WHEN YOU LOOK AT THE NORTHERN SKY, you look away from the densely populated Galactic center, so the northern sky generally appears less bright than the southern sky. Among the best-known sights in the northern sky are the constellations, Ursa Major (the Great Bear), and Orion. Some ancient civilizations believed that the stars were fixed to a celestial sphere surrounding the Earth and modern maps of the sky are based on a similar idea.
The North and South Poles of this imaginary the celestial sphere is directly above the North and South Poles of the Earth, at the points where the Earth’s axis of rotation intersects the sphere. The celestial North Pole is at the center of the map shown here, and Polaris (the North Star) lies very close to it. The celestial equator marks a projection of the Earth’s equator on the sphere. The ecliptic marks the path of the Sun across the sky as the Earth orbits the Sun. The Moon and planets move against the background of the stars because the stars are much more distant; the nearest star outside the solar system (Proxima Centauri) is more than 50,000 times farther away than the planet Jupiter.
