# Goodnight Scientist

Chad Orzel nerds out on the children's book Goodnight Moon:

[W]e know that the full moon in the sky covers an angle of about half a degree of arc. On our big copy of the book, the diameter of the moon in the final illustration is just about 7/8" (I only have an English-unit tape measure here), while the distance from the corner of the window to the outer edge of the moon is 2 and 5/8", exactly three times the diameter. So the moon has moved through about 1.5 degrees in the course of the story.

Now, the Earth rotates through 360 degrees in just about 24 hours, which is 15 degrees per hour (the Moon's motion is slightly slower, owing to its orbital motion, but it's not a significant difference for our purposes). This suggests that the bunny's goodnight ritual takes about 0.1 hour, or six minutes. Coincidentally, this is approximately the time required to read the book to [my daughter] at bedtime, as she points out all the important features of every picture ("Mouse right there! Mouse is sneaky!").

Of course, there's another way to estimate the passage of time in the book, which is the clocks shown in the various pictures. The clock in the first picture shows almost exactly 7:00, while the clock in the final picture shows approximately 8:10, for an hour and ten minute duration. Coincidentally, this is approximately the time it takes to get [my daughter] to go to sleep after reading Goodnight Moon ...