Online Astronomy eText: The Sky
The Celestial Sphere
(also see The Moon Illusion and Astronomical Coordinates)

The Celestial Sphere
      (the Celestial Sphere, as the simplest way of showing the motion of the sky; comparison to planetarium dome)


The Celestial Sphere

'Observing' the Celestial Sphere
     The celestial sphere is an imaginary globe, centered on the observer, which allows him (or her) to visualize the positions of celestial objects in the sky in terms of their positions on the sphere.
     Normally, only the upper half of the sphere is visible, as the Earth is in the way, for objects below the horizon. In fact, depending upon whether there are buildings, trees, hills or other objects which rise above the horizon, that part of the sky which is visible may be less than half of the sphere. Technically, the dividing line between the part of the sky which is visible, and those objects on or near the horizon which block the view of the sky, is the skyline, but it is very common for the skyline to be confused with the horizon.
     Astronomically, the horizon is a perfectly horizontal circle, going all around the observer, near the skyline. Unlike the skyline, which can go up and down relative to the sky, the horizon is exactly halfway between the zenith -- the point directly above the observer -- and the nadir -- the point directly below the observer. The nadir is, of course, like the rest of the lower part of the celestial sphere, not normally visible, but its direction can be determined by dropping an object, and seeing how it falls. If it has no sideways motion, so that it falls "straight down", then it is moving toward the nadir. The opposite direction, or "straight up", is the direction toward the zenith.
     To determine the position of the astronomical horizon, try to visualize your "eye level" -- a horizontal plane, at the same height as your eye. If you look at the distant horizon/skyline, it is difficult to tell exactly where eye level is, but if you look at nearby trees or buildings, you can usually gauge fairly well, what height or position on those objects corresponds to your eye level. Your line of sight, extended at that height, all the way to the celestial sphere, represents one point on the horizon. If you were to turn all the way around, so that your eye-level line of sight swept all around the sky, that would define the astronomical horizon.
     Similarly, you can find the zenith by looking straight up; but as it happens, most people, when supposedly looking straight up, are actually looking a little forward of, or lower than straight up, because we aren't used to doing that, and it can be uncomfortable -- particularly if you have any neck or back problems -- to look straight up. To make sure you are looking at the Zenith, you can use the following procedure.
     Look or point at what you think is the overhead point, or zenith, directly above you. Then, without changing the direction of your line of sight, turn halfway around a vertical axis, so your body is facing the opposite horizon. If you really are looking straight up, you will still be looking at the exact same place in the sky; while if you are looking a little forward of overhead in one direction, and the same amount forward of overhead in the opposite direction, then straight up -- the direction to the zenith -- will lie halfway between the two directions you were looking at, before and after turning around.

North, South, East and West Points
     In the diagram above, two straight lines are shown going through the observer's position. One, running east and west, represents that part of his parallel of latitude which extends out to the distant horizon. Where it meets the horizon is referred to as the East Point, or "true East", or "due East", on the eastern side of the observer, and as the West Point, or "true West", or "due West", on the western side of the observer.
     The other line running through the observer is that part of his meridian of longitude which runs off to the north and south, along the ground, and meets the northern horizon at the North Point (= true North = due North), and the southern horizon at the South Point (= true South = due South).

The Meridian
     Rising vertically at the North and South Points, and passing directly above the observer at the Zenith, is The Meridian, an arc which divides the sky into an eastern half, in which stars are rising, and a western half, in which they are setting. Throughout the eastern half of their paths, stars continually rise, higher and higher in the sky, until they reach The Meridian; then, as they cross The Meridian, and enter the western half of their paths, they continually sink, lower and lower. The point where a star's path crosses The Meridian, from east to west, is the highest point in its path, and as a result, is sometimes referred to as upper culmination. At the time the star crosses The Meridian, it is said to be transiting, and a telescope which is designed to look at the stars only as they cross The Meridian is referred to as a transit telescope. (It may sound odd that someone would want to use such a telescope, but particularly in earlier times, it was hard to tell exactly where a telescope was pointing unless it was very firmly fixed in position, so measuring stellar positions was more accurate when done with fixed telescopes, or transit telescopes.)
     The Meridian can also be thought of as your meridian of longitude, projected upward, into the sky. Imagine that we cut the Earth open, along your meridian of longitude, and placed a very bright light bulb directly below you, so that its light could shine through your meridian of longitude, into the sky. The path that it would trace out would be the same as The Meridian in the sky.

(Much more to follow, when time permits, as indicated by the notes below)
(discussion of the positions and labels on the Celestial Sphere, as in the diagram above)

(discussion and diagram(s?) showing how the position of the NCP/SCP/CE depends upon latitude -- mention of flattened dome of sky, and link to Moon Illusion; mention of various ways of defining coordinate systems on the CS, and link(s))

(rising of stars to east of Meridian, setting of stars to west of Meridian, etc?) (animation showing motion of Earth to east, and apparent resulting motion of stars to west, as in class; discussion of W horizon rising, E horizon setting (park bench?))