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Janice VanCleave's Science for Fun
Surprising Science Facts
Time: A 25-Hour Day
Fun Experiment to Try at Home!
To compare clock time to sun time during and after daylight-saving time.
- wooden block that measures 2x4x6 inches
- long nail, such as a 16d nail
1.  Create a sundial by hammering the tip of the nail into the center of the wooden block. The nail should be as firm and vertical as possible, without being driven in too deeply.
2. On Monday during the last week of daylight-saving time, take the sundial outdoors on the hour, and use a compass to determine the sundial's direction. Mark the shadow of the nail where it falls across the wooden block, and write the clock time next to the shadow mark. (For example, if you take the clock outside at noon, write "12 p.m." next to the line.)
3. Determine the sun time for the shadow, and write it next to the clock time. During daylight-savings time, the sun time is one hour less than the clock time. For example, if you take the clock outside at noon, the sun time is 11 a.m.
4. Repeat steps 3 and 4 several times before the end of daylight-savings time, making sure to use the compass each time to place the sundial in the exact same position.
5. The next week when standard time has returned, take the sundial outdoors at the same times you did the week before. Compare the nail's shadow now to where it was last week.
Sum It Up!
Because clock time and sun time are now the same again, the two times will be the same on the sundial. The one-hour difference you saw during daylight-saving time happened because clocks had been turned back an hour!
Daylight-saving time (DST) in the United States is the period from the first Sunday in April to the last Sunday in October.
We set our clocks forward one hour in April and then back one hour in October. If you get confused about which way to turn your clock, here's an easy way to remember:
  • In the spring, you "spring" forward one hour.
  • In the fall, you "fall" back one hour.

Why do we practice daylight-saving time? Because it gives us more usable hours of daylight to have fun with outdoor activities. The sun doesn't slow down, of course, and the total amount of daylight remains the same, but on the first Sunday in April the day is considered to be 23 hours long instead of 24. Just by moving the hands on your clock you lose one hour!

On the last Sunday of October, the reverse happens. This day is extra long because the clocks "fall" back one hour, and you are back to standard or sun time. So it all works out, you lose one hour in April and gain it back in October.

For more information and experiments about daylight-saving time, see these Janice VanCleave books: Earth Science for Every Kid, Math for Every Kid, and Play and Find Out about Math.

Janice VanCleave's Science Around the Year Don't miss Janice VanCleave's new book
Science Around the Year.

Click here to visit Showboard for science fair materials.

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