The Celestial Sphere
From the surface of Earth, it seems as if all the stars are attached to a gigantic sphere in outer space.
![](http://jrjohnson.net/wp-content/uploads/2015/01/CelSph.jpg)
We call this imaginary sphere The Celestial Sphere.
Now the way the stars move across the sky every 24 hours (every 23 hours 56 minutes) seems to suggest one of 2 things: either the Celestial Sphere is stationary and the Earth spins... or, the Earth is stationary and the Celestial Sphere spins.
Below shows us the 2 versions. On the left we see the Earth is spinning... on the right it seems as if the Earth stays still and the Celestial Sphere is spinning.
Now the way the stars move across the sky every 24 hours (every 23 hours 56 minutes) seems to suggest one of 2 things: either the Celestial Sphere is stationary and the Earth spins... or, the Earth is stationary and the Celestial Sphere spins.
Below shows us the 2 versions. On the left we see the Earth is spinning... on the right it seems as if the Earth stays still and the Celestial Sphere is spinning.
Now we know on the Earth we have The North Pole, The South Pole, and The Equator.
![](http://need-media.smugmug.com/Graphics/Graphics/i-2snwCCq/1/L/earth%20Equator-L.jpg)
NORTH CELESTIAL POLE
If a person at the Earth's North Pole were to shine a torch straight up and it hit the Celestial Sphere, the point where it hits would be the North Celestial Pole. If the Celestial Sphere were actually spinning (not the Earth) then the North Celestial Pole would correspond to the point in the sky that doesn't move in circles (like the centre of a fan or the bit of a basketball where the person's finger is while spinning it). All the stars in the northern part of the sky would move in circles around this point.
SOUTH CELESTIAL POLE
If a person at the Earth's South Pole were to shine a laser torch straight up (which is down if you are an alien looking at the Earth from the standard position of North Pole at the top) and let it hit the Celestial Sphere, then the point it hits would be called the South Celestial Pole. If the celestial sphere was spinning, it would correspond to the other point (combined with the North Celestial Pole) that doesn't move in circles but just spins around.
CELESTIAL EQUATOR
If a person at the equator were to shine a torch directly up, then as the Earth did a 360 degree spin over a day, it would point to a whole lot of different places in the sky. All the points the laser pointed at would be on a circle of the celestial sphere that is equal distances from each pole and corresponds to the 'middle' of the celestial sphere (if the poles represent top and bottom). This circle on the Celestial Sphere is known as the Celestial Equator.
If a person at the Earth's North Pole were to shine a torch straight up and it hit the Celestial Sphere, the point where it hits would be the North Celestial Pole. If the Celestial Sphere were actually spinning (not the Earth) then the North Celestial Pole would correspond to the point in the sky that doesn't move in circles (like the centre of a fan or the bit of a basketball where the person's finger is while spinning it). All the stars in the northern part of the sky would move in circles around this point.
SOUTH CELESTIAL POLE
If a person at the Earth's South Pole were to shine a laser torch straight up (which is down if you are an alien looking at the Earth from the standard position of North Pole at the top) and let it hit the Celestial Sphere, then the point it hits would be called the South Celestial Pole. If the celestial sphere was spinning, it would correspond to the other point (combined with the North Celestial Pole) that doesn't move in circles but just spins around.
CELESTIAL EQUATOR
If a person at the equator were to shine a torch directly up, then as the Earth did a 360 degree spin over a day, it would point to a whole lot of different places in the sky. All the points the laser pointed at would be on a circle of the celestial sphere that is equal distances from each pole and corresponds to the 'middle' of the celestial sphere (if the poles represent top and bottom). This circle on the Celestial Sphere is known as the Celestial Equator.
![](http://www.keplersdiscovery.com/Images/Celestial-sphere.jpg)
Some people like to explain it this way. If you could blow up the Earth like a balloon until it was the size of the Celestial Sphere, then the Earth's North Pole would match up with the North Celestial Pole and the Earth's South Pole would match up with the South Celestial Pole and the Earth's Equator would match up at every point with the points of the Celestial Equator.
The Celestial Poles and the Celestial Equator can be found easily because:
The Celestial Poles are important because:
The Celestial Equator is important because:
The Celestial Poles and the Celestial Equator can be found easily because:
- all the stars either move in circles around the North or Celestial Pole
- the stars on the Celestial Equator move around in the biggest circles of all and are equal distances from each pole.
The Celestial Poles are important because:
- being the pivot points of the apparently moving celestial sphere, they correspond to the Earth's poles. On Earth, you can verify you are at the North Pole (or South Pole) because when you look straight up, you will see the Celestial Pole with all the stars moving around it.
The Celestial Equator is important because:
- it is equal distances from each pole... when you are on Earth's equator, the North and South Celestial Poles should be on your horizon with half the stars above you going around the NCP and half the stars going around the SCP.
- A sailor will know they have reached the equator because the stars will behave in this way.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/3/30/Globespin.gif/220px-Globespin.gif)
The North and South Pole of Earth are the 2 points which point move around in circles as the Earth spins, but simply rotate around. They correspond to the centre of a spinning fan (pivot point) and the point on a spinning basketball where a person's finger is (or the point on top which is opposite this). The North and South Pole are opposite each other. This means that if you put a massive stick through the North Pole that came out at the South Pole, this stick would go through the centre of the Earth. Such an imaginary stick is called the Earth's Axis.
![](http://www.bbc.co.uk/staticarchive/2572f798d515d6c361e25f0476a631e8fa2db8e7.gif)
We say the Earth rotates on its axis to mean that as the Earth rotates in such a way as if someone (a giant bigger than Jupiter) had a hand on either side of this massive stick and was spinning it to make the Earth spin.
Now, the equator is a set of points that move in larger circles than any other point on Earth as it spins. In the picture below, Notice the white arrow shows the circular motion of points at the equator. Note these points are equal distances from the poles.
Now, the equator is a set of points that move in larger circles than any other point on Earth as it spins. In the picture below, Notice the white arrow shows the circular motion of points at the equator. Note these points are equal distances from the poles.
![](http://hyperphysics.phy-astr.gsu.edu/hbase/solar/picsol/earthprecess.jpg)
Now we know that Earth spins and the celestial sphere (which isn't even real) stays still.
![](http://upload.wikimedia.org/wikipedia/commons/1/12/Earth_within_celestial_sphere.gif)
But to someone on Earth, it seems as if the Earth is still and the celestial sphere is moving.