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Geek Trivia: Every other month

What are the other three types of astronomical months besides sidereal and synodic?

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Here's an easy question: How many months are there in a year? The instinctive answer, at least for everyone using conventional calendars, is 12. The lengths and names may change based on whether you're using the Gregorian, Julian, Indian National (not to be confused with Hindu), Hebrew, Islamic, or Persian calendars, but each of these time-measurement systems divides the year into twelve months.

Of course, the armchair astronomers would likely remind us all that the concept of a month comes from an etymologically similar word, moon, which has various definitions besides the 12 months of the year. That's really what I'm getting at—how many kinds of months are there? Within astronomical circles (no pun intended), the term month has many possible meanings based on what modifier you place before the word itself.

Case in point: A synodic month indicates the period of time required for the moon to cycle through all of its visible phases. Laymen often refer to this as a lunar month, but that can lead to confusion, especially when compared to a sidereal month, which is how long it takes for the moon to complete one full orbit around the Earth.

So what's with the complicated distinction for essentially the same period of time? Well, that's because it actually isn't the same period of time.

A synodic month is roughly 29.5 days, while a sidereal month is roughly 27.3 days. The reason for the discrepancy is the Earth's and moon's collective motions around the sun.

The moon's phases are a result of the relative position of the moon between the Earth and sun. Even though the moon can complete an orbit around the Earth in 27.3 days, the Earth has continued in its own orbit around the sun, changing the angles of reflection between the three celestial bodies. The moon needs an extra 2.2 days to make up the difference—and complete its full cycle of visible phases.

You can see why astronomers need several kinds of months just to keep things straight—five kinds of months, actually, counting the two we just mentioned, leaving three we haven't talked about.


Of the five types of months measured in astronomy, what are the remaining three besides sidereal and synodic months?

You may have never heard of them, but astronomers actively recognize and measure tropical months, anomalistic months, and (my personal favorite) draconic months.

Tropical months are almost precisely as long as sidereal months—a sidereal month is usually less than 7 seconds longer than a tropical month, on average—but science measures the two differently. Astronomers measure a tropical month based on how long it takes the moon to reach the same position in the sky relative to the sun's position during the vernal equinox.

The only reason to measure this type of month is within the context of the tropical year, which is the period of time it takes the sun to return to the same point on the ecliptic path. While most often measured from the vernal equinox point, a tropical year can take its start from three other points on the ecliptic, and the length of a tropical year depends on the point from which you start measuring. In other words, a tropical month is trivial, even by astronomical standards.

Anomalistic months measures the time between two passages of the Moon through its orbital perigee. Like all celestial bodies, the moon's orbit is elliptical, not circular, which means that it moves not just around the Earth, but closer to and farther from it. An anomalistic month measures how long it takes for the moon to reach the same orbital distance from Earth, a period of roughly 27.5 days, slightly longer than a sidereal month.

Draconic months are arguably the most complicated—and the most astronomically significant. The plane of the moon's orbit around the Earth is not the same as the Earth's orbit around the sun; the moon's orbit is tilted by roughly five degrees.

Astronomers call the points where these two planes intersect—when the Earth, moon, and sun are on the same plane—nodes. The length of time it takes for the moon to return to the same node—a period of roughly 27.2 days—is a draconic month.

So, why a "draconic" month? In order for a full eclipse to occur, the moon must be in a node where it's on the same plane as the Earth and sun and can therefore block the sunlight passing between the two.

According to Chinese mythology, the cause of a solar eclipse is because a dragon is "eating" the sun. Thus, the period of time it takes for the "dragon" (the moon) to move into position to potentially "eat" (eclipse) the sun is a draconic month. Now that's what I call some techno-mystical Geek Trivia.

The Quibble of the Week

If you uncover a questionable fact or debatable aspect of this week's Geek Trivia, just post it in the discussion area of the article. Every week, yours truly will choose the best post from the assembled masses and discuss it in the next edition of Geek Trivia.

This week's quibble comes from the March 30 edition of Geek Trivia, "Pitching a perfect game(show)." TechRepublic member J_denham quibbled with my incomplete list of parodies for the pre-Alex-Trebek-era version of the Jeopardy! game show.

"If you want another parody of the Art Fleming version, check out the song/music video for Weird Al Yankovic's 'I Lost on Jeopardy.'"

Quite right, dear reader: This song does indeed include a voice cameo by the classic Jeopardy! announcer Don Pardo, which more than qualifies as a referential parody. Who knew Weird Al had such drawing power?

For more, check out the Geek Trivia Archive.

The Trivia Geek, also known as Jay Garmon, is a former advertising copywriter and Web developer who's duped TechRepublic into underwriting his affinity for movies, sci-fi, comic books, technology, and all things geekish or subcultural.

About Jay Garmon

Jay Garmon has a vast and terrifying knowledge of all things obscure, obtuse, and irrelevant. One day, he hopes to write science fiction, but for now he'll settle for something stranger — amusing and abusing IT pros. Read his full profile. You can a...

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