Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Does the Earth move? photographing the night sky

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.

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


a 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.
 
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).

b Encourage students who are interested to make a photograph themselves.

time-lapse photo of the stars around the Pole Star. Photo by Lucy HollisPhoto by Lucy Hollis aged 15, assisted by her father. Photo taken in Bermuda. Exposure time about 45 minutes.

 

 

Teaching notes


1 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.
 
2 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.
changes in position of stars from November to DecemberA view of the stars through a window in Britain at the same time of night, but one month apart. Why have their positions shifted? You could also see them rotate through the same angle if you waited up on 6 November.
This experiment was safety-checked in April 2007