With the help of this method we
measure the distance of planets
and stars which are very far away from Earth. To understand this method, we
have to do an exercise. First of all, wherever you are, straighten your hand
with your face. Now close one of your eyes and look at the thumb of your hand
with the other eye, then without moving your hand with the eye you are looking
at now close it and look at your thumb with the other eye. Do you feel like
your thumb has moved from its place to another place? This happens because
there are a few centimeters between your eyes and when you look at that thumb
with separate eyes, the angle of vision formed on your eye with that thumb
changes and this phenomenon is called parallax. And we call the change in the position of the body by
looking with a separate eye as parallax shift.
Repeat the same activity by
bringing your thumb closer to your nose and then moving away from it. In this
case, you will notice that as the thumb gets closer to your nose, the parallax
change increases. And as the thumb
moves away from your nose the parallax change decreases. This means that the
parallax change of a distant body is less than that of a nearby body. It is also
possible that we may not notice this parallax change. A simple example of this
is that whenever we look at the sun while traveling in a bus or car, we see it
moving with us while the houses and trees along the road recede.
As the sun is so far
away from us, the parallax change in it is about a millionth of an arc degree
used in the measurement of a circle.However,
due to the closeness of houses and trees, the parallax change produced in them
reaches 180 arc degrees very quickly, and because of this, we see the trees
going back and the sun moving with us.With the
help of this activity we try to measure the distance of a distant star.
Using this method, we can only measure the distance of stars within 350
light-years and this range depends on the capabilities of the telescope.
If we want to measure the distance of more distant stars,
then we have to use other methods. This method is very important in astronomy.
Imagine that there is a star at
a place called P. First of all, we take an image of that star
with the help of a telescope
at any point A on Earth and save it. We all know that our Earth completes one
revolution around the Sun in twelve months Then after six months
when our earth will reach the east side of the station A on the other side of
the sun at the station B. Then once
again we will get the image of that star. As we have seen earlier, some parallax change occurs in
the thumb by looking at the different positions of the eye. Similarly, we
should see the parallax change in the images obtained of the star
from two different positions of the Earth. Now in these two images we do not
see any parallax change in the background of star
P which is thousands of light years away from the sun.And
these stars will be in the same places in both these pictures.But
if the stars whose distances we want to measure are within 350 light years, we
will see a parallax shift in them.And we
measure the change in the parallax angle of the star
by comparing it with the stars in the background of the two images obtained
with the telescope.
Suppose that for a star
this change comes to 0.62 arc seconds. If we convert this change to radians, it will be
0.000003 radians. Now, to measure the distance of this star from the Earth, we
will imagine a circle whose center is at the point P of this star and the
points A and B of the earth are on its circumference. And the radius of this
circle will be the distance from the earth to that star. From the basic equations of geometry, we know that for
any circle, the radius of the circle is equal to the arc between two points on
its circumference and the ratio of the angles formed between these points. This
Means the radius is obtained by dividing the arc by the angle. radius = arc/angle
We
already know that the angle of parallax formed between these two points A and B
of the Earth is 0.000003 radians and the distance between these two points (A
and B) will be twice the distance from the Earth to the Sun.
We
know that the distance from the earth to the sun is equal to an astronomical
unit whose value in kilometers is 14,95,97,871 km. And by doubling this value,
the value will be 29,91,95,742 km. If we divide 29,91,95,742 km by 0.000003
radian, the resulting distance from Earth is the distance of the star,
which is approximately 10 thousand billion100000000000000 km
and if we divide this distance by 946,10,30,00,000 km to convert it into light
years, the value obtained is 10 light years. And this value is the distance of the star
called 61 Cygni from our earth, which was found by Friedrich Bessel for the
first time
around 180 years ago using the parallax method with the help of a heliometer.
This means that our star is P 61-Cygni.
Read Also:
3. Composition of Earth's
Atmosphere
