When most people think about astronomy,
they usually think about stars.
So hopefully you've actually had a chance
to go out and observe the night sky.
I really hope you have 'cause it is
stunning, at least when it's clear
not cloudy and not a lot of light
pollution around.
So constellations are usually what we
think of
when dealing with stars in the sky.
According to the book, the constellation
is a group of stars named from antiquity.
To a modern astronomer, a constellation
is actually a region of sky
that contains those stars.
I realize it's a bit nit-picky, but I did
want to point it out.
You're hopefully familiar with at least
a couple of constellations,
for example the Big Dipper.
The Big Dipper is actually part of 
Ursa Major.
"Ursa" means "bear." So Ursa Major
is the Big Bear.
Now what constellations we actually see
at night changes throughout the year.
This is because of the orbit of the earth
around the Sun.
The Sun is so bright that during the day,
when it's up,
it actually causes the entire sky to glow.
If you remember back when talking
about light,
we discussed, "Why is the sky blue?"
It's all because of sunlight.
So we aren't really able to see the stars
during the day.
They are up. They do still exist, but we
don't actually see them.
So we have to wait until after sunset
to be able to see the stars.
And because the earth goes around the Sun,
what stars are up
at the same time as the Sun is going
to change throughout the year.
So for example, the constellation Orion,
which is notable because of the
three bright belt stars, is up during
the winter months.
The constellation Scorpius is up
during the summer months.
So we actually see this change of the sky
as the earth goes around the Sun.
As I mentioned already, the Big Dipper
is one
that hopefully you're familiar with.
It's always in the Northern part
of the sky.
You can actually use the Big Dipper
to help find Polaris, the North Star.
If you take these two stars,
they're the kind of front of the spoon,
so this would be the handle,
here's the bowl of the spoon.
Those two stars always point
to Polaris, the North Star.
And the Big Dipper is actually 
circumpolar.
What that means is it doesn't actually
go below the horizon.
It's actually up all year all the time.
So no matter when, as long as you can
identify the Big Dipper,
you're able to find True North.
Now although the stars of the Big Dipper
kind of appear
to all be the same distance,
they're really not.
Just looking up at the sky gives you
no sense of distance.
So for example, the closest one is
at 53 light years,
and the farthest is at 360,
and yet they all kind of seem like
they're the same distance.
This is actually an ongoing problem
that we have in astronomy
is trying to figure out exactly 
how far away these things are
just by looking up at them in the sky.
We do have many clever ways
of doing this,
but it is an ongoing problem.
In this image, we're looking at
a long exposure.
So what they did is they set up the camera
and opened up the shutters
to just allow it to record
for a couple of hours.
Each of these streaks that you see
are actually stars.
These are known as star trails.
This is showing how the earth is rotating.
Just off camera up here
is probably the North Star.
