The Self-Adjusting Mystery Clock

Dec 12, 2005

RAY: This puzzler was sent in by a fellow named Mark Sylvia.

Here it is:

It used to be that when we changed into or out of Daylight Savings Time, you had to manually change all the clocks and appliances in your house.

Now, many of them are wired into sources that change them for us. So, for example, you change your wall clock and your microwave clock, but your computer, your cell phone, and your cable TV clocks automatically show the newly changed time without you having to do anything.

There is, however, a certain clock in my house that is not wired into anything. In fact, it runs on a battery, and it reads correctly all the time without my having to change it every spring and fall at the time changes.

The question is, how does it do this?
RAY: The hint I gave is that Mark said he didn't have one of these when he lived in Kansas.

TOM: Yeah, I said to myself, "In your house on the Cape you have a tide clock."

RAY: Right, and the tide doesn't care whether it's Daylight Savings Time or not. The tide is the tide is the tide.

TOM: You have that clock in your kitchen, but what is a tide clock?

RAY: It tells you when it's high tide.

TOM: How does it know?

RAY: It's an analog clock that has only one hand, the hour hand. It runs slower than a normal clock because the tides don't come at six-hour intervals. High tides are anywhere from 12 hours and two minutes apart to 12 hours and 50 minutes apart.

TOM: And you certainly don't have to change it when they change all the other clocks.

RAY: Exactly. Do we have a winner?

TOM: The winner this week is Jane Douglas from Carbondale, Colorado. And for having her answer selected at random from among all the correct answers that we got Jane's going to get a 26-dollar gift certificate to the Shameless Commerce Division at, where she can get our brand-new CD collection of calls about cars and animals.

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