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The Celestial Sphere

The Celestial Sphere

Early observers reasoned that when they looked at the sky they were standing still because their senses gave them no signs that they were moving. They feltas if they were standing still. Therefore, they interpreted the changing positions of celestial objects to mean that the celestial objectswere moving. They visualized all celestial objects as revolving around a motionless Earth.

One effect of apparent daily motion is that the sky appears to move as if it were a single object. Here's a simple analogy. If you watch a truck with "Moving Van" painted on its side roll past you, the word "Moving" doesn't get closer to the word "Van" just because the truck is moving in that direction. All of the letters in the two words stay in a fixed pattern even though they are all moving because they are part of a single object ­ the sign. In much the same way, the stars in the sky stay in a fixed pattern even as you observe them moving through the sky.

It is not surprising, then, that early observers imagined that the sky wasa single object ­ a huge dome. Since the "dome" of the sky was in motion, and new parts would come into view as others dropped out of sight, these observers imagined that the dome extended beyond the horizon. As they followed through on this model, they realized that, if the dome were extended far enough, it would form a hollow ball, or sphere, surrounding Earth. They imagined a huge "sky ball," or celestial sphere,slowly spinning around a motionless Earth. The these observers the Sun, Moon, and stars were either holes in the celestial sphere or objects attached to it.

The celestial sphere, an imaginary sphere surrounding the Earth. The most you see at any one time is half of this sphere. Certain reference points on the celestial sphere are defined in relation to reference points on Earth. The celestial poles lie directly over Earth's poles; the celestial equator lies over Earth's equator midway between the celestial poles. Other points are defined by their positions in relation to the observer; the zenith is a point directly above the observer, the celestial meridian is the circle that runs through the celestial poles and the zenith. As Earth rotates from west to east, all objects in the sky appear to move from east to west, revolving around the north celestial pole.

The "celestial sphere" was a nice model because it accounted for many observations. It explained why objects appeared, arced across the sky, disappeared, and then reappeared the next day. Imagine it as a ball tied to a rope and swing in a circle around your head. First the ball arcs across your line of sight as you swing it in front of you, next it disappears as it swings around behind you, and then it reappears as it swings around in front of you again. This model explained why all of the celestial objects moved in the same direction at the same speed. It also explained why the stars remained in fixed positions relative to one another. This Earth­centered, or geocentric,model of the universe was used successfully for thousands of years to explain most observations of celestial objects.

Even though we now know that the motion of celestial objects is due to Earth's rotation, it is still sometimes useful, when discussing objects in the sky, to think of them as part of a sphere surrounding Earth. The most that an observer would see at any one time would be half of this sphere; but we still refer to this imaginary half­sphere, or dome, visible over our heads as the celestial sphere. The circle formed by the intersection of the celestial sphere and the ground is called the horizon.The point on the celestial sphere that is right over an observer's head at any given time is the zenith.The imaginary circle that passes through the north and south points on the horizon and through the zenith is the celestial meridian.

A Simple Celestial Coordinate System

A useful coordinate system for locating objects on the celestial sphere can be set up by projecting Earth's Equator and poles onto the sky. As shown in the figure below, Earth's Equator, North Pole, and South Pole correspond to a "celestial equator" and "north and south celestial poles" on the celestial sphere. Celestial objects can be located in the sky by their positions in relation to these celestial reference points.

Projection of Earth's Latitude­Longitude System onto the Celestial Sphere.

The star Polaris is located very close to the north celestial pole, making it a convenient reference point for determining the north­south positions of celestial objects in the Northern Hemisphere. Polaris can be located by following the "pointer stars," Dubhe and Merak, in the bowl of the Big Dipper in the constellation Ursa Major.

The "Pointer Stars," Dubhe and Merak, in the Bowl of the Big Dipper. Use these two stars to find the North Star, Polaris, and also to judge angular distances; they are about 5oapart.

Another way to estimate angular distances is to hold a fist out at arm's length; the fist marks about 10o.

A convenient reference point for determining the east­west positions of objects on the celestial sphere is the Sun. Objects to the west of the Sun on the celestial sphere will "rise" before the Sun and "set" before it. Likewise, objects to the east of the Sun trail behind it and will "rise" after the Sun and "set" after it.

The Sun's path on the solstices and the equinoxes. The Sun follows the celestial equator.

DMU Timestamp: February 15, 2016 23:03





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