Altitude and azimuth: finding things in the sky

Star charts give positions in coordinates fixed to the sky (right ascension and declination). For finding something from where you stand, it is easier to use two angles relative to your own horizon: azimuth and altitude. Astrotempo’s tables and the 3D sky view use them.

Azimuth: which direction

Azimuth is the compass direction, measured along the horizon from north towards east:

Azimuth	Direction
0°	North
90°	East
180°	South
270°	West

So an azimuth of 225° means southwest. On the tonight page each planet’s best direction is given both as a compass point and in degrees.

A note on compasses: a magnetic compass points to magnetic north, which differs from true (geographic) north by an amount that depends on where you are. Astronomical azimuths are measured from true north.

Altitude: how high

Altitude (also called elevation) is the angle above the horizon:

• 0° is the horizon,
• 90° is the zenith, the point directly overhead,
• negative values mean the object is below the horizon.

Objects low in the sky are harder to see: their light passes through much more air, which dims and reddens them, and trees, buildings and haze often hide the lowest few degrees. That is why Astrotempo only counts a planet as visible when it is at least 10° high.

Near the horizon the atmosphere also bends light upwards, by about 34 arcminutes on average according to the US Naval Observatory, so objects there appear slightly higher than they geometrically are. Rise and set times include this effect.

Why positions change

Altitude and azimuth are tied to your horizon, so they depend on where you are and change continuously as Earth rotates: objects rise in the east, reach their highest point when they cross the meridian (due south or due north), and set in the west. Stars close enough to the celestial pole never set. They are called circumpolar.

The Pole Star and your latitude

The north celestial pole is at an altitude equal to your latitude, due north. Polaris is less than 1° from the pole, so from the northern hemisphere its altitude is roughly your latitude: about 46° in Ljubljana or 40° in Madrid, and close to the horizon near the equator. From the southern hemisphere Polaris is never visible.

Try it

The 3D sky view shows the sky for any place and time with a horizon and compass directions. On a phone you can turn on the compass mode and point the phone at the sky to see what is there.

Sources

• US Naval Observatory: Rise, set and twilight definitions (horizon, zenith, meridian transit, refraction)
• Star positions: HYG database (Polaris declination +89.26°, J2000). Coordinates are computed with Astronomy Engine. See methodology.