Observing The Night Sky

Part I: The Celestial Sphere
The Sun and its system of planets are embedded in an enormous, pancake-shaped
assemblage of billions of stars known as the Milky Way galaxy. We view these stars from
a moving platform, Earth. From our vantage point, stars can be seen in every direction.
Introducing the Celestial Sphere
A powerful tool to help us orient ourselves to the night sky is known as the “Celestial
Sphere.” The Celestial Sphere is a giant, imaginary sphere centered on the Earth. The
stars in the Milky Way are so far away that they appear “fixed,” i.e., their positions
relative to one another don’t change noticeably. (In practice, they do move, but it takes
tens of thousands of years for the small changes to become visible to the unaided eye.)
Because of this, we can imagine (as ancient astronomers did) that the stars are all located
on a giant sphere with the Earth at the center.

Let’s sketch the Celestial Sphere. The small circle below represents Earth, with the
North Pole (NP) straight up, the South Pole (SP) straight down, and the equator running
through the middle. Draw a large circle representing the Celestial Sphere, centered on the
Earth. Make it take up the whole space! Now use a ruler to connect the North and South
Poles, and extend the line (the Earth’s spin axis!) until it touches the top and bottom
of the Celestial Sphere. Label the two points where the axis touches the Celestial Sphere
“NCP” and “SCP” (North Celestial Pole and South Celestial Pole). Like the Earth, the
Celestial Sphere also has an equator. Draw it in, and label it.

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