If we extend this imaginary line outward from Earth, the points where this line intersects the celestial sphere are called the north celestial pole and the south celestial pole. This is Earth’s axis, and Earth rotates about this line. Imagine a line going through Earth, connecting the North and South Poles. To help orient us in the turning sky, astronomers use a system that extends Earth’s axis points into the sky. (In reality, it is Earth that rotates.) (Credit: ESO/Iztok Bončina) This long-exposure photo shows trails left by stars as a result of the apparent rotation of the celestial sphere around the south celestial pole. Celestial Poles and Celestial Equatorįigure 3: Circling the South Celestial Pole. Rather, they are small pieces of cosmic dust, burning up as they hit Earth’s atmosphere.) We can use the fact that the entire celestial sphere seems to turn together to help us set up systems for keeping track of what things are visible in the sky and where they happen to be at a given time. (This is because they are not stars at all. Only meteors-brief “shooting stars” that flash into view for just a few seconds-move appreciably with respect to other objects on the celestial sphere. During a single night, even objects we know to have significant motions of their own, such as the nearby planets, seem fixed relative to the stars. A grouping of stars such as the Big Dipper has the same shape during the course of the night, although it turns with the sky. There is even a special theater, called a planetarium, in which we project a simulation of the stars and planets onto a white dome.Īs the celestial sphere rotates, the objects on it maintain their positions with respect to one another. Nevertheless, it is sometimes still convenient to talk about the celestial dome or sphere to help us keep track of objects in the sky. Today, we know that these celestial objects are not really on a dome, but at greatly varying distances from us in space. It is because Earth turns on this axis every 24 hours that we see the Sun, Moon, and stars rise and set with clockwork regularity. We can put an imaginary stick through Earth’s North and South Poles, representing our planet’s axis. Today, we know that it is not the celestial sphere that turns as night and day proceed, but rather the planet on which we live. The apparent motion of celestial objects in the sky around the pole is shown by the circular arrow. Note that Earth in this picture has been tilted so that your location is at the top and the North Pole is where the N is. In reality, it is Earth that turns around this axis, creating the illusion that the sky revolves around us. Here we show the (imaginary) celestial sphere around Earth, on which objects are fixed, and which rotates around Earth on an axis. From the sea or a flat prairie, it is easy to see the horizon as a circle around you, but from most places where people live today, the horizon is at least partially hidden by mountains, trees, buildings, or smog.įigure 2: Circles on the Celestial Sphere. The top of that dome, the point directly above your head, is called the zenith, and where the dome meets Earth is called the horizon. Gazing up, you get the impression that the sky is a great hollow dome with you at the center (Figure 1), and all the stars are an equal distance from you on the surface of the dome. If you go on a camping trip or live far from city lights, your view of the sky on a clear night is pretty much identical to that seen by people all over the world before the invention of the telescope. Let us, therefore, take a look at the steps by which we reevaluated the place of our world in the cosmic order. One of the great themes of our intellectual history is the overthrow of the geocentric perspective. However, the geocentric view happens to be wrong. Furthermore, the geocentric perspective reinforced those philosophical and religious systems that taught the unique role of human beings as the central focus of the cosmos. After all, it is simple, logical, and seemingly self-evident. This geocentric (Earth-centered) view was what almost everyone believed until the European Renaissance. Our senses suggest to us that Earth is the center of the universe-the hub around which the heavens turn. Understand the modern meaning of the term constellation.Describe how motions of the Sun, Moon, and planets appear to us on Earth.Describe how motions of the stars appear to us on Earth.Explain the system astronomers use to describe the sky.Define the main features of the celestial sphere.By the end of this section, you will be able to:
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