Altitude and Azimuth: Find Your Way Around the Sky

One angle around, another angle up

Altitude and azimuth describe a direction in your local sky. Azimuth tells you which way to face around the horizon. Altitude, also called elevation, tells you how far above the horizon to look. Neither number tells you how far away the object is.

In the convention used by JPL's observer tables, azimuth starts at true north and increases clockwise: east is 90 degrees, south 180, and west 270. Altitude is 0 degrees on the geometric horizon and 90 degrees at the zenith, directly overhead. A negative altitude places the direction below that horizon.

Reference: NASA JPL: Observer Coordinate FAQ

Translate a coordinate pair into a view

Imagine a prediction of azimuth 225 degrees and altitude 30 degrees. Face southwest, halfway between south and west, then look about a third of the angular distance from the horizon to overhead. The numbers are an illustrative direction, not a forecast for a particular object tonight.

Now change only the altitude to 5 degrees. The target remains southwest but lies very low. A building or tree may hide it even though the prediction correctly places it above the geometric horizon. Positive altitude does not guarantee a clear line of sight.

Keep location and time with the numbers

Earth's rotation changes a star's local direction through the night. Moving to another observing location changes the horizon too. A satellite adds rapid orbital motion, making an old coordinate pair especially unhelpful. Always preserve the observing location, date, time, and time zone with a prediction.

Check whether a compass shows magnetic or true north, and whether a detailed ephemeris includes atmospheric refraction. Right ascension and declination use a different celestial grid; their numbers cannot simply be read as altitude and azimuth. JPL Horizons documents these settings so that observers know which quantity they are using.

Reference: NASA JPL: Horizons Manual

Practice on an obvious target first

Choose the Moon after sunset or a bright star high above nearby obstructions. The aim is to learn the coordinate system before trying to locate something faint or fast-moving.

1. Confirm the location, clock, and north convention in your observing tool.
2. Turn to the azimuth first, then raise your gaze to the altitude.
3. Compare the predicted direction with the target and note anything blocking the view.
4. Repeat later and notice that the local coordinates change while the object remains the same.