Orbits
Earth and Space

Does the Earth move? photographing the night sky

Practical Activity for 14-16 PRACTICAL PHYISCS

Demonstration

Using a time exposure photograph to illustrate the apparent motion of the stars.

Apparatus and Materials

  • Tripod, or other means of holding camera still during exposure
  • Camera with B (open shutter) setting

Health & Safety and Technical Notes

Use a torch when setting up the camera and tripod. If students do this at home, they should make arrangements with parents or guardians to do it in a safe place.

Read our standard health & safety guidance


It is a good idea to cool down the camera by leaving it outside for some time before the exposure is set up, so that no condensation forms inside the camera.

The photograph will be more impressive if the picture includes the silhouette of the school building or of well-known trees near by. Avoid doing this at a time of month near a full Moon.

To get a B (Bulb) setting (open shutter) on a digital camera, you need to have your camera on manual setting and then decrease shutter speed. You will also need a cable release that you can lock. Otherwise the shutter only stays open as long as you keep your finger down on the button!

Have the lens aperture as wide open as possible so that you photograph more than just the brightest stars.

A digital camera or colour film will show the different colours of the stars.

Use a torch when setting up the camera and tripod. If students do this at home, they should make arrangements with parents or guardians to do it in a safe place.

Procedure

  1. Take a photograph of the night sky by exposing a film in a rigidly fixed camera for an hour or more, and make it available for discussion.
  2. To take such a photograph, attach a simple camera with an ordinary lens (not telephoto) to a firm stand or tripod. Point it towards the Pole Star, open the shutter on a setting that keeps it open indefinitely (though the aperture will usually have to be found by trial), and leave undisturbed for the period chosen (at least 2 hours, preferably 4 to 8 hours).
  3. Encourage students who are interested to make a photograph themselves.

Teaching Notes

  • The photograph will show arcs of a circle as the stars in the northern hemisphere appear to revolve around the Pole Star. The length of the arc, as a fraction of the circumference of the circle of which it forms a part indicates the time for the exposure as a fraction of 24 hours.
  • For the southern hemisphere, there is no bright star close to the celestial pole. The southern pole star, Sigma Octantis, is only of the 5th magnitude, so the direction to point the camera will have to be judged from other stars.

This experiment was safety-tested in April 2007

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