Constellations in the Sky Between sunset and sunrise on a clear night, we can see some 3000 points of light. Include the view from the opposite side of Earth, and nearly 6000 stars are visible to the unaided eye. A natural human tendency is to see patterns and relationships between objects even when no true connection exists, and people long ago connected the brightest stars into configurations called constellations. In the Northern Hemisphere, most constellations were named after mythological heroes and animals. Figure P.1 shows a constellation especially prominent in the northern night sky from October through March: the hunter Orion, named for a mythical Greek hero famed, among other things, for his amorous pursuit of the Pleiades, the seven daughters of the giant Atlas. According to Greek mythology, in order to protect the Pleiades from Orion, the gods placed them among the stars, where Orion now nightly stalks them across the sky. Many other constellations have similarly fabulous connections with ancient cultures.

Figure P.2 The Celestial Sphere Planet Earth sits fixed at the hub of the celestial sphere, which contains all the stars. This is one of the simplest possible models of the universe, but it doesn’t agree with the facts that astronomers now know about the cosmos.

The stars making up a particular constellation are generally not close together in space. They merely are bright enough to observe with the naked eye and happen to lie in the same direction in the sky as seen from Earth. Figure P.1(c) illustrates this point for Orion, showing the true relationships between that constellation’s brightest stars. Yet despite the fact that constellation patterns have no real significance, the terminology is still used today. Constellations provide a convenient means for astronomers to specify large areas of the sky, much as geologists use continents or politicians use voting precincts to identify certain localities on Earth. In all, there are 88 constellations, most of them visible from North America at some time during the year.

The Celestial Sphere

Figure P.3 The Northern Sky A time-lapse photograph of the northern sky. Each trail is the path of a single star across the night sky. The duration of the exposure is about five hours. (How can you tell that this is so?) The center of the concentric circles is near the North Star, Polaris, whose short, bright arc is prominently visible. (AURA)

Over the course of a night, the constellations appear to move across the sky from east to west. However, ancient sky-watchers noted that the relative positions of stars remained unchanged as this nightly march took place. It was natural for those first astronomers to conclude that the stars were attached to a celestial sphere surrounding Earth—a canopy of stars like an astronomical painting on a heavenly ceiling. Figure P.2 shows how early astronomers pictured the stars as moving with this celestial sphere as it turned around a fixed, unmoving Earth. Figure P.3 shows how stars appear to move in circles around a point in the sky very close to the star Polaris (better known as the Pole Star or the North Star). To the ancients, this point represented the axis around which the celestial sphere turned.