in a sudden, split second as soon as the moon's edge "touches" them. It's really exciting to see. Were there an atmosphere, the star would slowly fade and then disappear.
Dschubba is actually a very close double star about 400 light-years from Earth. The main star or "primary" is nearly seven times the sun's diameter sun and 13 times as massive. Since at least the year 2000, the primary has been flinging hot, luminous gas into space, causing it to brighten. For a time it nearly rivaled first magnitude Antares, the brightest star in Scorpius.
That's why I always keep an observant eye on Dschubba because you never know what its next move might be. Tonight, however, it's the moon's turn to make a move. I hope you see it!
"Astro" Bob King is a freelance writer for the Duluth News Tribune.