Moon And Winter Hexagon Bright On Bright Tonight

The nearly full moon will be surrounded by a hexagon of bright stars tonight. The map shows the sky facing southeaster around 9 o'clock. Created with Stellarium

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.

During December's lunar eclipse NASA's LRO made temperature measurements of the moon's surface during its orbit. Seen from the moon, the Earth eclipsed the sun that morning. Credit: NASA with additions of my own

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.

LRO's primary purpose it to map the moon incredible detail. In this photo we see the Apollo 17 lunar descent module, astronaut tracks and the parallel tracks made by the lunar rover driven by the astronauts. Credit: NASA

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.

12 Responses

  1. caralex

    Bob, can you link to a list of what you considere the 25 brightest stars? Some lists on the net have slight differences, with Adhara or Gacrux either included or left out. When I was a child, I remember learning the 20, not 25 brightest. The others, although first magnitude, were considered 2nd magnitude for some reason!

    1. astrobob

      Hi Carol,
      Yes, those fractional magnitudes can sometimes make one star replace another on different lists. I usually go with Jim Kaler’s list only because I’ve read many of his books. He’s an expert on stars. Regulus (mag. 1.35) is clearly 1st magnitude and 21st on the list. Adhara at mag. 1.50 is 22nd on his list, so you can consider it either the faintest of the 1st magnitude stars or the brightest 2nd magnitude star. After Adhara is Castor at 1.58, a tad closer to second. Gacrux comes in at 24th.

        1. astrobob

          You’ve got to make a cutoff somewhere. I should probably define my terms. Strictly speaking, 2.0 is second magnitude, but a more general description of second magnitude, at least as I interpret it, would certainly include stars from about 1.5 mag. to 2.5 mag.

  2. veronica

    any one know what a harvest moon looks like? And how often does it occur? And when was the last time it occured? tonight i was looking up at the moon and it was really bright and a circle around the moon anyone know why?

    1. astrobob

      Hi Veronica,
      The Harvest Moon is the full moon closest to the first day of fall. The circle you saw tonight is a lunar halo and caused by light bent by tiny ice crystals in high, thin clouds. I just blogged on a lunar halo I saw a couple nights ago. You can read more about halos here:

  3. Bewil

    I just wanted to say that I have found myself reading your post more and more as time time goes on. My 11 yr old has even started reading them. They are very informative and detailed. Thank you very much! It is a please. Not to mention that it is better then NASA’s website by far!

  4. Hi Bob!
    It’s time for me to re-publish my own image of the winter hexagon as I saw it on Dec 21 2010. Which you remember was the last eclipse of a full moon on the solstice for the next 4 centuries. And the first in the previous 4 centuries, if you’re keeping score.

    Anyhoo, your winter hex is so much finer than mine. May I use it on my blog, with attribution and a link back to this page?

    Cheers. Happy New Year


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