Once In A Blue Moon

This year January has two full moons, February has none, and March has two, a celestial even that happens only "once in a blue moon."

| February/March 1999

  • once in a blue moon
    February won't provide one full moon this year, much less two. However, January will have two and March will have two, which only happens "once in a blue moon." The crescent moon from top to bottom: Feb. 21, Feb. 20, Feb. 19, Feb. 18, Feb. 17. The planets at dusk, Feb. 23, after sunset: Saturn top center, then Venus, Jupiter, and Mercury. On Feb. 18, Venus and Jupiter will be farther apart but lie not far to the lower right of the moon, with Jupiter higher than Venus.

  • once in a blue moon
This February won't provide us with a full moon, though the sky will compensate us in March with the rare (and long-fabled) "blue moon."

The time between one full moon and the next averages (with very slight variation) about 29 1/2 days. So there can never be two truly full moons in February not even in leap year, when February is still a half day too short. Our longer months, on the other hand, can have two full moons. In fact, this happens once every few years. When it does, the second full moon of the month is called a "blue moon." However, I suspect that this definition of a blue moon came into being in recent years or decades. The very much older phrase "once in a blue moon" probably originated with an event much more rare than a two full-moon month: initially, it probably referred to those rare instances when the moon literally looks blue.

What causes the moon and sometimes the sun to look blue (or green or bronze) is a pall containing particles of just the right size to preferentially scatter the blue wavelength of moonlight or sunlight. Such a cloud may be produced by widespread forest fires or by vast volcanic eruptions. The only really widespread and striking case of blue moon (and sun) in the U.S. during this century was the one in September 1950, which was visible across New York, New England, and part of the Mid-Atlantic states. It was caused by Canadian forest fires.

If you want rare, however, the pattern of a two-full-moons-January, no-full-moon February, and two-full-moons March is pretty rare. Instances of it can never occur less than 19 years apart, but it doesn't always occur that often. This year's instance is only the fourth of the 20th century, and the last time it happened was in 1961. It won't happen again until 19 years from now, in 2018.

Actually, I do need to qualify the above statements. They are the correct years if you figure the exact times of full moons in the astronomers' worldwide time system, universal or Greenwich time. This year's episode of two fulls, no full, two fulls happens in both universal time and in American time zones. But that is not always true. Nevertheless, whatever time zone you calculate these events for, they average about one per generation.

Thrilling Months for the Planets

The two full moons of March will look more or less like any other full moons. But in the west, at dusk, during this February and March, there are sights not just of great intellectual interest but also of great visual beauty. There is one of the finest arrangements of moon and planets in years and what is arguably the closest really visible conjunction (meeting) of the two brightest planets in decades.

February begins with the two most brilliant planets, Venus and Jupiter, still about 22° apart. When we talk about degrees in the sky, we mean fractions of the 180° of the half circle or dome that we perceive the sky to be. Thus, from horizon to overhead — halfway across the sky — is half of 180°, which is 90°. You can measure these "angular distances" in the sky by using your hand: your fist held out at full arm's length is, from your viewpoint, about 10° wide.


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