On a clear, moonless night far from city lights, several thousand stars are visible to the naked eye. The faint band of the Milky Way arcs across the sky, made of countless distant suns too close together to be seen separately. Planets can be picked out because they shine with a steady light, while true stars twinkle as their light passes through the turbulent air. Ancient observers grouped the brighter stars into constellations and used them to mark the seasons and guide their travels.
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