If Earth keeps moving, how can we point to the same galaxy?
Move through time and watch how RA / Dec stays fixed while the direction and altitude in your local sky change.
How to explore
Press play or drag the timeline, then orbit the 3D dome to see how direction and altitude work together.
View from London
3D coordinate overlay · M31
The fixed RA / Dec grid is projected onto your changing local Alt / Az sky.
Right Ascension · Fixed
00h 42m 44s
Declination · Fixed
+41° 16′
Azimuth · Direction
42.9°
North-east
Altitude · Height
16.5°
Above the horizon
Simulated time
17 Oct 2026, 15:00 UTC
Play time to watch the violet celestial grid turn against the cyan horizon grid while RA / Dec stays unchanged.
Space coordinates · RA / Dec
Stays fixedRight Ascension
00h 42m 44s
Declination
+41° 16′
Think of this as an address on the sky map. It uses a reference frame based on very distant objects, so it does not rotate with the ground beneath us.
Local sky coordinates · Alt / Az
Keeps changingAzimuth
42.9°
Altitude
16.5°
These coordinates tell a telescope which direction to face and how high to tilt right now, so they must be recalculated for the observer's location and time.
A simple way to remember it
1 · Start with a fixed address
Choose a galaxy by its RA / Dec.
2 · Add Earth context
Use the date, time, and observer location.
3 · Point the telescope
Convert the result to Azimuth / Altitude.