Rising Stars

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If you see a star near a tree tonight, will it be there again tomorrow? Or later the same night? Let's observe whether or not stars appear to move by doing the following.

  1. Choose a star to observe somewhere near the eastern horizon which is near some fixed object like a mountain peak, tree, or building. Sometimes just moving a short distance will allow you to position it close to a nearby object, like a telephone pole. Try go get the star within a moon-width or so (1°) of the object on the ground, or at least know its position to that precision. Now make a note of 1) approximately what direction it is to the the star (like north-east), 2) just where the star is relative to your chosen fixed object so you can mentally visualize there again in case it moves, 3) exactly where you were standing, and 4) what time it is to the nearest minute. Now enter this data:

    Direction to StarObserving Latitude
    Far North (Canada, Northern Europe)
    Mid-latitudes North (U.S.A., Mediterranean)
    Near Equator (Northern Brazil, Africa)
    Mid-latitudes South (Argentina, Australia)

  2. One hour later (with 4 minutes), find the star again. (Hint: It will have risen somewhat, in a path similar to what the sun would do if it were located there.) Now find the Pointer Stars of the Big Dipper (See "Finding North" Exercise), or the Southern Cross, if in Southern Hemisphere. Remembering that the Pointers are 5° apart from each other (or Cross-bars 6°), estimate how far the star has moved in an hour to a precision of about 1°. Which of the following best describes the star's motion?
    Direction of Star's MotionAmount of Star's Motion
    rising toward the south
    rising vertically
    rising toward the north

    That complete's the exercise. We see that the stars rise and set very much like the sun and for the same reason: the earth is turning. Stars that rise in the east in the evening will set in the west about dawn. For a follow up, the next time you see the sun rising on whatever actual horizon you have, note the time if you are going to be at that location for one hour. Then estimate (without the benefit of the pointer stars) how many degrees it moves in an hour. Was it about the same as the star? If not, why not?

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