Today, Tuesday September 22, 2009 is the autumnal equinox. The equinox is the half way point between the summer and winter solstice and marks the first day of fall. The equinox is a great excuse to get outside with your students and take advantage of the sun before the weather gets too cold for outdoor activities. First, a few quick things everyone should know about the equinox:
1) Each place on earth will receive an equal length of daylight (sort of).
2) The sun will be directly overhead an observer at the equator (0 latitude).
3) On the equinoxes, the sun will rise exactly east and set exactly west.
4) The length of daylight will begin to get shorter after the autumnal equinox and longer after the vernal equinox due to earth’s tilt (23.5 degrees). This statement only applies to the northern hemisphere.
Measuring Shadow Length
As the Earth progresses in its yearly orbit around the Sun, shadows cast by the Sun vary in length due to the tilt of the Earth’s axis. Recording the length of noonday shadows is a good way to demonstrate how the angle of the sun and your latitude affect the length of shadows.
For this activity, students stand in a an open area and have a partner measure the length (in cm) of the shadow they cast. Students will return each month to record the length of their shadow until at least the winter solstice (December). The recorded lengths can then be used to create a graph of shadow lengths versus month.
A great extension to this is to have students try and simulate the changing length of their shadow over time using a flashlight and an action figure. It will lead students to notice that the higher the flashlight, the shorter the shadow and vice-versa.
If your students have digital cameras, a tripod and some paint your class can create time-lapse videos of the shadows at your school. You’ll need an area where students can return to repeatedly throughout the year to take photos. You can use the paint to mark where the tripod is located so students can replace the tripod in the exact same spot again. If they take enough pictures they can play them together and observe the change in light and shadow throughout the seasons. Here is an example of a time-lapse over one day.
If you do not have the technology to create movies, students can just compare pictures taken at the same time of day over the course of several months and describe how the position of the sun and the length of the shadows appears to change.
Make a Sundial
Using pre-made templates such as this sundial from the Sandburg Planetarium(designed for 39N latitude), students can create sundials to study the relationship between the length of the shadow cast by the gnomon and the time of day. This will lead to discussions about how the earth rotates and you could have students hypothesize what will happen to their sundial shadow as the months change.
More information on sundials.
Get outside and enjoy this bi-annual event with your students. If you have any questions please post them in the comments below.
National Geographic Video
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