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March 21, 2006 > TechKnow Talk

TechKnow Talk

by Todd Griffin

My aunt uses the expression "once in a blue moon." What is a blue moon?

Curiously, the answer is not as straightforward as many folks may think. What your aunt means of course is "rarely" or "very infrequently." So a blue moon doesn't occur very often. There are at least three definitions of the term that fit this criterion.

Let's start with the most common definition: a blue moon is the second full moon in a given month. Since full moons are spaced about 29.5 days apart, this occurs in months of 30 or 31 days, when the moon is full in the first day or two of the month and full again in the final day or two. The moon comes full twice in the same month every two to three years.

It is important to note that the true full moon occurs not when it appears full to the eye, which may encompass two or three days, but at the precise astronomical moment when the sun and the moon are on exactly opposite sides of the earth. Should such an instant occur twice in the same month, the second is referred to as a blue moon. It happened most recently in July 2004, and will occur next in June 2007. Approximately every 19 years, there are two blue moons in the same year, almost always in January and March: 1999 was the last, 2018 will be the next.

This "twice in a month" definition is quite recent, however, and does not appear to have been in use prior to the 1940s. Before that, a blue moon was something a little different. It was the third full moon of a season containing four full moons. The formula to calculate this is complex. It is tied to the Christian ecclesiastical calendar, which dates back hundreds of years, and is predicated on four distinct seasons, starting not on the first of January, but on the winter solstice (December 21 or 22). A typical season (three months) would have three full moons. If a season contained a fourth, the third was known as a blue moon. When such an event occurs, it is always on the 20th-23rd of February, May, August, or November.

Though this may seem rather arcane to us, the passing of the seasons, as marked by the cycles of the moon, were very important in agricultural societies. Each of the twelve full moons had seasonal and/or religious significance, and was named appropriately: Harvest Moon, Hunter's Moon, Hay Moon, Lenten Moon, etc. Being able to account for the occasional interloper--the unnamed 13th full moon in a year-as a blue moon, allowed people to stay on track with the seasonal cycles.

This "four in a season" interpretation was in common use as recently as the 1950s, and in fact may still be in use in some agrarian cultures. Since both definitions identify a blue moon in a year that contains 13 rather than 12 full moons, it is not surprising that the frequency of occurrence is the same: every two to three years. The next blue moon by the "four in a season" definition will be in May 2008.

There is yet another type of blue moon, and this one occurs very rarely indeed. It is the literal definition: a moon that appears to be blue in color. This has happened several times in recorded history, though not necessarily as a global phenomenon. It is caused by a distribution of fine particles of dust or smoke in the atmosphere. These particles may filter out or scatter some of the longer wavelength, reddish light, causing the moon to appear bluish in hue. This effect was observed worldwide in 1883, following the volcanic eruption of Mt. Krakatoa in Indonesia. It was also reported regionally in the mid-20th century following widespread forest fires in North America and Europe.

Mars is called the red planet. Is it really red?

It certainly is. If there was ever any doubt, the thousands of pictures taken by the hardy NASA Mars rovers the past couple of years offer indisputable evidence. It is red everywhere: rocks, soil, craters, canyons, and mountains. If your eyesight is very good and the atmosphere very clear, you can even detect a reddish tinge to Mars as observed from Earth.

Why is it so red? Mars is rusted. The soil and rocks contain a great deal of iron oxide, which is simply rust, no different than the rust we see on an old iron nail. Mars is literally covered by it. Some scientists have used this fact to argue for the historical presence of water on Mars. But recent experiments have proven that iron can oxidize-form rust-through the action of oxidizers other than water, especially in the presence of ample ultraviolet light, which the Martian surface receives in abundance. So the scientific jury is still out on whether water was a catalyst in the rusting of Mars.

By the way, at this writing a new IMAX movie featuring the amazing Mars rovers is playing in the Bay Area. It is well worth a look, offering a glimpse behind the scenes as engineers design, manufacture, and launch the rovers, as well as stunning visual images of an alien world. Unlike the moon, Mars has an atmosphere, which provides interesting effects such as whirlwinds or dust devils. And of course there's that wonderful, red scenery.

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