Celestial Sphere: Sky Globe (page 2)

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Author: Janice VanCleave

Design Your Own Experiment

    1. On Earth we create imaginary lines to help us locate places on Earth. These lines, called longitude lines (imaginary lines running around a celestial body or the celestial sphere from pole to pole that measure angular distances east and west of a designated 0° line) and latitude lines (running parallel to the equator), are seen on maps and globes. Using this idea, astronomers have created imaginary lines on the celestial globe to help them locate celestial bodies. Hour circles are great circles (circles around a sphere that have the same center point as the sphere), also called celestial longitude lines on a celestial globe that run from pole to pole perpendicular to the equator. These circles compare to Earth's lines of longitude. Design an experiment to show hour circles on your celestial globe, such as running thin strips of tape from rim to rim across the center of the outside of the bowl. Run another strip at a right angle to this strip, dividing the bowl into four equal parts.
    2. Hour circles measure a celestial body's right ascension, or its eastwest position. Right ascension is the celestial equivalent of longitude on Earth. It is measured in hours (h), with 1 hour equaling 15°. More precisely, right ascension is the angular distance of a celestial body from the vernal equinox (position of the Sun on or about March 21 in the northern hemisphere when it crosses the celestial equator). Mark right ascension on your model. Place a dot on the celestial equator below one of the hour circles. Label the dot "0h." Mark seven more dots evenly spaced along the celestial equator. Label the dots to the right of 0h as 3h, 6h, 9h, and so on to 21h as in Figure 22.2.
    3. Declination is the celestial equivalent to latitude on Earth. This angular distance is shown by imaginary lines circling the celestial sphere parallel to the celestial equator. A celestial body's declination is its angular distance in degrees north or south of the celestial equator. Mark a dot on the spot where the four hour circles cross the north pole (marked X). Label this dot "+90°." Mark the four hour circles where they cross the celestial equator at "0° ." Mark two dots, evenly spaced, between 0° and +90° on each section of hour circle. Label the lower dots "+30°" and the upper ones "+60°." Note: In the northern hemisphere, the declinations are positive (+). In the southern hemisphere, they are negative (–).
    4. Coordinates are two numbers that identify a location. Right ascension and declination make up a method of locating celestial bodies called the equatorial coordinate System. Demonstrate how to find the equatorial coordinates of stars. Stick stars on the bowl here and there. Tape the end of a 12-inch (30-cm) string at 90°, the North Pole. Pull the string down the side of the bowl so that it crosses one of the stars. Estimate the hours of its right ascension and the degrees of its declination. Star coordinates are written with the right ascension first. For example, the coordinates of the star in Figure 22.2 are 3h, +30°. Science Fair Hint: Identify stars in your display by their equatorial coordinates. (See Fig. 22.2)
  2. From where you live, can you see stars in both the northern and southern hemispheres? Observe the sky during as many different seasons as possible. Find coordinates and identify the stars. Make lists and maps of the stars in your region of the celestial sphere. For information about star identification, coordinates, and star maps, see Robin Kerrod, The Star Guide (New York: Macmillan, 1993).

Celestial Sphere: Sky Globe

Get the Facts

  1. The horizontal coordinate system uses the horizon as the reference circle. The zero point is north. Altazimuth coordinates are a combination of a celestial object's altitude (degrees above the horizon) and azimuth (degrees eastward around the horizon from north). For information, see David H. Levy, Sky Watch (San Francisco: The Nature Company, 1995), pp. 80–81.
  2. To make an instrument to measure altazimuth coordinates, see Janice VanCleave's Constellations for Every Kid (New York: Wiley, 1997), pp. 156–160.
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