The moon’s been getting brighter and brighter with each passing night this week. Tonight we’re just a day away from the Full Wolf Moon, and if you look up around 9 o’clock or later, the moon will be surrounded by the Winter Hexagon, an enormous six-sided figure formed by the season’s brightest stars. The figure reaches from Sirius, low in the southeast, all the way up to Capella near the top of the sky.
People often remark that the stars of winter are so much brighter than those in other seasons and assume it’s because the sky is exceptionally clear. The real reason the winter sky seems sparklier is simple – there are more bright stars concentrated there than in any other region of the sky for observers at mid-northern latitudes. Nine of the 25 brightest stars are found in and around the constellation Orion. There are an additional nine 2nd magnitude stars if you add in Orion’s Belt, the two remaining stars in his outline and four in the nearby constellation of Canis Major the Greater Dog. That’s 18 sparklers in all. How can your attention not be drawn to the southern sky in winter? Toss in the moon and your retinas runneth over with radiance.
Sunday night’s full moon is named after the howling wolf packs heard on cold winter nights. Your calendars may show the full moon date as the 9th, but since full phase happens at 1:30 a.m. CST Monday, the moon will appear closer to perfectly round tomorrow night compared to Monday night.
You’ll recall that last month’s full moon was eclipsed by Earth’s shadow for much of the central and western U.S., Canada, Alaska and points west. While some of us were waking up in the wee hours hoping to see the event, NASA’s Lunar Reconnaissance Orbiter (LRO) was busy measuring the temperature drop on the surface as the moon slid into shadow. We’ve all experienced an “eclipse temperature drop” when a cloud passes over the sun. Not only does the cloud eclipse the sun but it momentarily blocks some of the sun’s heat. If it’s early spring and there’s a breeze about, you can really feel the drop in temperature.
The same thing happens on the moon but with more dramatic effect, because there’s no atmosphere to hold onto the heat. LRO observations from the earlier June 15, 2011 total eclipse showed that temperatures dropped some 180 degrees F over parts of the moon’s surface in just 1 1/2 hours and then just as quickly shot back up when the sun returned at the end of the eclipse. Talk about extreme chill – nothing on Earth compares.
Since large boulders cool more slowly than a fine-grained or dusty surface, LRO’s Diviner temperature measuring instrument was able to see what areas are covered with boulders and what regions are blanketed by dust. The degree of cooling also depends on how rocky the surface is, how densely packed the soil is, and its mineral composition. Scientists hope the temperature data gathered from December’s eclipse will help us understand just a little bit more about that shiny thing in the sky tonight.