December 12
The Blue Shoulder, Again
Sun Position
The Sun is in Sagittarius, near 23.4 degrees south declination, approaching the solstice minimum. Northern Hemisphere nights are among the year's longest; southern days are among the longest.
Sky Highlight
The Geminid meteor shower is approaching its peak on December 13-14; tonight's activity is notably elevated, with rates building toward the shower's maximum. The radiant in Gemini is above the horizon all night for northern observers, and Gemini rises late evening in the Southern Hemisphere too.
Deep Sky Object
M35, an open cluster in Gemini about 2,800 light-years away, pairs well with the nearby NGC 2158 in the same binocular or wide-field telescope view; NGC 2158 is a much older, denser cluster five times farther away that appears as a faint blur beside M35's looser, brighter stars.
Featured Star
Bellatrix (γ Ori) is a B2III blue giant about 243 light-years away, appearing again this month as Orion's left shoulder. It is one of the hotter stars visible to the naked eye, with a surface temperature around 22,000 K, the blue-white light it emits reflects a star still relatively young and burning fierce.
Around This Date
- December 12, 1970Uhuru's launch opened a new observational window on the universe; within months it detected Cygnus X-1, the first strong black hole candidate, establishing X-ray binaries as a major class of astronomical objects.
- December 13, 1920Michelson's angular diameter measurement of Betelgeuse yielded a value that, combined with its known distance, confirmed it was a supergiant hundreds of times larger than the Sun, the first observational proof of the giant and supergiant categories predicted by stellar theory.
Both shoulders of Orion face the night (one red and cooling, one blue and hot) making the hunter a study in where stars end up.