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[Sabrina] 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.  

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?  

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.

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.

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