The Celestial Sphere

On the next clear night, leave the burdens of the day behind, dress warmly, and wander outside.  Find a place with a clear view of most of the sky and, if possible, away from direct light.  Look up. You will notice the sky takes on the appearance of a vast hemispherical dome with stars fixed to its inner surface.  If the Earth were transparent, you would see the stars on the other half of this starry dome, below your feet, and you’d get the impression you were standing at the center of a velvety-black sphere speckled with stars. Astronomers call this the celestial sphere.

The celestial sphere showing the position of the north celestial pole (NCP), south celes-tial pole (SCP) and celestial equator

The celestial sphere showing the position of the north celestial pole (NCP), south celes-tial pole (SCP) and celestial equator

While it appears the stars are fixed to this celestial sphere, they are in fact at very different distances, but you cannot directly see this simply by looking into the sky.  Ancient stargazers mused the stars may be tens or hundreds of miles away, and thought the stars were holes in the sky to let through the light of heaven.  Now we know more.  The stars are tens of trillions of miles away, and they are balls of burning gas sustaining themselves from the energy of nuclear reactions in the cores.

The horizon, where the sky appears to meet the Earth, and the zenith, directly overhead

The horizon, where the sky appears to meet the Earth, and the zenith, directly overhead

But let’s get back to the sky.  Surrounding you is the full circle at which the earth’s surface and the sky appear to meet.  This is called the horizon.  If you’re surrounded by structures, trees, and hills, it may be hard to see down to the horizon.  If you’re on a prairie or desert or the ocean, you should have little trouble seeing the sky down to the horizon.

The imaginary point on the celestial sphere that is directly overhead, and therefore 90 degrees above the horizon, is called the zenith.  The point that is 90 degrees below the horizon, which of course you cannot see, is called the nadir.

The imaginary points on the horizon which indicate the main directions, north, south, east, and west are known as cardinal points.  When you can, find out which way lies north.  This will come in useful later.  Use a compass, a smartphone with GPS, or ask a friend.