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2.1 The sky above

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Learning objectives

By the end of this section, you will be able to:

  • Define the main features of the celestial sphere
  • Explain the system astronomers use to describe the sky
  • Describe how motions of the stars appear to us on Earth
  • Describe how motions of the Sun, Moon, and planets appear to us on Earth
  • Understand the modern meaning of the term constellation

Our senses suggest to us that Earth is the center of the universe—the hub around which the heavens turn. This geocentric    (Earth-centered) view was what almost everyone believed until the European Renaissance. After all, it is simple, logical, and seemingly self-evident. 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. However, the geocentric view happens to be wrong. One of the great themes of our intellectual history is the overthrow of the geocentric perspective. Let us, therefore, take a look at the steps by which we reevaluated the place of our world in the cosmic order.

The celestial sphere

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. Gazing up, you get the impression that the sky is a great hollow dome with you at the center ( [link] ), and all the stars are an equal distance from you on the surface of the dome. 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 . 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.

The sky around us.

Diagram of the Horizon and the Zenith. In the center of this illustration a human figure stands looking upward. She is standing at the center of a series of concentric circles representing the ground, the outermost circle is labeled the “Horizon”. The sky is represented as a dome enclosing the figure and the ground the figure stands on. Thus, the dome meets the ground at the horizon. A line is drawn vertically upward from the figure to the top of the dome directly over the figure’s head, and is labeled the “Zenith”.
The horizon is where the sky meets the ground; an observer’s zenith is the point directly overhead.

If you lie back in an open field and observe the night sky for hours, as ancient shepherds and travelers regularly did, you will see stars rising on the eastern horizon (just as the Sun and Moon do), moving across the dome of the sky in the course of the night, and setting on the western horizon. Watching the sky turn like this night after night, you might eventually get the idea that the dome of the sky is really part of a great sphere that is turning around you, bringing different stars into view as it turns. The early Greeks regarded the sky as just such a celestial sphere    ( [link] ). Some thought of it as an actual sphere of transparent crystalline material, with the stars embedded in it like tiny jewels.

Circles on the celestial sphere.

Circles on the Celestial Sphere. At the center of this figure the Earth is shown with the Equator, North, and South poles labeled. The Earth is tilted so that the North Pole is pointing toward the upper right. The Earth is embedded within a sphere representing the sky. A white line is drawn projecting from the North Pole onto the sky, at which point it is labeled the “North celestial pole”. A white circular arrow is drawn counter-clockwise around the North celestial pole indicating the apparent motion of the stars. The equator is projected onto the sky, drawn in white and is labeled the “Celestial equator”. An oversized human figure stands in North America, with a vertical line drawn upward and intersects the sky sphere at a point labeled “Your zenith”, and is drawn in yellow. The horizon as seen from the vantage point of the figure is projected onto the sky, labeled “Your horizon”, and is drawn in red. This horizon line splits the sky roughly in half from the observer’s point of view.
Here we show the (imaginary) celestial sphere around Earth, on which objects are fixed, and which rotates around Earth on an axis. In reality, it is Earth that turns around this axis, creating the illusion that the sky revolves around us. 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. The apparent motion of celestial objects in the sky around the pole is shown by the circular arrow.
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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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