Flower Pot Sundial

* Flower Pot Sundial
This is a very fun and simple experiment that highlights an important astronomy principle – the earth’s rotation around the sun. Even better, this science experiment it lets your child create a very cool clock that dates back to ancient times!

* What you’ll need:
A flower pot (I usually use a terra cotta flower pot because the shadows show up so well)
A long dowel or straight stick – long enough that it sticks up at least 8″ (20 cm) from the top of the flower pot. This is known as the gnomon of the sundial.

*Step One: Put the dowel into the hole at the bottom of the flower pot. If the dowel does not fit tightly, fill the bottom of the flower pot with a few inches of sand or pebbles. If you want this to be even more permanent, glue the dowel into place before filling the bottom of the flower pot. Make sure that the dowel is as straight as possible.

*Step Two: Place the flower pot in a sunny location. Make sure to choose a spot that remains sunny throughout the entire day.

*Step Three: Each hour, exactly on the hour, put a mark on the flower pot, and write down the hour beside the mark. Some of the kids I have worked with like to use different colored, fine tip markers for each hour. For example, they might use a red, fine tip marker to record 10:00, a blue marker for 11:00, etc. The result is very cool!

*Step Four: Keep in mind, this might be a work in progress. Some of the kids I worked with forgot a few hours the first day. So, they would simply record the next hour when it happened. Then they would go back the following day and fill in the missing hours. After a few days, everyone had complete sun clocks!

* Why does this work?
This sundial style clock works because of the earth’s rotation around the sun. The earth rotates on its axis, and revolves in an elliptic orbit around the sun. As the sun hits objects, a shadow is cast; for non-moving objects this shadow will move clockwise (assuming that you are in the northern hemisphere). At noon, the shadow is at its northernmost position, and will continue to move throughout the daylight hours. This experiment works especially well because the shadows really stand out on the sloped walls of the flower pot.

It is also important to note that because the earth is tilted as it orbits, the sundial will change with the seasons as the daylight hours change throughout the year. Try making a summer sundial with one set of marker colors. Then, come back to the same sundial in the autumn and mark the shadows with a different color. Notice the differences?