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# Azimuth: Horizontal Coordinate

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Author: Janice VanCleave

An axis is an imaginary line through a body or the north-to-south line through the center of a celestial body from pole to pole about which the body rotates. One factor that determines the position of celestial bodies in the sky as seen from Earth is Earth's daily rotation, which is the turning of a body on its axis. This motion of Earth from west to east causes an apparent east-to-west daily movement of celestial bodies, such as the Sun, the Moon, stars, and planets. The angular distance measured eastward along the horizon from the north point on the horizon to a celestial body is called the azimuth.

In this project, you will make a simple astronomical measuring tool to determine azimuth. You will measure changes in the Sun's azimuth during one day and from day to day, and you will measure changes in the azimuth of other celestial bodies.

### Materials

premium-strength paper dinner plate
protractor
marking pen
grape-size piece of modeling clay
3/16-by-12-inch (0.47-by-30-cm) dowel
clock or watch

### Procedure

1. Make a compass rose (a circle marked in degrees that is used to indicate azimuth) by turning the paper plate upside down. Then use the protractor and pen to mark every 10° around the edge of the paper plate as shown in Figure 4.1, starting with 0°. Label 0°N, 90°E, 180° S, and 270°W.
2. Use the clay to stand the dowel vertically in the center of the plate as shown in Figure 4.2.
3. At 30 minutes or more before noon standard time, set the plate out-doors in a sunny area and on a level surface. (Standard time is clock time disre garding daylight saving time.)
4. Watch the dowel's shadow and determine the time when the shadow is shortest. At this time on the following day, position the plate so that the shadow falls across the 0° mark.
5. Stand so that you face the direction of the shadow. You are facing an azimuth of 0° .
6. Imagine the shadow continuing until it touches the horizon. Make note of the physical structures at this point on the horizon.

### Results

You have determined the direction of an azimuth of 0° and identified physical structures on the horizon to mark this direction.

### Why?

When the Sun is at its highest altitude in the sky, shadows are shortest. This time is called high noon. At high noon in the Northern Hemisphere, the Sun is at its highest position above the southern horizon. So the shadow of the dowel is cast toward an azimuth of 0°, which is true north (the direction of Earth's geographic North Pole, which is at the north end of Earth's axis).

The horizon makes a 360° circle. Each compass direction is 90° around the circle, so starting with 0° for true north and going eastward, true east is 90°, true south is 180°, and true west is 270°. The angular distance measured clockwise along the horizon from true north is called the azimuth. In this experiment, the dowel's shadow at high noon points to true north and leads to a point on the horizon with an azimuth of 0°. All celestial bodies on a line from the zenith to this point on the horizon have an azimuth of 0°.

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