Celestial Sphere

All of the stars and other objects that might be seen in the sky are at varying distances from us. Since even the nearest of these objects are so far away, our mind perceives them as all being at the same indeterminable distance away. For this reason and for the purpose of this amateur stargazers, the sky and all that it holds can be thought of as the celestial sphere – much like the inside of a planetarium dome. Stars are fixed points upon this sphere that all move together across the sky. Imagine the earth at the center of this sphere, and that the sphere surrounds the entire earth.

CelSphIn reality, the earth spins on its axis beneath the celestial sphere. The point on the earth’s surface from which we happen to be observing circles the earth’s once each day, and we observe the celestial objects passing through our field of view, or the sky, from east to west. Our perception is that the earth is fixed, and the celestial sphere turns above us. For casual observational purposes, and for ease of explanation, the remainder of this discussion will assume the perceived motions.

© James R. Johnson, 2015.


There are two ways to think of constellations. First, and foremost, a constellation is a grouping of stars that usually represent some mythological being. In the first chart Orion appears as a stick figure torso with a belt of three stars and holding what is depicted as a raised club and perhaps a dead lion. The interconnecting lines  aid in identifying the spatial relationships between the constellation’s stars by guiding our eye from one star to the next. Secondarily, a constellation is an area of the sky, which is delineated in the second chart by the white field. If the Sun, Moon, a planet, or some other celestial object happens to appear within a constellation’s borders, it is said to be located in that constellation.

Orion. Accessed by Jim Johnson on January 7, 2015.
IAU and Sky & Telescope magazine (Roger Sinnott and Rick Fienberg), June 5, 2011.
IAU and Sky & Telescope magazine (Roger Sinnott and Rick Fienberg), June 5, 2011.


© James R. Johnson, 2015.


The Meridian. Accessed by Jim Johnson on January 31, 2015.The meridian is a special line in the sky that is related to an observer’s location. It is defined by three points: 1) the north point on the horizon, 2) the zenith, and 3) the south point on the horizon. By extending an arm horizontally and pointing north, then swinging the arm upward until it is pointing straight up overhead, and continuing in that direction, which is now a downward motion, until reaching the south point on the horizon, an observer has traced the meridian.

The meridian remains stationary while the celestial sphere appears to rotate past it as the earth rotates about its axis. All objects that we observe on the celestial sphere appear to move perpendicular to the meridian. When an object on the same declination as the observer’s latitude rises from the east and ascends to the meridian, it has reached its zenith. From then on, the object descends toward the western horizon. Objects that are at a greater or lesser declination than the observer’s latitude have also reached their highest point when crossing the meridian, but will cross the meridian either north or south of the zenith.

The knowing what time an object reaches the meridian is important for an observer wanting to get the very best possible view of an object. The view toward the zenith is the clearest possible view, because the observer is looking through the thinnest possible cross section of the earth’s atmosphere. The view toward the horizon, however, is through the thickest possible cross section of the atmosphere, and this is where atmospheric haze will degrade viewing conditions the most. No matter the object’s declination, the time that it crosses the meridian is its closest approach to the zenith, and the very best possible time to observe.

Image Credit:
– The Meridian. Accessed by Jim Johnson on January 31, 2015.

© James R. Johnson, 2015.