Right ascension and declination describe a direction on the celestial sphere in a way that is largely independent of where an observer stands on Earth. Altitude and azimuth describe where that same direction appears in a particular observer’s local sky. The first system is useful for catalogues and star charts; the second tells you where to look now.
Right ascension measures eastward position around the celestial equator
Right ascension, usually abbreviated RA, plays a role similar to longitude on a globe. Astronomers normally express it in hours, minutes and seconds rather than degrees. Twenty-four hours of right ascension span a full 360° circle, so one hour of RA corresponds to 15°.
An object’s RA changes slowly because of effects such as precession and proper motion, but for ordinary observing it behaves like a catalogue coordinate tied to the celestial sphere.
Declination measures position north or south of the celestial equator
Declination is measured in degrees from the celestial equator. Positive declinations lie north of it and negative declinations lie south. The north celestial pole is +90° and the south celestial pole is −90°.
Declination has an immediate observing implication: your latitude controls which declinations can rise above your horizon and which objects can become circumpolar.
Altitude and azimuth describe the local sky
Altitude measures angle above the horizon. The horizon is 0° and the zenith directly overhead is 90°. Azimuth measures direction around the horizon, conventionally from north through east, south and west.
Unlike RA and declination, altitude and azimuth change continuously as Earth rotates. Two observers at different locations will generally measure different altitude and azimuth for the same object at the same moment.
Local sidereal time connects the systems
Local sidereal time tells you which right ascension is approximately crossing your local meridian. When an object’s RA is close to your local sidereal time, the object is near transit and usually near its highest altitude for that date.
Why observers care about altitude
Objects near the horizon are viewed through a longer path of atmosphere. Extinction, turbulence, haze and local obstructions can reduce contrast and sharpness. An object that technically rises may still be a poor target until it climbs higher.
This is why the Night Observation Planner treats altitude as part of observing context rather than simply declaring a planet visible as soon as it crosses the geometric horizon.