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Wednesday, November 4 Dr. Bob Schweikert
Sundials and Seasons:
Making A Sundial
[REMEMBER that all students should prepare questions to ask presenter during videoconference.]
Have you ever wondered how people worked out their ideas of telling the time? They used the position of the Sun in the sky. They used sundials to tell the time by looking at the shadow cast by the Sun as it shines on the pointer of a sundial.
A sundial works by casting a shadow in different positions, at different times of the day. You can try this for yourself by making a sundial. In this videoconference pre-activity you will construct a sundial (sun clock) that can show the time using a shadow line. You will also learn the design factors that are important for the accuracy of your sundial. You'll notice that at different times of the day, the shadow is in different positions, and it changes length.
In which direction do they appear to travel? They appear to move in a counterclockwise motion. Why? Because the Earth spins counterclockwise.
The Sun is highest in the sky at midday and casts a short shadow. In the afternoon, when the Sun is lower in the sky, the shadow is longer. The length of the shadow is also affected by the seasons. Winter shadows are longer than Summer shadows. This is because the Sun is lower in the sky in Winter.
SEASONS:
In Summer the Sun appears to be high in the sky and feels very warm. In Winter the Sun appears a lot lower in the sky and feels a lot cooler than in Summer. The Earth's tilt has an effect on our seasons in the angle at which the sunlight strikes the surface of the Earth. The greater the angle, the more dispersed and less concentrated the sunlight is on that particular point. To illustrate this, take a flashlight and shine it straight down. Most of the light is focused in a circle on the floor. Now, slowly rotate you wrist so the light shines at an angle. The same amount of light now covers a larger area and is a little less intense in that same spot. This is what happens to the sun's energy as the Earth is tilted away or towards the sun. When the northern hemisphere is tilted away from the sun (winter) the sun is lower in the horizon resulting in a greater sun angle.
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