ASTR 100 Diameter of the Sun Activity (Project 4)

ASTR 100 Diameter of the Sun Activity (Project 4)
  • Description

Brief Overview of Activity: A pinhole can form an image in much the same way as a lens. Measuring the size of the Sun’s projected image and the distance between the pinhole and the image, you will be able to calculate the diameter of the Sun.

Required Items: a friend to help you, a broom handle (or mop handle or long straight piece of wood of similar dimensions), a ruler (marked in centimeters), two envelopes, a pencil, masking tape, one stick-pin.

Number of Observations needed: 1

Timing of Observations: near noon on a bright sunny day


Preparation: Use the stick-pin to poke a small hole near the center of one of the envelopes. Mark a location near the top of the broom handle with masking tape (this is where your friend will hold the envelope with the pin-hole). Mark another location near the end of the broom handle with masking tape (this is where you will observe and mark the image). Carefully measure the distance between your two marked locations on your broom handle. Make your measurement to the nearest 0.1 centimeter and record here: __________.

Observation: Gather your friend, marked broom handle, two envelopes, pencil, and then head outside. With your friend holding the envelope with the pin-hole at the upper marked position and you holding the other envelope at the lower marked location, align the broom handle such that a small faint image of the Sun’s disk is seen on the lower envelope. You may find it convenient to actually sit on the ground for this procedure. With a pencil, carefully mark the location of opposite sides of the Sun’s disk.

Calculation: From your marked envelope, carefully measure the size of the projected image of the Sun’s disk to the nearest 0.1 centimeter and record here: __________.

Next, use the relationship below to calculate the Sun’s diameter in kilometers. Note that the distance to the Sun is 1.5 x 10 8 km.


Sun’s diameter in kilometers                      image diameter in centimeters

—————————————————-         =       ————————————————

Distance to the Sun in kilometers             distance between image and pin-hole in cm

Substituting the values;

Sun’s diameter in kilometers                      40.5 cm

——————————————–        =        ———————

(1.5 X 108)                                            0.4 cm


Sun’s diameter =   (1.5 X 108) X 0.4



Record your calculated value for the diameter of the Sun

= ________________________


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