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# Circumpolar: Stars Above the Horizon

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Author: Janice VanCleave

Different stars are visible from different locations on Earth. Some appear every night and never set below the horizon. These are the circumpolar stars.

In this project, you will model the motion of circumpolar stars. You will determine which stars appear circumpolar to observers at different latitudes. You will find out how an observer's latitude affects the altitude of stars. You will also learn about northern and southern circumpolar stars that are easily seen.

### Getting Started

Purpose: To model the motion of circumpolar stars at Earth's poles.

### Materials

drawing compass
ruler
9-inch (22.5-cm)-square piece of poster board
9-inch (22.5-cm)-square piece of blue, transparent plastic (report folder works well)
black fine-point permanent marker or ballpoint pen
scissors
transparent tape
sheet of white copy paper

### Procedure

1. Draw a circle with a diameter of 3 inches (7.5 cm) in the center of the poster board.
2. Lay the sheet of blue plastic over the poster board. Use the marker to trace the circle onto it. Cut out the circle and discard it.
3. Lay the plastic over the poster board again. Line up the cutout with the circle. Tape the plastic and the poster board together at the top edge. Cutting through both layers of the right-hand side, remove a semicircle about 1 inch (2.5 cm) deep at its deepest point
4. Using the compass, draw a circle 8 inches (20 cm) in diameter on the white copy paper. Cut out the circle.
5. Lift the plastic and center the paper circle on the poster board. With the compass point, punch a hole through the center of the poster board and the paper circle. Secure the paper circle and the poster board together with the brad.
6. Lower the plastic. Mark 20 to 25 dots in the exposed area of the paper circle, not too close to the outer edge (see Figure 24.1).
7. Turn the paper circle counterclockwise and observe the movement of the dots.
8. Turn the paper circle clockwise and observe the movement of the dots.

### Results

The dots move, but remain within the opening of the sheet of plastic.

### Why?

The brad represents a celestial pole (one of two ends of the axis of the celestial sphere). The dots in the opening represent the stars visible to an observer at latitude 900N or 900 S, the North Pole or South Pole of Earth. (The stars seen at the two poles are different.) The blue edge represents the horizon, where the sky meets Earth. The circumpolar stars are always visible above the horizon from a given observation point on Earth.

From the North Pole, all visible stars are circumpolar. They appear to move counterclockwise around the north celestial pole. From the South Pole, all visible stars are circumpolar. They appear to move clockwise around the south celestial pole.

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