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Zodiac Band: Background for the Ecliptic

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Author: Janice VanCleave

From Earth, celestial bodies appear to move across the sky. This apparent motion is predictable from day to day. The position of each body can be described in relation to the zodiac, a belt of stars bordering the ecliptic, the Sun's apparent annual path.

In this project, you will use the zodiac to predict the location of the Sun and the naked-eye visible planets—Mercury, Venus, Mars, Jupiter, and Saturn—for any date. You will also learn how to predict whether Venus or Mercury will appear as a morning or evening "star."

Getting Started

Purpose: To demonstrate the position of the Sun during the year.

    Materials

      scissors
      ruler
      10-inch (25-cm)-square piece of white poster board
      one-hole paper punch
      drawing compass
      protractor
      pen
      paper brad

Procedure

  1. Cut a 1-by-7-inch (2.5-by-17.5-cm) strip from one edge of the poster board.
  2. Punch a hole in the center and 2 inches (10 cm) from the end of the strip.
  3. On the larger piece of the poster board, draw a circle 8 inches (20 cm) in diameter. Cut out the circle. This circle is your constellation wheel.
  4. Use the protractor, ruler, and pen to divide the constellation wheel into twelve 30° pie-shape sections.
  5. Around the edge of the wheel, write the names and dates of the 12 constellations from Table 23.1, one in each section, as shown in Figure 23.1.
  6. Use the point of the compass to make a hole in the center of the wheel.
  7. Place the hole of the strip over the hole in the wheel. Push the paper brad through the holes and secure.
  8. Draw a sun design on the end of the strip even with the edge of the wheel. Write "Sun" by the design. On the strip, write "Earth" near the brad.
  9. Hold the constellation wheel stationary while you rotate the strip so the Sun moves through the constellations in chronological (time) order. Note the following:
    • The direction the Sun moves in relation to the constellations
    • The position of the Sun in relation to the constellations
    • The time interval between the constellations

Zodiac Band: Background for the Ecliptic

Results

The Sun appears to move counterclockwise and eastward in relation to the constellations. As it moves, it passes a different constellation about every 30 days.

Why?

If you could look down on the North Pole from space, you would see Earth revolving counterclockwise around the Sun. To observers on Earth, the Sun appears to move eastward among the stars along a path called the ecliptic. The ecliptic runs through a band of constellations that circle the celestial sphere. This band is called the zodiac, and the constellations (a group of stars forming a pattern) along the band are called zodiac constellations. In ancient times, skywatchers divided this band into 12 segments, each about 30° wide, with a constellation in each section.

As seen from Earth, the zodiac constellations provide a background for the Sun. At specified times, the Sun is said to be "in" a specific constellation. In this position, an imaginary straight line stretches from Earth through the Sun to the constellation

When the Sun is in a constellation, its light is so bright that the stars in the constellation cannot be seen. The Sun and the constellation appear to move from east to west From day to day, the Sun's apparent motion is about 4 minutes slower than that of the constellations. Thus, the constellations are about 1° farther west than the Sun each day. At the end of 30 days, the Sun is in the constellation 30° to the east This makes the Sun appear to move eastward through the zodiac constellations.

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