This year's full Snow Moon is Saturday night. The Snow Moon will shine all night long, rising over the eastern mountains at sunset and setting behind the western mountains at sunrise.

Courtesy/Jimmy Westlake

This year's full Snow Moon is Saturday night. The Snow Moon will shine all night long, rising over the eastern mountains at sunset and setting behind the western mountains at sunrise.

Jimmy Westlake: The full moons of 2013

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Jimmy Westlake

Jimmy Westlake's Celestial News column appears Tuesdays in the Steamboat Today.

Find more columns by Westlake here.

— Ordinarily, there are three full moons in each season of the year and each has one or more nicknames that trace back to early American or Native American folklore. Here are some examples of common names for the 12 seasonal full moons: winter has its Moon After Yule, Snow Moon and Crow Moon; spring has the Egg Moon, Milk Moon and Flower Moon; summer has its Thunder Moon, Green Corn Moon, and Fruit Moon; and autumn has the Harvest Moon, Hunter’s Moon and Long Night Moon.

The first full moon of this winter season was Dec. 28, so January’s full moon will be the Snow Moon, the second of the season. Look for this brilliant full moon rising over the eastern mountains at dusk on Jan. 26. The light of the Snow Moon across the snowy landscape will give it the “lustre of mid-day,” as it was so beautifully described in Clement Moore’s classic poem “Twas the Night Before Christmas.”

Sometimes, when the full moon is seen low on the horizon, it looks much larger than usual. This peculiar effect is called the “moon illusion,” and psychologists aren’t sure what causes it. Simple measurements prove the moon is no larger in size when seen near the horizon than when seen overhead, but perhaps the comparison with nearby foreground objects makes it seem so.

Some full moons really are larger than others, depending on the moon’s distance from Earth at the time of the full phase. The largest full moon of 2013, called the perigee full moon, will be June’s Thunder Moon. The smallest full moon of 2013, the apogee full moon, will be December’s Long Night Moon. The maximum difference in apparent size between the perigee and apogee full moons is only about 13 percent — not easy to detect with the human eye.

June’s big Thunder Moon will happen only three days into the season of summer, so we will have time to fit in a fourth full moon this summer before autumn begins. There are no traditional folk names for an unusual fourth full moon in a season, so what do we call it? It is customary to call the third full moon in a season with four full moons a Blue Moon, so the full moon that falls on Aug. 20 this year will be a Blue Moon by that definition.

This month’s Snow Moon provides a wonderful opportunity for a snowshoe hike after sunset. If there are high cirrus clouds in the sky, look for a ring or halo around the moon caused by ice crystals refracting the bright moonlight. Old timers might tell you the number of stars seen inside the ring is the number of days before the next big snow. There is some truth to this, because high cirrus clouds often precede an advancing weather system.

Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. Check out Jimmy’s astrophotography website at www.jwestlake.com.

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