(1a)   The Celestial Sphere

An introduction to the celestial sphere, describing its
apparent rotation and the special role of the pole star.

Part of a high school course on astronomy, Newtonian mechanics and spaceflight
by David P. Stern

 This lesson plan supplements the web page listed below. Originally named "The sky above us", it was split into two parts, each of which now has a lesson plan. This lesson covers the second of the two: "The Celestial Sphere," section #1a : on disk Scelsph.htm, on the web           http://www.phy6.org/stargaze/Scelsph.htm Part of "From Stargazers to Starships" home page and index: on disk Sintro.htm, on the web           http://www.phy6.org/stargaze/Sintro.htm

 Goals: The student will Understand the apparent rotation of the night sky, the special role of the Pole Star and the reason which sets that star apart. The student will also realize that other celestial objects--Sun, Moon and planets--share the rotation (and hence rise and set), even though their positions among the stars slowly change. [Optional: Understand the mounting of astronomical telescopes.] Terms: East, west, south, north, horizon, equinox, winter solstice, summer solstice, elevation of the Sun )above the horizon). Celestial sphere, constellation, zodiac, celestial pole, the Pole Star ("Polaris"), celestial equator, rotation of the Earth, planets [theodolite, equatorial axis]. Stories and extras: Psalm 19 (A poetic impression of the sky)     Start the class with a discussion of the night sky: how many have seen the sky on a really dark night, away from streetlights and with no Moon? What was it like?     What is a constellation? Ask the class: did anyone recognize some constellations? Did you see the Big dipper? The Little Dipper? (Both featured in the next lesson plan) Orion? Cassiopeia? [The Pleiades? This is a compact group of white-hot stars, roughly kite-shaped. Many people call them "the 7 sisters" but in fact the group also contains many more less-bright stars.]     If students have seen the Pleiades: could they count its stars? Tidbit: In Japanese, the Pleiades are called "Subaru." Next time you see a "Subaru" car, check its factory's logo--it has a picture of the constellation. How many stars on it?     A giant Japanese telescope, atop the extinct volcano Mauna Kea on Hawaii, is also named "Subaru." Did you see the Milky Way?    (Do you know what it is? Billions of distant stars in our galaxy. The galaxy is a flat wheel, and we are looking through it side-on.). We now know that at the center of this wheel is an enormous black hole--a collapsed star with mass 3-4 million times that of the Sun, and gravity so strong that even light cannot escape.     [For the teacher: you might want to look up the web page "The Black Hole at the Center of our Galaxy".]     Has anyone visited a planetarium show? What was it like?     The quoted part of psalm 19 may be read by someone at a proper moment     After this, present the material. The questions below may be used in the presentation, the review afterwards or both --What is "the celestial sphere"? When we view the stars at night, it seems as if we are at the center of a huge sphere and the stars are points of light distributed across it. That is "the celestial sphere." In ancient times people believed such a sphere actually existed and was rotating around us. We know better now, stars are actually very far away, and the Earth is the object that rotates. Still, the celestial sphere remains a convenient tool for describing the positions of the stars in the sky.) --Are the positions of the stars on the celestial sphere fixed with respect to each other? Yes, which is why we have patterns ("constellations"), of fixed shapes. Over time the stars themselves move, but the distances to them are so great, the movement appears to be small. A few "planets" (wanderers) move quickly among them, including Venus and Jupiter, brighter than any fixed star. --At night, how do stars appear to move, and why? They rise and set, like the Sun and Moon, because of the rotation of the Earth. --What do you know about the Earth's rotation around its axis? The Earth rotates around an imaginary line connecting its north pole and south pole.[This may be illustrated using a globe, ball, apple or any round object] --How long does it take the Earth to make a full rotation around this axis? About 24 hours--more accurately, 4 minutes less than 24 hours, as will be explained in a later class. --When viewed from above the north pole, does the Earth rotate clockwise or counterclockwise? Counterclockwise. When it is 12 o'clock in New York and that part of the Earth directly faces the Sun, it is only 9 o'clock in California. In 3 hours California will have rotated to the position where New York was. One may explain that "clockwise" and "counterclockwise" is relative, it depends on the place from which one views the motion--it is not something absolute. Viewed from the far south, the Earth seems to rotate clockwise.     To demonstrate: Hold a plate vertically in your left hand, on your left side, with the top of the plate facing the class, Slowly circle a finger around it in a clockwise direction. Then gradually swing the arm to your right, with the finger continuing in the same circle. The class now watches the bottom of the plate, and the same motion seems to be counterclockwise.] --What makes the Pole Star special? The Pole Star is the star around which all other stars in the Northern Hemisphere appear to rotate. --Do all stars set? No. On clear nights, in the continental US or Europe, the Pole Star and the ones near it may be seen at any time. --Can any star visible from Earth, be seen from the US? No, stars near the southern pole of the sky never rise. Similarly, the Pole Star is never visible from Australia. --What happens if you aim a telescope at a star and hold the telescopes direction fixed? Stars will appear to rush through your field of view. When you see the Sun rise or set, it seems to move very slowly, as do stars that rise or set, but a telescope greatly magnifies this motion and makes it appear fast. [Note: The book "First Light" by Richard Preston describes how astronomers at the 200-inch telescope on Mt. Palomar--for many years the largest in the world--deliberately clamped down the telescope, to make it sweep across long strips of the sky, searching for distant galaxies. A recommended, well-written book by a writer who spent some months living with the astronomers at the observatory.] --What is an equatorial mounting? The mounting of a telescope, allowing it to rotate around an axis parallel to that of the Earth. It keeps the star in view by rotating the telescope at exactly the same rate as the Earth rotates. Without this, any star being viewed quickly moves across the field of view and out of sight.

Guides to teachers...       A newer one           An older one             Timeline         Glossary

Author and Curator:   Dr. David P. Stern
Mail to Dr.Stern:   stargaze("at" symbol)phy6.org .

Last updated: 27 August 2004

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