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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
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

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.