Azimuth: Horizontal Coordinate (page 2)
Try New Approaches
- Once the azimuth of 0° (true north) is identified, other azimuth readings can be determined. With the 0° mark on the plate pointing toward azimuth 0°, true north, make note of structures along the horizon at the azimuth readings of 90° (east), 180° (south), and 270°(west).
- Imaginary lines of azimuth extending from points on the horizon to the zenith can be used to measure the azimuth of celestial bodies at different altitudes. Use the structures noted in the previous experiment as markers to determine if the azimuth of stars changes during the night. Select four brighter stars in each of the four directions: north, east, south, and west While standing in the same position, measure the azimuth of the four stars every 30 minutes for 2 or more hours. You can record a +, a –, or a 0 for an increase, a decrease, or no change, respectively in the azimuth, for each star. From your results, determine if there is any change in the azimuth of the stars and if so, if the change varies depending on where the star is located in the sky.
Design Your Own Experiment
- 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. To an observer on Earth, the Sun appears to move across the sky because of Earth's rotation, which is the turning of a body on its axis. Design an experiment to measure the azimuth of the Sun over the course of a day without looking directly at the Sun. One way is to prepare a stationary compass rose. Do this by using a marker to draw a circle around the middle of two 3/8-by-8 inch (0.94-by-20-cm) dowels. Outdoors on a level section of ground, use a hammer to drive one of the dowels into the ground up to the mark. This will be dowel A Save the second dowel for later. Use the marker and ruler to draw a line across the center of a 12-inch (30-cm)-square piece of thick corrugated cardboard. Label one end of the line "N" for north and the other end "S" for south. Use a drawing compass to draw a 6-inch (l5-cm)–diameter circle in the center of the cardboard. Center the circle on the N-to-S line. Using the compass and a pen, mark every 5° clockwise around the circle as shown in Figure 4.3, starting at N. Label 0°N, 90°E, 180°S, and 270°W. You have drawn a compass rose.
- From the information in the previous experiment, determine the apparent speed (angular distance per time) of the Sun across the sky during the day. Do this using the following formula.
- speed = total change in azimuth ÷ total time
- Does the apparent speed of the Sun change from day to day? Discover the answer by determining the daily speed of the Sun one or more days during a month for as many months as possible during one year. The longer you collect data, the more conclusive your results will be.
- Is the azimuth of the Sun at sunrise and sunset the same each day of the year? Discover the answer by graphing the azimuth of the rising and setting Sun 1 day a week for as many weeks as possible during 1 year.
Use a nail to make two holes in the cardboard large enough for the dowels to easily slip through. Make one hole in the center of the circle and the other on the line and about 1 inch (2.5 cm) from the N. Slip the center hole in the cardboard over the dowel that is in the ground. Slide the cardboard to the ground. Turn the cardboard so that the north end of the line points in the direction of true north as determined in the original experiment. Insert the second dowel in the open hole in the cardboard. Use the hammer to drive the dowel into the ground up to the mark. This will be dowel B. In this position, the N points to true north. Remove the cardboard, leaving the dowels in place. CAUTION: Cover each dowel with a bucket or a chair to prevent accidents. The next day, at or near sunrise, put the cardboard back on the dowels. Determine where the first dowel's shadow crosses the compass rose.
The Sun's position is 180° from the direction of a shadow cast by the Sun. So, to determine the azimuth of the Sun at a particular time, subtract 180° from the azimuth of the dowel's shadow. For example, if the azimuth of the shadow is 300° the azimuth of the Sun is 300° – 180° = 120° Take measurement of the azimuth of the dowel's shadow as often as possible, preferably every hour, between sunrise and sunset. In a table such as Table 4.1, record the time for each shadow measurement and the calculated azimuth of the Sun.
For example, if the first and last azimuth measurements are 110° at 8:00 A.M. and 240° at 6:00 P.M., respectively, the total change in azimuth would be 130° over a 10-hour time period. Thus, the apparent speed of the Sun would be 130° ÷ 10/hour = 13°/hour.
Get the Facts
Topocentric coordinates are also called horizon or altazimuth coordinates. One of the coordinates is the azimuth and the other the altitude. How do these coordinates compare for observers at different locations on Earth? For information see the National Audubon Society Field Guide to the Night Sky (New York: Knopf, 1991) pp. 64–66.
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