Monday, May 24, 2010

Sky Navigation: Altitude and Azimuth

Whatever the celestial object you desire to observe, you first need to find it(obviously), when we talk about stars (or any other celestial objects) we need to have a system for identifying where they are in the sky.

Of course, we can say that the star Betelgeuse is in the constellation Orion. But to specify exactly where Betelgeuse is and be able to compare readily where it is with the location of any other celestial object or region requires a system that involves numerical values. The simplest such scheme is called the altazimuth system. It tells us where an object is in the hemisphere above the land and sea that we call the sky. Azimuth is the sideways measure in the sky. Altitude is the up–down measure in the sky. Don't confuse this kind of altitude, which is angular altitude given in degrees (as in 360° in a circle) with altitude in Earth’s atmosphere - which is given in feet or meters or miles or kilometers above Earth’s surface.

Azimuth is also angular, also given in degrees. Angular altitude is perhaps a little simpler than azimuth. Two fundamental positions serve as limits at the extremes of altitude: the horizon (the line that forms the border between sky and land or sea) and the zenith (the point exactly overhead). The horizon is set at 0°. If we draw a line from one horizon point straight up through the zenith to the opposite horizon, this distance would be 180°—a dome or half-sphere that is one-half of the full 360° circle that passes both above our heads and below our feet.

And since the distance from horizon to zenith alone is half of 180°-90°-we say that the altitude at the zenith is 90°. Thus, in the altazimuth system, a rising star that is right on the horizon would be at an altitude of 0°; a star that is exactly overhead would be at an altitude of 90°. Any angular height in the sky between these extremes would be a value in degrees between 0° and 90°, of course. Azimuth is the sideways frame of reference in the sky-that is to say, where an object is in relation to the cardinal directions of north, east, south, and west.

That is, by the way, the progression of directions used and, to numerize them, we set due north as equal to 0° azimuth, east as 90°, south as 180°, west as 270°, and, to complete the circle around the sky, north is 360°—or, which is the same thing, 0° again. Thus, instead of having to say that the star Regulus is currently a little south of south-southeast, we could say with much greater precision and economy of expression that the current azimuth of Regulus is 165°.

The altitude and azimuth of all celestial objects must be given for a specific time because these figures keep changing as Earth carries us around with its rotation making stars and other celestial objects appear to move during the course of a night or day. The only object that wouldn’t change altitude and azimuth would be one located exactly above (north of) or below (south of) Earth’s axis of rotation. The bright star one of 2nd magnitude, which comes closest to meeting this condition is a famous one: it is Polaris, the North Star. The mention of Polaris brings us to the other most important, in fact, much more important, positional system used by astronomers: declination and right ascension.

A good instruction on how to find and object
using Altitude-Azimuth Coordinates

You may also want to check these articles:

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