Download PowerPoint on finding the distance to a star 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.
.
Notice the apparent change in the position of your finger relative to
the object at the end of the room.
11/15/99
Norm Herr (sample file)
.
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.
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Norm Herr (sample file)
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).
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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.
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Norm Herr (sample file)
• 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.
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Norm Herr (sample file)
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.
•
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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)
• 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.
11/15/99
Norm Herr (sample file)