Time

Shadow Stick

4.7 - Understand how to determine the time of local noon using shadows, including use of a shadow stick

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4.19 - Be able to use shadow-stick data and the Equation of Time to determine longitude

An interesting project is to plot the shadow of the Sun over the course of a day at various times of year.

Every hour place a mark with the time at the end of the shadow of the stick. Plot more frequently over the midday period. You may find the shortest shadow does not occur at midday on your watch. The stick is showing apparent solar time.

At the end of the day, connect the marks and you will see  an interesting pattern.

Repeat the exercise on another date but this time put another date or use another pencil so you can see the difference between the dates.

See the animation for further instructions.

The point of this that the Sun casts different shadows at the same time of day throughout the year. This happens because of Earth's elliptical orbit and axis.

To work out your longitude take the reading of the point when the sun was highest in the sky. This should be apparent solar time of midday. Have an accurate watch set to GMT. If your noon happened before 12.00pm GMT you are east of the Prime Meridian, if it happened after you are west.

Note the difference in time. For every four minutes difference the longitude is 1 degree away from the Prime Meridian.

Example 1: Difference was 8 minutes after GMT noon. Observer is 2° west. 8 divide 4 = 2. 

Example 2: Difference was 1 hour, 10 minutes. Equal to 80 minutes. Observer is 20° east. 80 divide 4 = 20.