THE CELESTIAL SPHERE

Background

As you look up at the night sky and see some of the recognizable constellations, it is important to learn that these stars are not really grouped together relative to each other in real space. They only appear that way to us from our vantage point on earth. In reality, these stars are widely scattered from us in terms of distance away, but appear to be grouped together in the sky because their light is reaching the earth at the same time. The seven stars of the Big Dipper vary greatly in their distance from earth, but as their light reaches us, they appear to be a group shaped like a giant water dipper at some giant punch bowl. Astronomers have developed a system to tell other astronomers where to find things of interest in the night sky. In the mid 1700's, Charles Messier was trying to construct a map of the night sky and include a list of fuzzy objects where were always present, and not to be confused as a new comet. He compiled a list of 110 such objects, ordered M1 to M110. Each object has celestial coordinates so anyone can look in a very specific place in the sky and find them. This practice of using coordinates to make maps and navigate was used by sailors when traversing the oceans. One only needed to know the latitude and longitude and then navigate by the stars to such a location and in would be there. This page and accompanying website are designed to help you become familiar with the celestial coordinate system. You may never use these numbers, but if you get excited by the course and purchase a telescope, knowledge of the celestial sphere will be essential if you want to try to find interesting stuff in space.

The Celestial Sphere

The stars in the night sky look like they are points of light scattered on a black dome which rotates around the earth. Actually, it is the earth which is rotating, thus causing the stars to appear to turn in a great arc from east to west, and there is no black dome, but blackness with stars as far as the most powerful telescopes can probe. We call this black "dome" the Celestial Sphere. This is not a huge, rotating, glass sphere, but a term for how the night appears to move. The pivots about which the sky seems to rotate are called the celestial poles, and are aligned with earth's geographic north and south poles. The celestial equator is an imaginary line around the sky directly above the earth's equator. Seen from Minnesota, the celestial equator runs from the east point on the horizon up across the southern sky and down to the west point.
The location of the celestial poles and equator in our sky depends on our latitude. If we were standing on the North Pole (perhaps hand-delivering our list to Santa), the north celestial pole would be straight overhead, and the celestial equator on the horizon. We would look over our heads and see Polaris, the North Star, and throughout the night the stars would rotate around Polaris. As we would move southward, the celestial equator would rise in front of us. As we walk further south, the angle between the north celestial pole and the northern horizon always equals our latitude ... which is around 44o here in Minneapolis/St Paul. This is how navigators were always able to measure where they were in latitude on the open sea.
The star Polaris, which is the tail star of the Little Dipper, happens to be very close to the north celestial pole, and this it moves very little as the sky rotates. At any time of the night, in any season of the year, from anywhere in the earth's northern hemisphere, Polaris always stands above the northern horizon, and is thus known as the North Star. Constellations which rotate around the North Star and are visible during any season of the year are called circumpolar constellations. Ursa Major, Ursa Minor, Cassiopea, and Draco are examples of circumpolar constellations.

The Ecliptic and the Zodiac

The sun appears to rise from the east and set in the west because the earth rotates counterclockwise in 24 hours. This rotation of the earth causes the sun to appear to move in the sky, and the path which the sun takes through the sky is called the ecliptic. If you were to trace the sun's path during the complete 24 hour day, it would move through 12 different constellations, of which 6 are visible during the night and 6 are hidden by the sun's glare during the day. These 12 constellations make up the Zodiac. Since the earth's spin makes it appear as if the sun and moon are moving in the sky, it also causes the stars in the night to appear to move from east toward west. As mentioned earlier, circumpolar constellations circle around Polaris during the night. It would be an interesting experiment for you to take a camera outside at night, open the shutter for the duration, and look at the star trails circling the North Star. Stars which are closer toward east-west and southward appear to rise from the east, move southward and progressively higher in the sky, and set westward. An open-shutter camera would reveal long arcing star trails throughout the night. Orion and Taurus are still Orion and Taurus, but as you will see, a frustrated Orion the hunter remains ever the hunter and never the killer, since he never catches the bull night after night after night.

More About the Celestial Sphere and the Coordinate System

At this time, please click on the image at the left of this paragraph to enter a nice website which teaches different aspects of the Celestial Sphere. You will be responsible to know what declination, right ascension, altitude, and azimuth are. You will be expected to find the zenith stars for anywhere on Earth at any time, and how to determine the angular separation between stars in the sky. Follow the website pages to learn more about the Celestial Sphere and you will then be asked to use some of the information you learn there on your observation runs. When you think you have mastered this information, move ahead to History of Astronomy or go back to the Introduction to the Starry Sky., the Syllabus, or the Home page. REMEMBER, you are expected to complete the Constellation Observation.


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