D: Yaël, do you notice the days changing length over the course of the year?
Y: Sure, Don, it stays light later in the summer and gets dark earlier in the winter.
D: No, that's not it. I mean the whole day feels longer.
Y: I'm not sure that's possible, Don--aren't days always 24 hours long?
D: Well, it actually takes 23 hours, 56 minutes, and 4 seconds for the Earth to rotate on its axis. But in fact the Earth's rotation is slowing down all the time because of the gravitational pull of the moon.
Y: So we're still spinning around, only more slowly. Does this mean that the days really do feel longer?
D: Not likely to you or me, Yaël. The moon slows us down by about two milliseconds a century, so that even 1 million years ago days were only twenty seconds shorter.
Y: I doubt I would have noticed that.
D: Maybe not a million years ago. But 400 million years ago days were 21-and-a-half hours long.
Y: That I would have noticed.
D: What you really would have noticed is that the time it takes the Earth to get around the sun hasn't changed, so that back then there were 400 days a year.
Y: 400 days a year! I hope the extra days were all vacation days.
D: I'm not sure about that. But scientists have found fossil evidence from tropical corals that existed at that time, and they have about 400 daily growth rings per year. So someone was enjoying all those extra sunny days.