Think you know how many days are in a year? Think again.

One of the great joys and headaches of writing about science for a D.C. audience is that there are a lot of brilliant people reading: engineers at NASA, doctors for the National Institutes of Health, paleontologists at the Smithsonian, etc. And they always notice when I make a mistake.

So I wasn't totally surprised when I arrived at work Thursday morning and found a disgruntled message on my answering machine. My story about the discovery of a nearby solar system with seven Earth-sized exoplanets had run in that day's paper. And Bert Schwarzschild, a particle physicist and former editor of the magazine Physics Today, had a bone to pick about one small number in the piece: 365.26.

That's how long we said a year on Earth lasts in a graphic that accompanied the story. The number came from NASA, so I felt pretty confident in it. But Schwarzschild said it wasn't right. A year is 365.24 days long — that's why we have to skip a leap day every 100 years.

Puzzled, I looked up the question online, only to end up more confused. No website I checked could agree on an exact number. Was it 365.25, as NASA states on this page? Or 365.242196, as explained by the National Institute of Standards and Technology? Or had our original illustration been right all along, as this Jet Propulsion Laboratory profile suggests?

I called up the U.S. Naval Observatory, home of America's master clock. (Along with NIST, the Naval Observatory also operates the website time.gov, which may be my new favorite government URL.) If anyone could tell me the correct length of the year, it was them.

“Well, it depends on what year you're referring to,” said Geoff Chester, a public affairs officer for USNO, when I explained my quandary to him.

What year? There's more than one?

“There are four principal years that are in use,” Chester told me. “Nothing’s as simple as it seems when it comes to this stuff.”

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First, there's the Julian year, which is exactly 365.25 days long. It's not very precise, since it's just a number someone decided on, rather than an exact measurement of an astrophysical phenomenon. But when it was introduced by the Julius Caesar in 46 B.C., it was revolutionary.

Before then, the Romans knew that it took about 365.25 days for the Earth to orbit the sun, but they decided to stick to a 355 day calendar anyway. Every so often the high priest of Rome would call for an “intercalary month” to put the calendar back on track. Theoretically these intercalations were supposed to be systematic, but they often were abused — priests would call for an extra month when their friends were in power, or omit a needed month if an enemy was consul. Things got pretty wonky pretty quickly, and the last years before Caesar swept in with his new system were known as “the years of confusion.”

We don't use the Julian year for calendars any more, but it is used to define the light-year as a measurement of distance.

Modern calendars are set according to the tropical year, which tracks the amount of time it takes to get from spring equinox to spring equinox — about 365 days, 5 hours, 48 minutes, and 46 seconds, or 365.2422 days. It's probably the most well-known measure of a year because it's the most useful for people here on Earth. If you want to know when the seasons will change, the tropical year will tell you.

But there's a not-so-tiny problem with the tropical year: the Earth wobbles. Instead of spinning like a globe on an axis, we turn like a top. This means that the orientation of Earth's equator is constantly shifting ever so slightly; thus the moment of the equinox (when the equator passes through the center of the sun) also changes. The consequence of all this wobbling is that a tropical year ends about 20 minutes before Earth actually completes an orbit of the sun.

If you want to know how long that journey takes, you've got to look at the anomalistic year. That's the measurement of the number of days it takes for the Earth to return to its perihelion — the point at which it is closest to the sun. It comes out to about 365.259636 days per year.

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But Earth's orbit doesn't stay the same each year. The large, looming presence of Jupiter in our solar system means that the ellipse that Earth circumscribes around the sun is distorted. If you're trying to do calculations across long time spans, or keep a telescope pointed at the same spot in the galaxy, then this measurement might lead you astray.

“Everything moves in the universe,” explained Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics. “It really causes problems.”

Astronomers' solution is to measure time by the largest, most fixed frame they can find: the entire cosmos. They abide by the sidereal year, the amount of time it takes for the sun to return to the same position relative to the fixed (most distant) stars. This year is 365 days, 6 hours, 9 minutes and 9 seconds, or about 365.26 days long. Since this is the measurement most useful to astronomers, it makes sense that NASA used it to compare Earth to the TRAPPIST-1 exoplanets.

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McDowell and his colleagues also live by the sidereal day, which is the amount of time it takes Earth to complete a single rotation relative to the fixed stars. This day is four minutes shorter than our 24-hour one, but astronomers don't mind.

“We don’t care about the sun — we’re never out when it’s up,” McDowell laughed. “We just want our observatories to point to the same spot in the sky they did yesterday.”

Weirdly, the days in a sidereal year are ordinary days, not sidereal ones. This is why people think astrophysics is hard. (Okay, maybe there are a few other reasons.)

But even the sidereal year is imperfect, McDowell said. After all, the so-called fixed stars aren't really fixed — everything in the universe is moving away from everything else at an accelerating rate. If you take the theory of general relativity into account, you'll find that time passes differently on Earth than it does elsewhere in space.

“Every aspect of time, from the very small to these big scales … all of these things are fraught with a, 'Yes, but actually' caveat,” McDowell said.

“Sure, we’ve refined and refined over the centuries,” he acknowledged — just ask the Romans who lived through the “years of confusion.” But clearly, the confusion isn't over yet.

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Think you know how many days are in a year? Think again.