The stars, planets, moon, and the sun all travel (as we see it) on a huge sphere around the earth. This sphere is known as the celestial sphere. The poles of this sphere, called the celestial poles, correspond with the projections of the earth's poles onto the sphere. The celestial equator corresponds to the projection of the earth's equator onto the celestial sphere. It is interesting to note that the altitude the celestial pole correspond to the observer's latitude.

Another point of interest is the zenith. The zenith is the point directly above the observer on the celestial sphere. Different stars and celestial objects will pass through the zenith. Others never will. An imaginary line from the northern horizon, through the zenith, and to the southern horizon is referred to as the celestial meridian. When a star or other object passes through the celestial meridian, it is as high in the sky as it will be. (Circumpolar objects are an exception since they pass through the meridian twice.)

The stars are located all across the sphere. In order to locate stars and other night time objects, we need a coordinate system to locate points. You may have learned about the xy coordinate system in math classes. The same idea holds here. Or you may know about longitude and latitude from geography classes. This is an even better analogy to what astronomers use. We use two coordinates called right ascension and declination. To supplement your reading, Sky and Telescope has an article to help you with celestial coordinates.

The stars are also grouped into 88 regions of the sky referred to as constellations. They also assist astronomers in locating objects in the sky, just as states or countries allow us to narrow a search for cities on a map. However, some of the "cities" on this map move. These include the planets (five of them are easily seen), the moon, and the sun. (Some stars will move also, but the motion is too small to detect without careful observations and expensive equipment.) Interestingly enough, all of these objects (not including the moving stars) tend to move through the same region of the sky. The twelve constellation through which they move are referred to as the zodiac. You probably associate this term with astrology, and rightly so. These objects and their motion through the zodiac are the basis for astrological predictions.

The sun's path through the celestial sphere is particularly important. It is called the ecliptic. The ecliptic is found by watching the sunset and seeing what constellation appears just after the sunset. The process is also repeated in the morning just before sunrise. Once these constellations are determined, the position of the sun can be interpolated. This path forms an "S" shape around the celestial equator. Note the curve in the chart below.

Click on the chart above for a clearer (but not real clear) image. But it may take a while to load.

If you note the dates along the ecliptic, you will see the time of year when the sun is in certain locations. Note also the seasonal position of the sun. During the summer months (in the northern hemisphere) the sun is north of the equator. It makes a transition to south of the equator in September. It remains south of the equator for 6 months, our winter time. (I'm ignoring fall and spring for simplicity, though in Wyoming they don't exist anyway.) In our sky, these changes manifest as alterations in the altitude of the sun in the sky. They also alter the time of rising and setting. Combined, these changes give us the seasons.

You can observe the change in altitude of the sun at noon and the rising and setting times by using the software included with your text and running the exercise Seasonal Suns.

Its interesting to note that common belief is that the seasons are due to the distance of the Earth from the sun. In fact if this were the case, the seasons would be the same all around the world. In actuality it has to do with the time the sun is in the sky, and more importantly, the angle at which the sunlight strikes the surface of the earth.

There are four points of note on the chart above. Two of them are the Vernal (Spring) and Autumnal (Fall) equinox. These are the points where the sun crosses the celestial equator. They correspond to the dates of the first day of Spring and Fall (in the northern hemisphere) as well. Note that the right ascension of 0 hours corresponds to the vernal equinox. The other positions are the Summer and Winter solstice. These correspond to the points where the sun is farthest north and south of the celestial equator, respectively. They are also the longest and shortest day of the year (in the northern hemisphere), respectively.

These dates will not stay the same. This is because of precession. Precession is caused by the gravitational pull of the moon and sun upon the Earth. The Earth is a spinning object with its rotational axis tilted by approximately 23.5 degrees. Gravity and this tilt causes the north celestial pole to move slowly in a coning motion. This motion is extremely slow. It takes 26,000 years to complete a cycle. So we will not live long enough to note a huge change. However, astronomers have been able to detect this change. In fact Hipparchus noted the change 2000 years ago. (More on him later.)

There are a couple of changes to note. One is the alteration of the position of the vernal equinox on the celestial sphere, and therefore the alteration in the right ascension of all astronomical objects.

The other is the position of the celestial poles in our sky. Currently, Polaris is near the position of the north celestial pole. But it is slowly changing as the orientation of the Earth's axis changes.

If you have the opportunity, spin a top and watch as the axis of the top spins. It will spin more slowly than the top itself is spinning, but it will be noticeable. The link here is for a movie of a wheel precessing.

This page was last updated on 06/06/01.