With this celestial globe you can introduce the concept of the celestial sphere, and show how we use it to specify the positions of celestial objects. You can also show the relationship between the earth’s axis of rotation and the plane in which the earth orbits around the sun (the ecliptic; vide infra), and the paths that objects take as they move across the sky.
When we look up at the night sky, we see (assuming that light from nearby cities or towns does not prevent it) a variety of stars, some of which form recognizable patterns. Ancient astronomers named patterns of stars that reminded them of particular things for the objects, animals or mythological figures that they resembled. These star patterns are called constellations. As they watched these patterns move across the sky, the astronomers imagined that the stars were fixed to a spherical surface, at whose center the earth sat, and which rotated about the Earth. They referred to this surface as the celestial sphere. The stars, even those in a given constellation, vary greatly in their distance from the Earth. Since the line from each star to our vantage point on Earth would pass through a particular point on an imaginary sphere set at some distance from the Earth, however, the concept of the celestial sphere is useful for specifying the locations of the various stars (and other objects) in the sky. Since the Earth is our frame of reference, the celestial sphere is essentially an extension of the coordinate system we use on Earth to the universe.
Objects in the sky appear to move from east to west, and in the northern hemisphere they appear to move in circular paths about a point that is very close to the star Polaris, which is also known as the Pole Star or North Star. Polaris is so named, because it sits above the north pole, almost in line with the Earth’s axis of rotation. The celestial sphere rotates from east to west (opposite the direction of the Earth’s rotation), about the same axis on which the earth rotates; the north celestial pole is directly above the earth’s north pole, and the south celestial pole is directly above the earth’s south pole. The celestial equator, which lies midway between the celestial poles, is in the plane of the Earth’s equator. We may define a system of celestial coordinates by which we can specify the precise location of a celestial object. These are right ascension (RA), which is analogous to longitude, and declination (dec), which is similar to latitude. An object’s declination is the angle of its position with respect to the celestial equator. Declination is measured in degrees north or south of the celestial equator, and is positive north of the celestial equator and negative south of it. The celestial equator is at a declination of 0°, the north celestial pole is at +90° and the south celestial pole is at -90°. Since the celestial sphere is rotating with respect to the Earth (or vice versa), right ascension is measured in time relative to a specific reference point, in hours (h), minutes (m) and seconds (s), and it increases toward the east. The reference point for zero right ascension is by convention taken as the position of the sun in the sky at the instant of the vernal equinox.
The vernal equinox is when the Sun crosses the celestial equator from south to north, which occurs on or about March 21, when it is spring in the northern hemisphere. It is so called, because at that point in the Earth’s orbit, the plane in which the Earth’s axis lies is perpendicular to the line between the Earth and the Sun, and the periods of night and day are equal. (At the opposite point in the orbit is the autumnal equinox, which occurs when the Sun crosses the celestial equator from north to south on September 21.) The Earth’s rotation causes it to bulge slightly at the equator. As a result, except near the equinoxes, the gravitational force between the Earth and the Sun exerts a torque about the center of the Earth, pulling the bulging equator toward the plane of the ecliptic. The Moon’s orbit lies at an angle of 5° to the ecliptic, so it, too exerts a torque in the same direction. This torque causes the Earth’s axis to precess slowly. Because of this precession, the equinoxes drift westward by 50.3 arc seconds per year, or about 0.14 arc seconds per night. For this reason, astronomers use the location of the equinox at some standard year, which is updated every 50 years. The last two standard equinoxes are from 1950 and 2000; the next one will be for 2050. (Some people refer to these as “epochs,” which is not right. The epoch is the year during which one observes the position of an object; the equinox is the reference frame for the equinox that one uses to make the measurement. See, for example, https://srmastro.uvacreate.virginia.edu/astr313/lectures/coordchange/coordchange.html or http://mingus.mmto.arizona.edu/~bjw/mmt/spectro_standards.html#:~:text=What's%20the%20difference%20between%20equinox,star%20was%20at%20year%202015.5..)
Since the Earth rotates through 360° in 24 hours, in one hour it rotates 15°, in one minute it rotates 0.25° (15 arc minutes, or 15′), and in one second it rotates 0.0042° (15 arc seconds, or 15″).
As the Earth orbits the Sun the position of the Sun relative to the stars changes over time. The Sun thus appears to move across the sky in a path called the ecliptic. Because the Earth’s axis is not perpendicular to the plane of its orbit around the Sun, but sits at an angle of 23.4° away from perpendicular, the ecliptic does not lie along the celestial equator, but is tilted at an angle of 23.4° relative to it.
The celestial globe and its features
The celestial globe shown above is a Universal Celestial Globe made by Hubbard Scientific, Inc. It consists of a transparent 12-inch-diameter plastic spherical shell, with a four-inch-diameter metal globe inside it to represent the Earth, and a 3/4-inch-diameter sphere representing the sun. The seam that joins the two hemispheres of the celestial globe marks the celestial equator, which lines up with the zero on the scale on the ring to which it is mounted at the poles. The globe has marked on it lines of right ascension, starting at zero at the vernal equinox and going in increments of one hour eastward around the globe. The ecliptic is marked with the months and their days. The globe shows the constellations with their names, the brightest stars, major nebulae, bright star clusters and the Milky Way. (The manual states that the globe shows stars whose brightness is magnitude five or lower. Decreasing magnitude corresponds to increasing brightness. Magnitude six corresponds to the faintest stars visible with the naked eye.)
At the north celestial pole is a scale with the months of the year, and a moveable dial with the time of day in hour increments. To set the date and time, you rotate the dial so that the desired time of day lines up with the desired date. A knob next to the north celestial pole allows you to set the sun to its position at any day of the year, except between about June 15 and June 28, where the Earth’s axis gets in the way of the wire on which the Sun is suspended.. To show how the system would look for a particular location, set the globe so that the angle of the north pole with respect to the horizontal plane (the north horizon) corresponds to the latitude of that location. (On the scale, this angle equals 90° minus the latitude angle.) For locations in the northern hemisphere, the north pole should be above the horizon and the celestial globe should be rotated until midnight on the dial at the north pole points to the zenith. For locations in the southern hemisphere, the north pole should be below the horizon and midnight should point toward the north horizon. Once everything is set as described, via the knob at the south pole rotate the Earth globe until the desired location is at the zenith (or, rather, the zenith is directly above that location). With the globe set up this way, by looking through the hemisphere that lies above your chosen location, you can see the sky as it would look from that location at the date and time that you have set – which objects are visible, and where in the sky they appear. If you are at the location for which you have set the globe, and you turn it so that the north pole faces north, then the objects on the celestial globe are as they should appear in the sky above you.
Though without a camera it is difficult or impossible to see the details on the celestial globe in a large lecture hall, you can still point to various objects to make their locations, and their positions relative to other objects, clear to the class and show how the system of celestial coordinates works.
References:
1) Chaisson, Eric and McMillan, Steve. Astronomy Today, Third Edition (Upper Saddle River, New Jersey: Prentice-Hall, Inc., 1999), pp. 8-12, 178, 386-387.
2) https://science.nasa.gov/learn/basics-of-space-flight/chapter2-2/