It’s trivial for me to recall the components of the book because it’s one of Thomas’s favorites. Meaning that I read it a lot. Meaning that, sometimes while I’m reading it, my mind wanders.

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One night last week, it wandered in a particular direction. I wonder, I wondered, if one could determine how long it took this rabbit to fall asleep based on the movement of the moon in his bedroom window. For, in addition to reading the same book over and over, much of the period during which one is trying to get a baby to go to bed is spent wondering why it takes so long for babies to go to bed.

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And so I decided to try to find out.

This is the progression of the moon in the window over the course of the book.

It seems as though it should be simple to figure out the relationship between the progress of the moon and the passage of time, just as one can use the movement of the sun to figure out how much time has elapsed. But as outlined to me by Paul Lynam, an astronomer at Lick Observatory near San Jose, it isn’t.

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“You would have to have a good understanding of the moon’s motion, the moon’s orbit, and you need to know a little something about the movement of the Earth and your position on the Earth,” he explained. You could figure it out, in other words, but it’s a lot more complicated than simply measuring the distance between point A and point B.

The apparent movement of the moon depends on where you are in the world, which was beneficial hundreds of years ago.

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“Years and years ago, when they were building empires and they were trying to navigate around the world — before the invention of the wristwatch or the spring chronometer — one of the serious proposals was to develop these lunar tables,” he said. “Ships would carry a lunar table and then make an observation of the position in the sky and then referring back to the tables they were able to work out their longitude.”

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For our purposes, though, it’s less helpful. Here, for example, is how the moon traverses the sky at the South Pole during the part of the year when the region experiences night 24 hours a day.

Where you are matters.

There are other complicating factors that make it tricky to figure out where the moon will travel and how quickly, Lynam said: The speed of the moon’s orbit (which picks up as it moves closer to the Earth), the wobbling motion of the moon itself (which means that we actually see a bit more than half of the moon’s face as it moves through the sky) and even continental drift, which can affect where we expect the moon to be over a much longer time period than one evening.

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(You may have heard that astronauts planted reflectors on the moon during one of their visits. The point of those reflectors was to measure that drift. By aiming lasers at the reflectors and measuring how long the beam of light took to return to Earth, scientists could measure the distance between points on Earth and the moon and, they thought, measure how the tectonic plates were moving.)

“I don’t want to sound flippant,” Lynam said about the complexity of the calculations, “but there’s probably an app that would potentially be able to do this.”

So there is.

Sun Surveyor is an app created by Adam Ratana, an amateur photographer who wanted to know when the sun and moon would be in the optimal position for exceptional photos. He created a tool that allows you to do precisely that.

This is what the sky above my back yard looked like Tuesday morning, viewed through Sun Surveyor. I’ve marked the actual moon with an arrow; it sits next to Sun Surveyor’s estimate of where the moon will be based on the geographic data it picks up from my phone’s sensors.

If we had access to the rabbit’s room in the book, we could point our phones at the window and actually scrub through the year to figure out when the moon in the app lined up with the moon in the window. Ratana likes to do this with Google Street View, he said when we spoke by phone Monday, lining up the sun in the sky with the date that Google’s car was roaming the streets, taking its pictures. (They’re often in mid-June, he said, since it makes more sense to hire people to make these trips on the longest days of the year.)

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Again, though, that motion is dependent on where the observer is in the world — and we don’t know where our little rabbit lives.

I guess this is a decent point at which to admit that “Goodnight Moon” is not a documentary work. It’s a children’s book, written by Margaret Wise Brown and illustrated by Clement Hurd, which makes no claims to a robust relationship to reality — including to the motion of the heavens.

That said!

Brown is a fascinating figure. The book was first published in 1947, at which time Brown was living with the poet Michael Strange in Manhattan in a small cottage called Cobble Court. The house was at 10 Gracie Square, near Gracie Mansion, the traditional home of the mayor of New York City, until it was moved to Greenwich Village in 1967.

Brown’s partner used a pen name. Michael Strange was born Blanche Marie Louise Oelrichs, and was formerly married to the actor John Barrymore (the grandfather of actress Drew). Brown herself later got engaged to a Rockefeller, but died at age 42 before they wed.

With Sun Surveyor, we can see what the moon would have looked like from Cobble Court on any given day when Brown and Strange lived there. At midnight on the day that “Goodnight Moon” was published, it hung over the East River, nearly full, before eventually setting to the west nine hours later.

Perhaps this was the moon that Clement Hurd illustrated at some point over the previous year.

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But we’re overthinking this, in case that wasn’t already clear.

Alex Filippenko, professor of astronomy at the University of California at Berkeley, emailed with a good rule of thumb for estimating how much time had passed based on the movement of the moon. Remember: The moon rises and sets based on the rotation of the Earth. So:

“The entire sky rotates 360 degrees from east to west in 24 hours, because of the turning of Earth on its axis. That’s 15 degrees per hour. So, if you just measure the angular motion of the Moon across the sky from east to west, in 1 hour it will move 15 degrees,” he wrote.

If you extend your arm and spread your fingers, the distance from the tip of your thumb to the tip of your pinkie is about 18 degrees, so the moon would move a little less than that over the course of 60 minutes. (Or actually slightly less, since the moon also moves east-to-west, he wrote, meaning that the overall movement is about 14.5 degrees.)

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That would be useful if we were with that rabbit at Cobble Court, which we are not. But we do know the angular diameter of the moon, the titular star of our book, as seen in the night sky: One-half of a degree.

Of course, this varies depending on where you are on Earth and where the moon is in the sky. The moon appears much larger when it is close to the horizon, which can throw off our calculations. But since we aren’t trying to land a rocket on it, we’ll use the half-degree standard for our calculations.

(In the Sun Surveyor photo in my back yard, the math works pretty well. The diameter of the arrow-marked moon is about one-thirtieth of the distance between the hour markers in the app.)

We soon have a problem, though, in our “Goodnight Moon” calculations. The moon at the start and end of the book is, in fact, depicted at different sizes — and, incorrectly, it’s shown as being larger once it’s higher in the sky.

So which moon is the half-degree moon? The one whose diameter is marked as a or the one marked as b?

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As it turns out, it doesn’t really matter. If we compare the size of moons a and b to the distance traveled, d, we learn that Filippenko’s rule of thumb doesn’t really work. If moon a is the correct size, about five minutes would have passed over the course of the book. If moon b is, only eight minutes would have.

How do we know this is wrong? Because, I realized belatedly, there’s a clock on the mantel.

Remember, I am reading this book when both the baby and I are tired.

Between the first visible moon and the final image in the book, about an hour passes.

That means that we can use the distance traveled to figure out not how much time passed, but how big the moon should actually appear out the window. And so we did: It’s that small pink dot on the right-hand side of the image.

Or, if we re-illustrate the final image: