Sky Roundup: February 9, 2007

Does the Moon ever appear this way?  Indeed it can! During a lunar eclipse, the Full Moon’s hue can range from orange to red, even to black! (Image by Steve Rismiller.)

Full Moon bright, orange or white?

During the colder months of the year, when many of us may not venture outside to look at the night sky, a Full Moon outdoors may attract our attention indoors, especially if there’s a fresh snowfall.  We look out the window and note the landscape looks brighter than in warmer months, partly due to snow all about, but also due to the Moon’s position high in the sky.  By contrast, in the warmer months of the year, we often see the Moon riding lower across the sky, appearing to glide slowly up from the horizon to give us, for example, the Harvest Moon near the start of Autumn. 

Is this real or an illusion?  In fact, the Moon’s position in the sky is due to the angle at which the Moon’s orbit is tilted with respect to the ecliptic (that is, the plane of the Earth’s orbit) and to our local horizon.  The ecliptic itself is tipped about 23.5º to the celestial equator (that projection of Earth’s own equator onto the night sky); the Moon’s orbit is tipped about 5º to the ecliptic.  In Winter, the ecliptic makes a fairly high angle to our horizon in Cleveland; so, too, does the Moon’s orbit, hence the Moon really climbs higher in the sky when it’s seasonally colder than when it’s seasonally warmer.

Most casual observers are familiar with the warm-weather name of Harvest Moon, which is the Full Moon seen closest to the autumnal equinox, or, the first day of Autumn.  In past agrarian times, this Moon provided additional light for farmers to work after sunset, hence the name.  Due to the relatively low angle of the Moon’s orbit to the horizon in Autumn, the Full Moon appears to rise gradually, spreading moonlight broadly across the landscape, giving the extra illumination needed to bring in the crops. 

Less well known is the Hunter’s Moon, which is typically the next Full Moon after the Harvest Moon.  After harvest time, one may have needed extra provisions for the coming winter, hence the name given to the Full Moon that provided the light at night to track one’s prey.

How about actual cold-weather names for the Full Moon?  In November, it’s the Frosty Moon; in December, the Cold Moon.  In January, it’s the Moon After Yule.  In February, it’s known as the Full Snow Moon, as we see this month with a Full Moon high above our own snowy landscape.  In March, it’s known as the Full Crust Moon, since the snow cover gets crusty due to the thawing by day and freezing by night.  If the weather begins to warm early in March, then it’s known as the Full Worm Moon, since earthworms are beginning to emerge with the return of robins.

It’s also part of the fascination with the Moon that casual observers may think the Full Moon changes color or size throughout the night.  It’s true that the Full Moon rising may appear orange in color.  Due to absorption by Earth’s atmosphere of part of the light reflected from the Moon surface, the Full Moon seen near the horizon will indeed appear a shade of orange; more to the point, since the Moon is viewed at a low angle, essentially through more atmosphere, the intervening airborne dust and local pollution can help to absorb the Moon’s light, making the Moon itself appear orange.

But the Moon doesn’t need to be near the horizon to appear colorful; if airborne dust, hazy smoke, industrial pollution, or summertime humidity are great enough, the Moon’s white light pales to a tinge of orange even when high in the sky.

In terms of color, the Full Moon can appear orange during a lunar eclipse.  When the Earth passes exactly between the Sun and the Moon, the shadow of the Earth falls across the surface of the Moon, so the name for this type of eclipse.  As seen from the Earth, the Full Moon gradually slips into the shadow of the Earth.  When fully immersed in this shadow, a Full Moon can take on a distinct hue, ranging from a coppery orange to a deep crimson or even darker. 

The next time to see this colorful phenomenon in Cleveland is March 3rd when a lunar eclipse will be visible, already in progress, as the Moon rises at sunset.  The astronomy staff of the Cleveland Museum of Natural History intends to observe this phenomenon at an outside location (to be announced in Sky Roundup later this month); so, weather permitting, you may be able to see an orange Full Moon not only for its nearness to the horizon but by its coloring from a lunar eclipse!

Does the Moon ever appear this way? Not really. This picture deliberately exaggerates the size of the rising Moon. The Moon can look large near the horizon, but not this large. The Full Moon’s usual apparent large size is simply due to its proximity to objects seen along the horizon. (Image by Keith Cooley.)

In addition to its orange tint, casual observers may claim that the Full Moon seen near the horizon looks large, far larger than it normally appears when seen later in the night.  Is this real or an illusion?  The Moon’s apparent big size is just an illusion.  The Moon is slightly larger high in the sky (when, in actuality, it’s slightly closer) than when near the horizon.  But when seen near objects along the horizon, such as houses and trees, the Moon seems huge; away from these landmarks, the Moon seems tiny.  The Moon’s real size hasn’t changed; rather, its appearance is influenced by perspective, mainly due to objects in the foreground as we view the Moon.

Does the Full Moon ever change its apparent size in the sky?  There are times when a Full Moon appears genuinely larger than at other times.  Since the Moon revolves around the Earth in an elliptical orbit, the Moon’s closest approach, or perigee, can coincide with the phase of the Full Moon, and, at that time, the Moon can indeed appear large.  Coincidentally, the Moon’s perigee will coincide with this year’s Harvest Moon, Hunter’s Moon, and Frosty Moon!  So look for a big Moon rising on September 26th, October 25th, and November 24th of this year.  Of these dates, the Hunter’s Moon on October 25th will be the largest Full Moon for 2007.