The Size of the Sun

Objective

This acitivity will teach you how to make a pinhole camera, which can be used to project images of bright objects onto paper. You will use the camera during class to measure the size of the Sun. For homework you will use the camera to determine the size of the Moon. As of this moment the next full Moon is on May 9, 2009 but the Moon will be visible after dark as early as April 27th.

To successfully complete this activity you must answer the following questions:

  1. Using a candle, describe what happens to the image on a piece of paper when you move the paper farther away, holding the pinhole camera in the same location.
  2. Using a candle, describe what happens to the image on a piece of paper when you move the pinhole camera farther away, holding the paper in the same location.
  3. Using the pinhole camera, make a drawing of the outline of the Sun and write down the distance in cm between the paper and the pinhole camera. Make a measurement of the diameter of the Sun on the paper in cm. Using these data, calculate the actual diameter of the Sun in miles and kilometers.
  4. Are there any sunspots on the Sun? If there are, draw them on your sketch of the Sun.
  5. What are sunspots? What do they tell us about the activity of the Sun? (this questions will require further research such as at: http://www.exploratorium.edu/solarmax/index.html (Exploratorium page on solar activity) and http://cse.ssl.berkeley.edu/SegwayEd/lessons/sunspots/ (Berkeley Center for Science Education sunspots page).

Materials


Background

The distance from the Earth to the Sun is 9.3 × 107 miles or 1.50 × 108 kilometers. In normal decimal notation that is 93,000,000 miles and 150,000,000 kilometers. The distance from the Earth to the Moon is 2.39 × 105 miles or 3.84 × 105 kilometers. In normal decimal notation that is 239,000 miles and 384,000 kilometers.

A simple proportion can be used to calculate the size of the Sun or the Moon:


    diameter of image
  --------------------- X distance to object = true diameter of the object
     distance from
    pinhole to paper

The answer comes out in the same units used to express the distance to the object.

Sunspots are areas on the Sun’s surface that are cooler than the rest of the surface of the Sun. That is why they appear dark. They are caused by the magnetic field of the Sun which pokes straight up out of the surface of the Sun from sunspots. The magnetic field impedes the transport of heat from the interior of the Sun. Sunspots are roughly the same size as the Earth. They are associated with overall solar activtiy and follow an eleven-year cycle. The more sunspots there are, the more active the Sun is. When the Sun is more active it produced more solar flares and coronal mass ejections. Both of these types of event can cause disruptions to the power grid, ground communication and satellite communication. Also, when the Sun is more active the norther lights are more active since they are caused by the arrival of material from the Sun.



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Procedure

Make a pinhole camera like the one your teacher shows you. It is made as follows:

  1. Cut a hole about 2 cm square in the center of your cardboard or paper.
  2. Cover the hole with a square of aluminum foil about 3 cm square and tape the aluminum in place.
  3. Poke as small a hole as possible in the center of the aluminum foil using a pin or tack.

Test the pinhole camera as follows:

size.of.sun.setup (4K)
  1. When everyone in the class is ready, turn out the lights and light a candle (or two).
  2. Project an image of the flame on a white piece of paper by holding the paper about 10 cm away from the pinhole camera. Write down a description of what you see. Is the image upside-down? How big is the image compared to the size of the actual flame?
  3. Keeping the pinhole camera in the same location, move the paper farther away. What happens to the image?
  4. Keeping the pinhole camera and paper about 10 cm apart, what happens to the image as you move closer and farther away from the candle?

When everyone is ready, go outside as a class and measure the size of the Sun. Bring a notebook, a piece of white paper, the pinhole camera, a pencil or pen, and a meter stick.

size.of.sun.setup (4K)
  1. Hold the pinhole camera so that an image of the Sun falls on a white piece of paper. Try to make the distance between the camera and the paper as large as possible. (Why do you think you should do this?)
  2. Carefully draw the outline of the image of the Sun..
  3. Without moving the pinhole camera, measure the distance between the camera and the paper in cm.
  4. Measure the diameter of the outline of the Sun’s image on the paper in cm.
  5. Use the formula given in the background section to calculate the diameter of the Sun.

Grading

A short report answering the questions in the Objective section is due. There must be one report per person. Grading will be based on the attractiveness of your presentation, the accuracy of your results, and the amount of detail in your answers. If there are sunspots, a drawing showing them is also required. You will be given enough time before the report is due to research the answers to the questions outside of class time. The due date is M 11/09/2009.



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Homework Assignment

You can use this technique to measure the size of the Moon. Your assignment is to do exactly that. Sunrise/set and Moonrise/set data from the 26th of April to the 9th of May are listed below:

 Sunday    Monday    Tuesday   Wednesday Thursday  Friday    Saturday
+---------+---------+---------+---------+---------+---------+---------+
| 26      | 27      | 28      | 29      | 30      |  1      |  2      |
|SR:  5:40|SR:  5:39|SR:  5:37|SR:  5:36|SR:  5:35|SR:  5:33|SR:  5:32|
|SS: 19:38|SS: 19:39|SS: 19:40|SS: 19:41|SS: 19:43|SS: 19:44|SS: 19:45|
|MR:  6:06|MR:  6:54|MR:  7:53|MR:  9:03|MR: 10:19|MR: 11:36|MR: 12:52|
|MS: 22:06|MS: 23:17|MS: None |MS:  0:17|MS:  1:05|MS:  1:43|MS:  2:14|
|         |         |         |         |         |FQ: 16:45|         |
+---------+---------+---------+---------+---------+---------+---------+
|  3      |  4      |  5      |  6      |  7      |  8      |  9      |
|SR:  5:30|SR:  5:29|SR:  5:28|SR:  5:26|SR:  5:25|SR:  5:24|SR:  5:23|
|SS: 19:46|SS: 19:47|SS: 19:48|SS: 19:50|SS: 19:51|SS: 19:52|SS: 19:53|
|MR: 14:05|MR: 15:16|MR: 16:26|MR: 17:35|MR: 18:44|MR: 19:52|MR: 20:58|
|MS:  2:39|MS:  3:02|MS:  3:23|MS:  3:45|MS:  4:09|MS:  4:36|MS:  5:07|
|         |         |         |         |         |         |FM:  0:02|
+---------+---------+---------+---------+---------+---------+---------+

The times given are all correct for daylight savings time. MR means moonrise, SR means sunrise. MS means moonset, SS means sunset. You will need to view the Moon at night to use your pinhole camera so only times after sunset are appropriate. This should be possible on any date after April 27th. The phase of the moon will have no effect as long as you measure the true diameter of the moon. First quarter (the half-moon phase halfway between new moon and full moon) is on May 1st. The moon is full May 9th.

Measure the diameter of the moon and the distance between the pinhole camera and the paper using the same units (cm work well but any other unit is fine). Hand in your drawing of the moon, your measurements and your calculation of the true diameter of the moon. You must also answer one question: Why is the diameter of the moon different from the diameter of the sun? After all, they appear to be about the same size in the sky! Due May 11th.



Inspired by the similar activity at this page
at the At Home Astronomy Page of
The Center for Science Education at Space Sciences Laboratory, UC Berkeley.
Last updated: Nov 02, 2009 Home