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Questions on the Earth in Space

Q15.

On a clear night, a camera was set up on a fixed stand pointing at the Pole Star. The camera shutter was opened and kept open for a number of hours. The diagram shows the paths of a number of stars appearing in the photograph.

(a)

(i) In the photograph, most of the stars appear as curved lines instead of dots. Why do the stars appear as curved lines?

The stars appear as curved lines because the Earth was spinning on its axis as it photographed the stationary picture of the stars.

Stars appear to move in a circle over a period of 24 hours - because we see them from a slightly different perspective as time progresses.

1 mark

(ii) The Pole Star appears as a bright dot in the middle, not as a curved line. Why does the Pole Star appear as a dot?

The pole star is positioned on the Earth's axis of rotation - it is directly above the North Pole of the Earth - therefore it does not appear to move at all.

1 mark

(b) Study the diagram carefully. For how many hours was the camera shutter kept open? (give your answer to the nearest hour).

Using a protractor you can measure the angle subtended by the arc that the star has drawn out - how much of the full circle the star would draw out in 24 hours has been completed.

To do that draw dashed lines from the end of any one arc to the centre (pole star) - see diagram and measuer the angle between them.

24 hours would be 360o

1 hour would be 15o

The angle subtended by the arc drawn by the path of the star is 93o - therefore the camera shutter has been open for just over 6 hours... in would need to be an angle of 105o for seven hours - so the answer is 6.

6 hours

 

1 mark

Maximum 3 marks