Celestial vs Horizon Coordinates

HK default

Note: Overlaps with RA/Dec and Alt/Az tools — use this for side-by-side comparison.

The same star can be described in two coordinate systems. Horizon (Alt/Az) depends on your location and time. Equatorial (RA/Dec) is fixed on the celestial sphere. Default observer: Hong Kong.

Altitude: 45°

Azimuth: 90°

Horizon view

Alt 45°, Az 90°

Equatorial view

RA 90°, Dec 30°

Before you start

Astronomers use two main coordinate systems. This tool shows the same star described both ways side by side.

Key idea

Horizon coordinates (Alt/Az) depend on where and when you observe. Equatorial coordinates (RA/Dec) are fixed to the stars.

What you see

Side-by-side horizon and equatorial diagrams
Tabs to adjust each system independently

Try this

Set Alt 45°, Az 90° on the horizon side
Switch to equatorial tab and adjust RA/Dec
Compare how each diagram represents the same direction in space

Why it matters in Hong Kong

Default observer is Hong Kong — your horizon is unique to your location on Earth.

Common mistakes

Expecting Alt/Az to stay constant as Earth rotates
Expecting RA/Dec to change with time (they don't for a given star)

Glossary

Local coordinates: Alt/Az — tied to the observer's horizon.
Equatorial coordinates: RA/Dec — tied to the celestial equator.