LAB 2: PLOTTING THE DAILY MOTION OF THE SUN

PURPOSE

To plot and discuss the Sun's apparent daily movement across the sky.

MATERIALS

plastic hemisphere

grease pencil

magnetic compass

hemisphere base diagram (attached)

transparent tape or stapler

cardboard sheet 20 cm x 20 cm (8 x 8 inches)

PREDICTION

Place the square rim of the plastic hemisphere flat on your desk. Imagine that the sky is the inside surface of the hemisphere. As an observer, you would be standing at the center of the circle at the base of the hemisphere. This is the spot marked X in Figure 1. You will draw the path of the Sun as it would appear from inside the hemisphere.

With the plastic hemisphere in front of you, choose a point on the base at one of the ridges, tag it as North, and label it “N.” Looking down on the dome and going clockwise from north, use a transparency pen to mark the other three ridges as East (E), South (S), and West (W).

2.1 a) Predict the following positions for the Sun for today by writing a letter on the dome. Use the letter r to show position of the Sun at sunrise, the letter n to indicate its position at noon; and the letter s to indicate its position at sunset.

b) Connect these points on the hemisphere with a curved line to show how you think the Sun will move across the sky on this day.

2.2 From what direction did the Sun rise this morning?

2.3 In what direction will the Sun set this evening?

2.4 Where in the sky is the Sun at noon?

2.5 How many hours of daylight will there be today?

You will repeat this activity another day. Meanwhile, store your hemisphere in a safe place to prevent loss, damage, or smudging.

OBSERVATION

1. Tape or staple the base diagram to the piece of cardboard. Then tape or staple the base of the hemisphere to the base sheet-cardboard combination so that the ridge marked “N” lines up with North on the base sheet. See Figure 2.

2. Place the hemisphere on a flat, horizontal surface in direct sunlight. With the aid of a magnetic compass, turn the hemisphere so the ridge marked “N” points north. NOTE: Be careful not to place your hemisphere near iron or steel objects since these metals will attract your compass needle and produce an inaccurate reading. Once the dome is set in place DO NOT MOVE IT! (Draw an outline around the cardboard with a piece of chalk just in case the hemisphere is accidentally moved.)

DO NOT STARE AT THE SUN IT CAN DAMAGE YOUR EYES.

Adapted from Project STAR, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138

3. Plot the Sun's position in the following way (see Figure 2):

a) Carefully move the tip of the grease pencil close to the plastic hemisphere but do not let the pencil touch the sphere.

b) Move the pencil around until the shadow cast by its tip fans directly on the + mark on the base sheet.

c) Touch the pencil tip to the dome and make a dot. The dot's shadow should fall directly on the + mark on the base sheet.

d) Repeat Steps (a) - (c) every hour from sunrise to sunset.

e) Connect the plotted points with a line. Draw this line on the inside of the hemisphere. Label the line with the date and time range. DO NOT ERASE THIS LINE.

DISCUSSION QUESTIONS

2.6 Discuss how the points and line you drew for Question 2.1 compare with the points and line plotted in this activity.

2.7 From what direction did the Sun rise?

2.8 Where in the sky was the Sun seen at noon?

2.9 In what direction did the Sun set?

When you have answered these questions, erase the line you drew for Question 2.1. Keep the line you plotted in step 3.

EXTENSION

1. Repeat this plotting of the Sun's daily motion on a clear day one month after the date of your original plot. Repeat this plotting for as many months as possible. Use a different color pen for each month.

2. Refer to an almanac or a calendar to determine the first day of each season. Plot the daily motion of the Sun on the hemisphere for these days. Use a different color pen for each day.

Adapted from Project STAR, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138