[Music] Ready for more star gazing? Last time, we talked about constellations -- clusters of stars in the sky that appear
together in a particular pattern and have been given a super cool name
like Draco or Hercules or Pegasus. And you might also remember that
astronomers can use constellations to help them find objects in the night sky. After all, a big shape in the
night sky made of lots of stars is way easier to find than a single, sparkly dot. But what if the thing you're trying
to spot is a constellation? Are all of the constellations always visible
in the sky from everywhere on Earth? Nope! To understand why, let's see how constellations
appear to move across the night sky. [Music] For one thing, the constellations you can see depend on where you are on
the Earth when you're looking. People above the Equator in the Northern
Hemisphere see different constellations than people below the equator
in the Southern Hemisphere. For example, this is Ursa Major
or the "Great Bear" constellation. Ursa Major is visible high in the sky in
most parts of the Northern Hemisphere. And this is Crux, sometimes
called the "Southern Cross," the smallest of the constellations. Crux is highly visible in the Southern Hemisphere. So, why is Ursa Major not
as visible below the equator and Crux not as visible above the Equator? [Music] Well, the Earth is round and
rotates on a tilted axis. So, certain parts of the world have better views
of certain constellations than other parts do. Observers at the North Pole, for example, can really only see constellations
visible north of the Equator. They can't see the ones below that
because the Earth is in the way. But, if you're lucky enough
to live near the Equator, you can see all the constellations
over the course of a year, which brings us to another point. The constellations you can see don't just
depend on where in the world you are. It also matters what time of
year you're looking for them. To see why this is, let's take
a look at Earth from space. Hey there home planet! Here's the Sun over here, and all those
twinkly things are stars and constellations. You'll notice the right side of the
Earth is darker than the left side. That's because it's nighttime
on that side of the Earth. So, people on this side of the planet have a great
view of the stars and constellations over here. As the Earth rotates on its axis, whichever part of the planet
is facing away from the Sun has a great view of these
stars during this season. Let's say it's winter if you're
in the Northern Hemisphere and summer if you're in the Southern Hemisphere. But, as you know, the Earth
isn't just spinning on its axis. It's also revolving around the Sun. So, as the Earth moves around the Sun,
the seasons change from, say, winter to spring, or in the Southern Hemisphere, summer to fall. And as the seasons change, so do the
constellations that are visible at night. So several months later, you're now
seeing these stars in this part of space. And, as the Earth continues around the Sun, the seasons change from spring to summer
or fall to winter in the south. And these stars are now visible to us at night. And finally, as that season fades and
the Earth continues its path around the Sun, giving us this group of stars. So, where the Earth is around the Sun
affects what season it is, and therefore, what constellations are on
display in the night sky. [Music] And that helps us tackle our big question, how do the constellations appear
to move across the night sky? Their change in position during the night and over the year is actually a
result of the Earth's motion. Depending on where in the world
you are and when you're looking, different constellations will have
moved into view in the night sky. What constellations are visible
where you live this time of year? To find out, just grab a few friends,
maybe some nice hot chocolate, and wait for dark to do some serious star gazing. [Music]
