Even if you weren’t a fan of Sex and the City, you probably know what Sarah Jessica Parker’s character on that show meant when she said, “Maybe you can't change a man, but once in a blue moon, you can change a woman.” “Once in a blue moon” is something that happens only rarely. Let’s wonder a bit about blue moons.

The phase of the Moon is constantly changing. On some nights, perhaps tonight, there will be a full moon—a big, bright, circular Moon that lights up the entire sky. Over the two weeks following, the Moon will become more and more of a sliver until it completely disappears. After that, a sliver of the Moon reappears, grows larger and larger, and in just about two more weeks, the Moon is full again. The complete process takes about a month.

Yes, our calendar months are tied to the time it takes for one full cycle of the Moon’s phases.

“Moon” comes from the same root as “month.” Yes, our calendar months are tied to the time it takes for one full cycle of the Moon’s phases. That full cycle is tied to the time it takes for the Moon to orbit once around the Earth.

A calendar month and the lunar cycle are not exactly the same, however. While a month on our calendar can be anywhere from 28 to 31 days long, the average time between full moons is relatively constant at about 29 days and 13 hours. So while there are 12 calendar months in a year, there are about 12 and one-half lunar cycles.

If there were exactly 12 lunar cycles in a year, there would almost be just one each calendar month, and three in each season. But because the lunar cycle and the calendar month don’t line up exactly, there are sometimes two full moons in one calendar month and sometimes four full moons in a particular season. The second full moon in a calendar month, or the third full moon in a season that has four, is termed a “blue moon.” Blue moons occur at a rate of about one every two to three years.

Wait. Can the Moon ever actually appear blue? Yes! We discovered when we wondered about the color of the sky that when light passes through the atmosphere, some colors are absorbed and other colors scattered. A volcanic eruption and sometimes a forest fire can produce clouds of particles of just the right size to scatter red and yellow light, leaving mostly blues. Some people reported seeing a blueish Moon after the eruption of Mount St. Helens in 1980.

The blue moon in 2018 will be a special one. There are only about 40 blue moons every 100 years, but the blue moon in January 2018 will be the first of two in that year. A single year with two blue moons only happens about once every 30 or 40 years.

Get ready for a blue moon! The next month with two full moons is January of 2018, and the next season with four full moons is spring of 2019. The blue moon in 2018 will be a special one. There are only about 40 blue moons every 100 years, but the blue moon in January 2018 will be the first of two in that year. A single year with two blue moons only happens about once every 30 or 40 years. So the blue moons in 2018 will be something rare indeed.

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Questions to ponder:

If you could look down at the solar system so that you could see the North Pole of the Earth, the Earth would orbit the Sun counterclockwise. From this vantage point, the Moon would orbit the Earth counterclockwise, and the Earth and the Moon would rotate counterclockwise. And the other planets? Also counterclockwise! What does this suggest about the formation of the solar system?

There are some oddballs in our solar system. Venus rotates its axis almost exactly clockwise, as does Neptune’s moon Triton. What does this suggest about the history of those objects?