Hi there.
Welcome to
the Cosmic Classroom.
Today we are going to
talk about Parallax, but
before talking about
Parallax, I need to
briefly explain to you
what are angular sizes.
What's an arc second,
what's an arc minute.
So let's start with that
before we move into Parallax.
If you would see
my first slide right here,
right there on, we have a
full circle and as you know
a full circle contains 360°.
Now imagine just looking
at one of this 360° it
would be something very
small, so you can't even
see that it's a pie shape
here because it's a little,
really small pie shape.
So one degree is a
pretty small angle.
However, objects in
the sky are so small
that they actually
appear smaller than that.
The moon and the sun
are the only things that
appear to be 1° across.
For that reason we
need to subdivide the
degree even further.
So we'll get this, this
1°, zoom way in and
subdivide this degree
into 60 equal parts.
Each one of those parts
will be called an arc minute.
So there are 60
arc minutes in 1°.
But objects are still
bigger than one arc
minute in general.
So we need to subdivide
that even further.
We subdivide one arc
minute,which you couldn't
possibly see here, into
60 equal parts again.
In each one of those
we call one arc second.
So there are 60 ARC
seconds in one arc minute.
This angular distance is
important because we can
measure both the size of
objects in the sky according
to the angle that they takes,
the angle that it takes in
the sky, but we can also
measure distances between
different objects in the sky.
So we'll get to that a
little bit more. Let's see why
is it that that's important.
So for example imagine a
flower, angular size of a flower.
The angular size is given
by this angle right here,
We use angular size when
we don't know the distance,
therefore we can say is
how big does it appear.
So this angle here is the
angle of size of the flower
when its far away and the
bigger angle is the,the
angular size of the flower
when it's close to you.
You can try it yourself.
Get your thumb, put right in
front of your face, and try
to cover me in your screen.
See if you can cover me.
Alright?
You probably won't be able
to because it's far from you.
So the angular size of
your thumb is pretty small
because it's far from you.
Now, if you get it
closer and closer
and closer to your
face you'll see that
the angular size
will increase,your
thumb will be able to
cover my entire face.
Maybe a good
part of the screen.
So the angular
size became bigger.
Things are closer the
angular size are bigger.
So the angular size tells
us something about the
distance from us,or from
the observant, in this case
your eyes to the object.
So you can use that
to measure, measure
distances in the sky.
You can use your,your
little finger to measure
about one arc minutes.
If you get three fingers
you, you are measuring
about five arc minutes.
A whole fist closed,you
will be measuring right
here right there
about ten arc minutes and
if you do your fingers like
this is about 15 arc minutes.
So you can use this to
measure distances
between stars in the sky.
So for example you can
observe a bit deeper,
and lets say you want
to show that to your
significant other you're
trying to impress them,
but the only star that you
can show in the Big Dipper
is this one right here.
Well, you can help them
find a star by saying,well
the other star is about
10 arc minutes from us.
So just look 10 arc
minutes ten arc
minutes east and you
should be able to see it.
So, so you can use
that, you can angular
measurements to measure
distances between objects.
Now let's understand
why is it that this is
important for parallax.
So, parallax is a method
used to the measure
distance to nearby objects.
When an object is close
enough to you,you'll see
it against the different
background depending on
where you see that object from.
So here is the Earth
orbiting the Sun.
So lets say here is the
Earth at,in June.
Right?
So here's the Earth in June.
If you observe this star
right here it'll be across,
it will be in front of a
certain background of stars.
If you take a pictures for, of
this star it will look like this.
However, if you wait six
months and now it's
December and you take
a picture of that same
star, it will appear against
a different background.
So it will appear as the
star moved, it really hasn't
moved, but it appears to,
to as if it has moved.
And again, you can try it.
So put your thumb again
in front of your face and
close one of your eyes.
As you close one of
your eyes try to notice
what's behind your finger.
Behind my finger is a killer.
Now I'll, I'll now
blink the other eye and
you'll notice that your eye,
your thumb will appear
to have moved with
respect to the background.
So do that a few times.
Your thumb hasn't really
moved it's just that you
see it from the, with the
right eye or you are
seeing with the left eye.
So it appears to have moved.
How much it appears to
have moved depends on
how close or far away
this finger is from you.
If a finger is very far
away from you, it will
be together with his
background and it will not
appear to have moved.
Another example of Parallax
can be seen every day.
Take a picture of a
building you may want
to change your angle.
Right?
Just reposition yourself
to take a better picture.
So in this case,this is
just moving a few steps
to the side you'll see
how the the post and
the bench appear to
move their positions.
Well the things are kind
of background like
the buildings, they remain
in the same place.
Nothing has really moved,
but it's apparent motion
tells you about how far
away you are from this post.
Going back to the Stellar
Parallax, if you want a little
bit of math to go with it,the
distance is related to this
angle called Parallax,which
is this angle right there.
So the distance is simply
one over the parallax angle.
But the, but this formula
holds if the distance to
the star is given in Parsecs.
[And I'm sorry that it cut
here, but it's in Parsecs]
And the parallax angle
is one arc seconds.
That's actually the
definition of a Parsec.
A star that appears to
move one arc second in
the sky has, it's at the
distance of one Parsec.
That was,how we
define a Parsec.
So one Parsec is the
distance from us to an
object that has a parallax
angle of one arc second
and it happens to be
also 3. 26 light years.
I hope that helped.
And let me know if you
have any more questions.
See you next time.
