Goals: The student will|
- Know the constellations of the Big Dipper and Cassiopeia, and their use in finding the Pole Star.
- 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.
Terms: Big Dipper, guide stars, Cassiopeia.
Possible assignment, given out in the preceding session of the class. Have students look up in a dictionary the word "cynosure" (usually pronounced "sino-sure"), an old word hardly used any more. What does it mean?
Stories and extras: The flag of Alaska, with the Big Dipper and Polaris, and the story of Benny Benson.
Other names of the Big Dipper, and its use by slaves escaping to freedom. The Greek legends reflected in the names of constellations.
Guiding questions and additional tidbits
(Suggested answers in parentheses, brackets for comments by the teacher or "optional")
This is a short lesson. Start by asking the class how to find the Pole Star. Then present the material, and use the questions to solidify the concepts.
The lesson might end by asking students if they know of other constellations. Depending on season, they may look for Orion in winter or fall, Leo in the spring, Scorpio in the summer. The Milky Way is bright near Scorpio.
After this, present the material. The questions below may be used in the presentation, the review afterwards or both
--Why are hikers (in places like the US and Europe) told to look for the pole star if they are lost at night, and not for any other star?
The pole star always occupies nearly the same position, to the north of the observers. Other stars rise and set, or at least move across the sky.
[Personal story: In army elementary training, our squad was brought at night to an uninhabited spot and told to find its way to a certain pick-up point. The squad leader then assembled us, pointed at a bright star and said--"I know which way we came. We will find our way by keeping this star on our right." Three hours later it became evident that the chosen star was setting and that we were completely lost.]
-- What is the Big Dipper? What other names is it given?
Astronomers call it "Ursa Major", the big bear, and that is the name in some languages. In England it's "the plough" (spelling in the US is "plow").
-- What is the state flag of Alaska?
Have a student draw it on the board, or do so yourself.
--Why do you think Alaska chose the pole star and the Big Dipper to be on its flag?
If no one suggests a valid answer, give a hint:
--You know Alaska is the US state closest to the north pole. Suppose you were at the north pole, on a clear night, could you see the pole star? (Yes!) And where? (Right overhead!)
--Could you see the Big Dipper (Yes, because it is close to the pole star!). At any time? (Probably!)
--Well.... if Alaska is near the pole, where would the pole star be? (Not be exactly overhead--but close!) Could you see it and the Big Dipper? (Probably, and at any time).
The teacher may add: New Zealand also has a constellation on its flag, but a different one.
Why? That country is well south of the equator--two large islands near Australia, and from it you can see neither the pole star nor the Big Dipper, they are always below the horizon. (On the equator, the pole star would be on the horizon, and the Big Dipper close to it. You will probably never see there the pole star and rarely see the Big Dipper.)
But you see from New Zealand other constellations, which are close to the southern pole of the sky, and which are never seen from the continental US. One such constellation is the Southern Cross, framed by 4 bright stars, and that is what you see on the New Zealand Flag.
-- How does the Big Dipper help you find the Pole Star?
There also exists a constellation of "the little dipper" or "the small bear" (Ursa Minor). It has a somewhat similar shape, except the "dipper" looks more like a square. It faces the opposite way from the big dipper, and the pole star is the last star in its "handle." Its other stars are rather dim--you need a clear night to see them all.
--[If any students looked up "cynosure": it originally referred to the "little dipper" or (more recently) the pole star. Used in English, it means a person who is the center of attention.]
-- Can you always see the Big Dipper on a clear night?
Depends where you live. In the "lower 48 states" of the US, it may be too low on the horizon. In Alaska, the answer is probably "yes."
Students who want to know more about the Big Dipper may look up two relevant message exchanges with users of "From Stargazers to Starships."
-- What constellation helps you find the pole star when the Big Dipper is not seen? How would you use it?
Draw Cassiopeia and the Pole Star on the blackboard.
-- Who was Benny Benson?
Have a student tell the story (see the cited web page; a bit more can be found elsewhere on the web) If time allows and a map of Alaska is handy, point out Unalaska, Seward and Kodiak.
Bit of trivia: Seward is at the end point of the Alaska railroad, south of Anchorage, the main city in Alaska. It is a fishing port, one of the more beautiful towns in Alaska, and is named for the person who oversaw the buying of Alaska from Russia. It sits at the tip of a deep bay: in 1964 an earthquake driven ocean wave ("tsunami") raced up that bay, getting higher as the bay became narrower and in the end destroying much of the town. Today signs in the city warn people to run for the hills if they hear the tsunami alarm.