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Altitude: Vertical Coordinate

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Author: Janice VanCleave

The celestial sphere is an imaginary sphere that has Earth at its center and on which all the other celestial bodies appear to be located. The altitude of celestial bodies above the horizon on the celestial sphere is measured in degrees, from 0° at the horizon to 90° at the zenith, the point directly overhead.

In this project, you will measure the approximate altitude of a star using your hands as the measuring tool. You will also use the angle of an object's shadow to determine the Sun's altitude.

Procedure

1. On a clear, moonless night, stand outdoors and select a bright star.
2. Use your hands and the method depicted in Figures 3.1 and 3.2 to measure the altitude of the star above the horizon. For example, if you measure a star at three fists and three fingers above the horizon, the star is at an altitude of about 35° above the horizon.

Results

The altitude of stars measured may vary. The one shown in the example is 35°.

Why?

A coordinate system called the altazimuth system is used in astronomy to locate celestial bodies by their altitude (angular height above the horizon) and azimuth (angular distance around the horizon). (See Chapter 4 for more information about the horizontal measurement of azimuth.) In describing this coordinate system, it is convenient to adopt the model of the celestial sphere (an imaginary sphere that has Earth at its center and all other celestial bodies scattered around the sphere). The coordinates (two numbers that identify a location) of altitude and azimuth are angles used to specify positions on the celestial sphere. In this experiment, the coordinate altitude is investigated. Altitude is a vertical measurement on the celestial sphere, measured in degrees above the horizon, from 0° at the horizon to 90° at the zenith. The width of different parts of your hand can be used to measure approximate altitudes of celestial bodies.

Try New Approaches

Altitude lines representing angular distances above the horizon can be imagined to form increasingly smaller, parallel circles from the horizon to the zenith of the celestial sphere. How do the lines of altitude compare to latitude lines. Latitude lines are imaginary parallel circles representing angular distances north and south of a celestial body's or the celestial sphere's equator (an imaginary line running east and west around the middle of a celestial body or the celestial sphere). On a clear, moonless night, face north and find the seven stars of the Big Dipper (see Figure 2.4 in Chapter 2). Follow the two stars in the bowl of the dipper to Polaris. Use your hands to measure the approximate altitude of Polaris above the horizon. Compare the altitude of Polaris to the latitude of your location on a map. Repeat this measurement at different latitudes, or ask friends who live in other parts of the country or world to take measurements for you. For more information about the comparison of celestial altitude and latitude lines, see Janice VanCleave's Constellations for Every Kid (New York: Wiley, 1997), pp. 64 –72.

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