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Finding the distance using Parallax

Finding the distance using Parallax. With your arm outstretched, hold up a finger so that, when viewed with your right eye, it is in line with something at the end of the other end of the room Now, keeping your finger steady, close your right eye and look at the finger with the left eye.

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Finding the distance using Parallax

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  1. Finding the distance using Parallax • With your arm outstretched, hold up a finger so that, when viewed with your right eye, it is in line with something at the end of the other end of the room • Now, keeping your finger steady, close your right eye and look at the finger with the left eye. . Notice the apparent change in the position of your finger relative to the object at the end of the room. Norm Herr (sample file)

  2. . The position of your finger or of the object at the end of the room have not changed but, because you look from a different place, they seem to move relative to each other. . This effect is called PARALLAX and it can be used to measure the distance to an object. . It is fairly easy to demonstrate in the classroom how parallax can be used to measure how far away certain stars are. Norm Herr (sample file)

  3. Finding the distance to nearby stars using Parallax: 1 . Set up the apparatus as shown so that the centre of the meter stick is directly in front of a spot on a distant wall in the lab and the stick is parallel to the wall. . View the spot through the straw. (Try to use relatively long straws if possible). . Read the angle between the metre stick and the straw using the protractor, which is under the straw (see inset top left). • Repeat this procedure at the other end of the metre stick. • The two angles should be very similar. Add and divide by two to get the average. Subtract this from 90 to get the angle P in degrees. Norm Herr (sample file)

  4. From the diagram we can see that, tan(P) = (0.5/distance). Therefore the distance = 0.5/tan(P) • Fill in the value for P to find your answer. Now measure the distance from the metre stick to the spot. • The two answers will be very close. • There are some obvious sources of error in this experiment, which lead to incorrect answers. Norm Herr (sample file)

  5. Finding the distance to nearby stars using Parallax: 2 • This experiment illustrates the basic idea involved in stellar parallax. • The first step is to identify a ‘nearby’ star by observing its parallax relative to the ‘fixed’ distant stars. • It is then possible to find angle P and, knowing the radius of the Earth’s orbit around the Sun, to calculate the distance from the Earth to the nearby star. Norm Herr (sample file)

  6. It is important to realise that the angles involved in this method are extremely small and so difficult to measure accurately. • For example, the parallax angle to the nearest star (other than the Sun, of course), Proxima Centauri, is 0.772 seconds of arc. • This is roughly the same as the angle subtended by an object of diameter 2 cm (e.g. a 5 cent coin) viewed from a distance of 5.3 km. Norm Herr (sample file)

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